CT Physics Final Exam Review

A 10 x 10 matrix consists of how many pixels?

100

What is the pitch in the following scenario: 16- slice scanner, 0.5 mm slice thickness, table movement of 12 mm per rotation?

12/(16 × 0.5) = 12/8 = 1.5

What analog-to-digital converter (ADC) is most accurate with respect to sampling an analog signal?

16-bit ADC

What mathematical technique is used for reconstruction of volumetric MDCT?

180 degree interpolation.

What is the DFOV used for a 320 x 320 square matrix with 0.75 mm pixels, in cm?

24 cm

How many parts will a 8-bit analog-to-digital converter will divide a signal into?

256 parts

How many pixels make up a computed tomography image whose matrix size is 512 x 512?

262144

The approximate inherent filtration of the CT amounts to what thickness of aluminum?

3 mm

The approximate inherent filtration of the CT x-ray amounts to what thickness of Al?

3.0 mm

What is the HU of soft bone?

400

What is the whole body dose limit for radiation workers in the United States?

50 mSv/year

What is the typical matrix size for a medical CT?

512x512

What is the ratio of heat to x-rays in the x-ray tube?

98% heat is produced and 2% x-rays are produced.

What is a voxel?

A 3-D picture element. An isotropic voxel is a cube. This allows for reconstruction in any plane. An anisotropic voxel is not a cube and you cannot process in any plane.

What is a slip ring device?

A brushlike apparatus that provides continuous electrical power and electronic communication across a rotating surface

What is a cupping artifact?

A darkening in the center of a relatively homogeneous structure surrounded by highly attenuating material. An example of this is darkening of the brain, which is surrounded by the skull.

How does image matrix size affect noise?

A decrease in image matrix size decreases noise.

What is contrast resolution?

A decreasing contrast resolution allows for the ability to distinguish between tissues of different densities. Ex) liver and spleen (low HU #'s) Increasing contrast resolution gives the ability to distinguish between tissues of very different densities. Ex) Bone vs muscle, mA's effect contrast resolution. An increase in mA gives better contrast resolution / an increase in kVp gives better contrast resolution.

What is smoothing?

A filter that is very good at low contrast resolution. It uses a smoothing alogrhythm. For example: the brain and abdomen have very little differences in change of density to see details/ change of tissue type.

What is edge enhancement?

A filter that sharpens and crisps images. It adds details to dense anatomy. For example: bones. Gives good spatial resolution.

What are algorithms?

A finite set of steps performed in a prescribed sequence to solve a problem

What kind of phantom is used to measure high-contrast spatial resolution?

A line-pair phantom.

What is interpolation?

A mathematical method of estimating an unknown value using the known values on either side of the unknown

What is Adaptive Statistical Iterative Reconstruction?

A newer method of image reconstruction. Complex method that computes projections from the image, compares it with the original projection data, and updates the image based on the difference. It can reduce image noise and pt dose

What is a pixel?

A picture element. Rows and columns of pixels makeup a matrix. Ex) 512x512 or 1024x1024 Pixel size = DFOV / Matrix size. An increase in matrix size increases the reolution of you have enough data to fill the pixels.

What is a set of ray sums is referred to as?

A projection

What is a partial helical scan?

A scan that collects data from 180° tube arc, plus the degree of arc of the fan angle

In CT image creation using a third- generation design, what is a complete set of ray sums is known as?

A view. A view in the CT process can be compared with observing an object from a single angle. It takes many views to obtain a true understanding of the shape of the object. Similarly, it takes many views to create the CT image.

What component of the CT system converts the electric signal supplied by the detectors into a digital format?

ADC

What converts electrical signals into digital?

ADC

What phantom insert would most likely be represented by -987 HU?

Air

Which of the following determines the total data acquisition time? I. gantry speed II. helical pitch III. detector coverage

All 3

What can be said of an ROI measurement with a standard deviation of 0?

All of the pixels in the ROI have the same value

Compared with conventional computed tomography scanners, slip-ring scanners offer which the following advantage(s): I. removal of cable wraparound II. elimination of start-stop action III. continuous rotation of x-ray tube and minimal interscan delays

All of them

Which of the following factor(s) changes in the attenuation of a heterogeneous beam of radiation?: I. the beam quality II. the beam quantity III. the beam intensity

All of them

Which of the following feature(s) does CT overcome the limitations of radiography and conventional tomography? I. reducing the problem of superimposition II. improving the contrast of the image III. using quantitative detectors and a computer

All of them

Attenuation of a beam of radiation depends on which of the following (or combination)?: I. the atomic number II. the electrons per gram of tissue III. the energy if the radiation

All of them.

Limitations of slice-by-slice sequential computed tomography scanning include: I. longer examination time II. inaccurate generation of three-dimensional images III. possible omitted anatomy because of patient respiration

All three

Which of the following mathematical functions may be used to quantify the spatial resolution of the scanner: PSF, MTF, LSF.

All three.

Put simply, what is a CT image a map of?

Attenuation coefficients.

During prospective ECG gated cardiac CTA studies, the scanning acquisition mode may be described as what?

Axial step and shoot.

Phantom measurements of the uniformity of a CT system is used to primarily assess the negative effects of what?

Beam hardening

What can be used to slow a patient's heart rate and make the rhythm more regular?

Beta blockers.

What is a result of calcifications on the aorta?

Blood backflows.

Does scatter affect spatial resolution or contrast?

CONTRAST!

When was the CT math developed? When was the first commercial scanner built? Why the long gap?

CT math was developed in 1910, but the first commercial scanner did not come out until 1972 because computing technology had to catch up.

What is the equation for the CT number?

CT# = 1000*(mu-mu_water)/mu_water

What two terms are used to describe patient dose in computed tomography?

CTDI & MSAD

What is CTDI_W (name and equation, wrt CTDI_100)?

CTDI_W = (1/3)CTDI_100,center + (2/3)CTDI_100,periphery

What is the equation for CTDI_vol? What does it represent?

CTDI_vol = CTDI_W/pitch ; CTDI_vol represents the average absorbed dose along the longitudinal axis from a series of contiguous CT exposures.

The process of scanning a phantom device of uniform density is known as what?

Calibration

What is the process of scanning a phantom device of known density to improve the accuracy of CT attenuation measurements referred to?

Calibration

How does motion cause artifacts?

Causes ghosting or lines.

What is the primary interaction between x-rays and tissues in CT imaging?

Compton scatter.

What is CTDI (acronym, definition)?

Computed Tomography Dose Index. Radiation dose measured from 14 contiguous slices and normalized to beam width.

Another name for surface rendering is?

Shaded surface display

What term is used to describe the ability of a CT scanner to differentiate objects with minimal differences in attenuation coefficients?

Contrast resolution.

What is the function of the digital- to- analog converters?

Converts the digital signal from the computer into an analog signal for the display monitor

What are kernals?

Convolution algorhythm; aka filters; software. Standard convolution, Edge enhancement, and smoothing.

How does DFOV effect noise?

DFOV effects pixel size. A decrease in pixel size decreases noise. And a decrease in DFOV decreases noise.

What is an SSP? What does it measure? What is it dependent on?

Slice sensitivity profile. It measures the cross-plane spatial resolution. Slice sensitivity is dependent on slice thickness.

Why is it necessary to convert the digitized data from the reconstruction processor to shades of gray?

So that an image can be displayed on the monitor

What two main features are assessed to measure image quality?

Spatial resolution and contrast resolution

What two main features are assessed to measure image quality?

Spatial resolution and contrast resolution.

What is the best way to observe high-frequency data?

Take the FT of it.

What does the Central Slice Theorem state?

The 1D FT of a parallel projection of an object obtained at angle theta equals a line in a 2D FT of the object taken at the same angle.

What two things does the Central Slice Theorem relate?

The 2D FT of an image to the 1D FT of its projection

During CT perfusion studies, the quantity of blood (in mL) contained within 100g of brain tissue is termed what?

The cerebral blood volume (CBV)

What is unique about 4th generation CT imager detectors?

The detectors made the entire ring instead of being just directly accross from the tube.

FWHM of a slice sensitivity profile diagram indicates what?

The effective section width.

What 5 things is the radial resolution determined by?

The focal spot size, the focal spot shape, detector aperture, scanner geometry, and reconstruction algorithm

What kind of transform allows for the reconstruction of an image from a sinogram?

The inverse Radon transform.

Why type of image reconstruction was used in the first prototype CT scanner?

The iterative technique.

What is window level?

The midpoint number of the window width. This tells what tissue we are looking at. Range = WL +/- 1/2WW

What is spatial frequency?

The number of line pairs visible per unit length

What 4 things is the azimuthal resolution determined by?

The number of projection views, the focal spot size, distance to the iso-center, and detector aperture.

Auto tube current modulation software is utilized by the CT system in an effort to do what?

Decrease radiation dose to the patient.

How does increasing mA affect noise?

Decreases noise, but increases patient dose.

How does an increase in kV affect noise?

Decreases noise.

What does decreasing detector size do to noise?

Decreases noise.

Does increasing or decreasing the matrix size result in decreasing noise?

Decreasing the matrix size decreases the amount of noise.

What orientation was the detector array in 2nd generation CT imagers? What about 3rd gen?

Detector arrays in 2nd gen CT imagers were flat whereas they were curved in 3rd gen CT imagers.

What is Display Field of View-DFOV?

Determines how much of the collected raw data are used to create an image. It changes to the DFOV affects image quality by changing pixel size. DFOV works in a manner similar to the zoom on a camera

What is Scan Field of View-SFOV

Determines the area, within the gantry, from which the raw are acquired and determines the number of detector cells collecting data. Anything outside SFOV is not imaged

Why was CTDI_100 developed?

Developed to adapt calculation to length of the pencil chamber used for measurement (100 mm in length)

What is DFOV?

Display field of view effects noise and resolution. A large DFOV uses more detected photons and decreases noise, but it also decreases the resolution. A small DFOV has increased resolution, and can increase noise, but using an increased mA will correct that. Here the image looks bigger. since increasing mA increases patient dose, you need to find the right balance.

What is DLP (acryonym and equation)?

Dose Length Product: DLP = CTDI_vol * scan length

Which company pioneered the development of the first clinically useful computed tomography scanner?

EMI

The detectors convert the attenuation data into what kind of signal?

Electric

Which image reconstruction technique is most used now?

Filtered back projection.

What are Filter Functions?

Filtering is used to minimize streaks on the image that result from back projection (smooth image/reduce artifacts). It is done by a series of complicated mathematical steps, often referred to as reconstruction algorithm. It can also be referred to as algorithms, convolution filters or kernels

What process converts signals in the spatial domain to the frequency domain?

Fourier Transform

Explain what is needed to create x-rays in an x-ray tube.

Free e- are needed to create x-rays. A seperate low voltage filament connects to the cathode filament. Activating the circuit causes intense heating of the filament and e- are released. All of the e- hanging around the filament are kept in equilibrium (not getting bigger or smaller) because the cathode is repelling them. This is called an e- cloud.

From what domain does a Fourier transform change an image and into what domain?

From the spatial domain to the frequency domain

What two type of detectors are used in CT?

Gas ionization and scintillation

Partial volume artifacts can be reduced by doing what?

Getting thinner slices

What contribution did Cormack make to the development of clinical computed tomography?

He developed the solutions to the mathematical problems in computed tomography.

Who developed the first clinically useful computed tomography scanner?

Hounsfield

When discussing the quality of a CT image, what is meant by temporal resolution?

How rapidly data are acquired; it is controlled by gantry rotation speed, the number of detector channels in the system, and the speed with which the system can record changing signals

What equation represents Lambert Beer's law?

I = I_0*e^(-µx)

The basic problem with the back-projection algorithm, prior to filtration, is which of the following factor(s): I. the image is blurred and not useful II. it compares assumptions with measured values III. the image suffers from the star pattern artifact

I and III

The overall goals of automatic tube current modulation are to: I. provide consistent image quality II. increase scan speed III. control dose to the patient

I and III

The photoelectric effect occurs more frequently in tissues with which of the following property(ies)?: I. high atomic number II. low atomic number III. high Z number IV. low Z number

I and III

What is the process by which additional rotations before and after the planned length are taken? I. overranging II. doubling scan III. z-overscanning

I and III

Which of the following removes the classical star pattern (blur) of the back-projection algorithm? I. filtered back projection II. interpolation III. convolution

I and III

Limitations of maximum intensity projections include: I. artifacts from pulsating vessels II. inability to show superimposed structures III. images are three-dimensionally ambiguous unless depth cues are provided

I, II, and III

Three-dimensional rendering algorithms include: I. virtual rendering II. surface rendering III. volume rendering

II and III

Which of the following belongs to the class of analytical algorithms for computed tomography? I. back projection II. filtered back projection III. Fourier reconstruction

II and III

Describe the beam hardening artifact.

If there is too much beam hardening, broad streaks occur that effect how the HU's are read.

What are the mathematical techniques used by the computer to reconstruct the computed tomography image known as?

Image Reconstruction Algorithms

What two technical adjustments would decrease the quantum noise of a CT image?

Increasing the mAs and increasing the section width

The central processing unit (CPU) performs what function?

Interprets computer program instructions and sequences tasks

Why two types of detectors are used in CT imaging?

Ion chambers and scintillators.

If the pitch increases, what happens to the dose?

It decreases

How does increasing the mAs affect image noise?

It decreases it.

How does increasing the slice thickness of a CT image affect noise?

It decreases it.

Describe CT contrast.

It has a higher atomic number than the organs and vessels. This leads to an increase in beam attenuation and shows up white in the image.

An image of a water phantom displays a range of pixel values. What can be said about this image?

It is "noisy."

A water phantom is scanned and several ROIs are placed in the resulting image. The standard deviation measurements from the ROIs range from 15 to 25. What can be said about the image?

It is noisy.

What is spatial resolution?

It is the ability to measure the smallest size of an object.

What is the tube current that is associated with an x-ray tube?

It is the number of e- in the e- cloud. The number associated with this is the mA (milliamperes) which is a direct reflection of the number of e-'s in an e- cloud and therefore the number of x-rays being produced.

On a CT image an ROI is placed within a structure and measured. Its standard deviation is 0. What can be determined about this structure?

It is very homogeneous.

The window width of a specific CT image is set at 300, and the level (or center) is set at 100. How is a structure with a measurement of 280 HU displayed?

It is white.

What is a disadvantage of a bone algorithm?

It reduces the visibility of soft tissue structures.

About how long did it take the first CT scanners to complete a scan? What part of the body was scanned?

It took about 4 minutes to scan the head, which was the only body part measured at the time of release.

Who developed the first whole-body computed tomography scanner?

Ledley

In an ROI on a CT slice, what two things are measured?

Linearity and section width

The broadening of the slice sensitivity profile inherent to helical CT acquisition is said to negatively affect what?

Longitudinal spatial resolution.

What is the equation for magnification?

M = SID/SOD

What is the quantity of radiation dose received by the patient form a series of CT scans referred to as?

MSAD (multiple-scan average dose)

THe quantity of radiation dose received by the patient from a series of CT scans is referred to as what?

MSAD - multiple scan average dose

What is the MTF in relation to the PSF?

MTF = the magnitude of the FT of the PSF

Describe metal artifacts.

Metal artifacts attenuate beam and give streaks in image.

8. What is it called when a patient breathes differently with each data acquisition and areas of anatomy seem to be misplaced in the z axis?

Misregistration

What is MTF? What is it defined as? What does it measure?

Modulation transfer function. It is defined as the ratio of the output modulation to the input modulation. It measures the response of a system to different frequencies.

How does an increase in patient size affect noise?

More dense tissue, and therefore increases noise.

What does MDCT stand for?

Multidetector CT

What is NPS?

Noise power spectrum.

Describe aliasing artifacts.

Not enough data is sampled. If the mA is not high enough to go through the patient it will cause streaking of the image. Happens in more dense tissue and is aka photon starvation.

What theory states that the sampling interval (pixel size) has to be sufficiently small to support the reconstruction and visualization of small objects?

Nyquist theorem

What is the equation for pixel size in terms of DFOV, matrix size, and slice thickness?

Pixel size = DFOV/matrix size

What are the 3 types of computer memory?

ROM-Read only Memory, RAM-Random-Access Memory and WORM-Write-once read many times memory

For performance analysis, it is often more convenient to separate the spatial resolution analysis into what directions?

Radial and azimuthal, which are orthogonal.

In 1917, who proved it possible to reconstruct 3-D objects from an infinite set of projections?

Radon.

Which is a disadvantage of a small focal spot size?

Reduced heat capacity

Increasing what will increase spatial resolution?

Sampling frequency

What does the greek word "tomos" mean?

Section (or slice)

If the linear attenuation coefficients for bone and water are 0.380 and 0.190, respectively, and the scaling factor of the scanner is 1000, what is the CT number for bone?

+1000

What is the HU of air?

-1,000

If the linear attenuation coefficients for fat and water are 0.185 and 0.205, respectively, and the scaling factor for the scanner is 1000, what is the CT number for fat?

-100

What is MSAD (acronym and def)?

-Multiple Scan Average Dose -The average radiation dose over the central scan of the CT study consisting of multiple parallel (axial) scans.

What are some problems with using CTDI (4)?

-Only 2 phantom sizes are used (16 cm and 32 cm) -Patients smaller than CTDI phantoms tend to lead to under-estimation of dose. -Patients larger than CTDI phantoms tend to lead to over-estimation of dose. -Beam widths are larger than the ion chamber (10 cm)

What is the HU of water?

0

What is the computed tomography number for water?

0

Two adjacent pixels are measured to have a difference of 1 hounsfield unit. This amounts to a tissue density difference of approximately what?

0.1%

If the field of view for an examination is 30 cm and the matrix size is 1024 x 1024, what is the approximate pixel size?

0.3 mm

What is the lead-equivalent thickness requirement for protective apparel?

0.5 mm

If the field of view is 30 cm with a matrix size of 512 x 512, what is the pixel size, in mm?

0.58 mm

A CT scanner capable of producing an image that is a perfect reproduction of the actual anatomic section is said to have an MTF of what?

1

What is the HU of hard bone?

1,000

When the mAs value for a CT acquisition is doubled, what happens to the patient dose?

The patient dose increases.

How is the probability of the photoelectric effect related to energy and Z-number?

The probability is proportional to Z^3 over E^3

What are the thousands of bits of data acquired by the CT system with each scan called?

The raw data

What was invented to fix the problem with rotating cables?

The slip ring.

Does convolution apply image filtration in the spatial domain or the frequency domain?

The spatial domain

What is kVp?

The voltage coming through the cathode which is equal to the kinetic energy of the e-'s.

The assignment of different generations of CT scanners is based on the configurations of what two things?

The x-ray tube and detectors.

Describe the filaments in an x-ray tube.

They are made of tungsten and are like the wiring in a light bulb. There are usually 2, one long one and one short one. The long one is used for long scanning times (it produces more e- because it has a greater surface area) because it can handle more heat, and the short one is used when you want an increase in resolution.

What do pre-patient and post-patient collimators do?

They help shape the beam. A pre-patient collimator keeps the beam width constant, which is equal to the slice thickness, and therefore the slice thickness is kept constant as well. A post-patient collimator works with other filters to keep slices at a constant width. It absorbs what gets scattered in a patients body.

Describe detector malfunction artifacts.

This produces ring artifacts. If the detector is not working properly it does not attenuate beam properly and causes rings. This will not allow for proper calculation of the linear attenuation coefficient.

What is Shannon's sampling theorem?

To avoid aliasing, the original data must be sampled at a rate of at least twice the highest spatial frequency contained in the signal.

How much anatomy (lengthwise) will be covered in a helical scan when the following parameters are selected: 15 seconds total acquisition time, 0.5 seconds gantry rotation time, 2 mm slice thickness, 4 slices per rotation, pitch of 1.5?

To calculate scan coverage use the formula: Pitch × total acquisition time × 1/rotation time × (slice thickness × slices per rotation). Therefore, 1.5 × 15 × 1/0.5 s × (2 mm × 4) = 360 mm.

What is the goal of helical interpolation methods?

To take the slant and the blur out of the helical image so that they closely resemble images taken from axial methods

What is noise?

Too few photons leads to an increase in noise. Not enough photons reach the detector and therefore you get noise that makes the picture appear grainy.

Compared with helical scanning, what are the primary disadvantages to the axial method of scanning?

Total examination time is longer; data reconstruction is more limited

What movement did the first generation CT scanner use? How many detectors were used?

Translate-rotate. One detector was used.

Explain generally how the CT works.

Tube and detectors rotate around the patient. When x-rays strike the detectors, the info is changed into electrical signal and then sent to a computer for processing.

When using a 3rd generation CT scanner, it is important that reference detectors positioned at peripheral portions of the detector array be exposed to what?

Unattenuated radiation.

What are collimators?

Used to help create the shape of the x-ray beam. It is moved in and out to do so. In CT this is called slice thickness. The width of the beam allowed through is equal to slice thickness. Collimators help absorb excess radiation so there is less scatter. Less scatter helps improve image contrast.

What is window width?

WW is the range of HU you get to see in a scan; or the total number shades of grey. For bones you would scan 100-1000. It is important to change the WW when imaging the brain to be able to tell the difference between white matter and grey matter. A narrow WW will give fewer shades of grey to allow you to see differences in tissues.

Which x-ray interaction do we like the most in CT imaging? Which do we like the least?

We like the photoelectric effect, but we don't like scatter.

What is a pulmonary embolism?

When a blood clot forms in the leg, breaks off, and travels to the lungs.

What is bolus triggering?

When the contrast bolus itself initiates the scan. Once adequate enhancement is seen in a ROI, image acquisition begins.

What occurs when the e-'s strike the anode?

When the e-'s hit the anode they hit it at what is called the focal spot. The anode continuously rotates to help reduce heat. The focal spot is made of tungsten because it has a very high melting point and it dissipates heat faster than other metals. KE is converted into electromagnetic radiation in the form of x-rays, this is called bremmstrahlung. A spectrum of enrgies ranging from 1/3 to 1/2 the peak energies occur. The peak energy is set on the control of the high voltage cathode.

What si the true test of the quality of a specific image?

Whether it serves the purpose for which it was acquired.

Describe the process of CT reconstruction.

X-ray beams go through the patient and strike the detector and then are converted with Fourier transform. Fourier transform calculates how many x-rays actually hit the detector and therefore how many were attenuated. These get recalculated into linear attenuation coefficents and quantitized. Quantization involves assigning a number based on the amount of attenuation. This number is called a CT number or Hounsfiled unit and represents the anatomy in different shades of gray. An increased density absorbs more x-rays and therefore has a higher HU.

How are the x-rays directions controlled?

X-rays are emitted in all directions from the anode. The anode is angled to where you want them, but they still go in all directions. Housing of the x-ray tube has metal that absorbs x-rays that we don't want (x-rays going in different directions from where we aim them). Also collimators help create the shape wanted from the x-ray beams.

What is interpolation?

a mathmatical method of estimating the value of an unknown function using the known value on either side of the function

A histogram in digital image processing is:

a plot of the number of pixels as a function of the number of gray levels

What is the true test of the quality of a specific image? a. Whether it serves the purpose for which it was acquired b. Whether it has a limiting resolution of 7 lp/mm or greater c. Whether it has an MTF of greater than 1 d. Whether pixel size is 0.5 mm or smaller

a. Although many objective measures of image quality exist, the true test of an image is whether it is useful in providing an accurate diagnosis. For example, an image of an infant using a very low technique may appear quite noisy, and by measurable standards the quality may be quite low, but it may still be adequate if the image is taken to follow up a large abnormality, such as an abscess.

Which has become the favored 3D imaging technique in CT? a. Volume rendering b. Surface rendering c. MIP d. MinIP

a. An advantage of VR over other 3D techniques is that all voxels contribute to the image. This allows VR images to display multiple tissues and show their relationship to one another.

All of the following were improvements necessary to make helical scanning possible EXCEPT: a. fourth- generation scanner design. b. gantries with slip ring designs. c. software that adjusts for table motion. d. improved raw data management.

a. Helical scanners can be of either the third- or fourth- generation design. However, because the fourth- generation design contains so many detectors, it could not be easily adapted to MDCT. Therefore, all scanners made today are of the third- generation design.

Which of the following are key aspects of the axial method of data acquisition? a. The table remains stationary while the x- ray tube rotates within the gantry, collecting data. b. Multiple parallel rows of detectors are needed. c. The table moves continuously throughout the data acquisition. d. Each slice is created from data acquired during a 180° rotation of the x- ray tube.

a. In an axial sequence the table moves to the correct position and then stops while the gantry rotates. Each slice is most often create from data acquired during a 360° gantry rotation. Axial scans can be performed with either SDCT or MDCT systems.

Which can be attributed to the uncoupling effect? a. Even when the mAs or kVp setting is too high, a good image results. b. When mAs or kVp settings are either too high or too low, quantum mottle results. c. mA and scan time have no relationship to the quantity of x- ray produced. d. Normal x- ray physics are reversed; mAs controls the quality of the x- ray beam, whereas the kVp controls the quantity of the x- ray beam.

a. Many technologists are not aware that with digital technology the image quality is largely uncoupled from the dose, so even when an mAs or kVp setting that is too high is used, a good image results. This effect can make it difficult to identify when a dose that is higher than necessary is used, so that adjustment can be made to future scans.

Using a standard 512 matrix for all studies, which of the following contains the smallest pixels? a. A scan of the internal auditory canals, in which DFOV is 16 cm b. A scan of the brain, in which DFOV is 25 cm c. A scan of the chest, in which DFOV is 35 cm d. A scan of the abdomen, in which DFOV is 42

a. Pixel size can be calculated by dividing the DFOV by the matrix size. Therefore, the pixel size of choice (a) is 0.31 mm; choice (b) is 0.49 mm, choice (c) is 0.68 mm, and choice (d) is 0.82 mm. The smaller the pixel size, the higher the spatial resolution of the image (holding all other factors constant).

In creating a surface-rendered image, what can happen if the threshold set is too narrow? a. Actual protruding structures can be imperceptible. b. Non-tissue materials, such as fluids, can be displayed as if they were tissue and can obscure protruding structures. c. Too much data are included so that less powerful computers may not be able to generate a display. d. The SR image generated cannot be rotated.

a. Setting the appropriate threshold CT values for surface rendering is critical. Setting the threshold too high will include unwanted structures that may obscure the area of interest; setting the threshold too low may cause important structures to be omitted from the display.

Which of the following does increasing the scan field of view do? a. increases the number of detector cells collecting data. b. increases the range of HU displayed on the image. c. decreases the pixel size. d. decreases the display field of view.

a. The SFOV of view determines the size of the fan beam, which, in turn, determines the number of detector cells that collect data. (Comprehensive Text Chapter 3 ; Heading: Overview of Image Reconstruction/Scan Field of View)

Which is a TRUE statement regarding beam hardening? a. The beam is hardened more by bone and less by fat. b. The beam is hardened more by air and less by bone. c. Nothing can be done to minimize beam- hardening artifacts. d. Lowering the kVp is the best method of reducing beam-hardening artifacts.

a. The beam is hardened more by dense objects. Increasing the kVp will increase the average photon energy of the beam, reducing beam hardening to some extent. The best strategy to minimize beam- hardening artifacts is to select the appropriate SFOV.

Which of the following components is NOT housed within the gantry? a. Three-phase generator b. High- frequency generator c. Slip rings d. Xenon gas detectors

a. Three-phase generators are standalone units located near the gantry

In what situation would overlapping reconstructions for subsequent image rendering not be indicated? a. Slice thickness = 0.5 mm, DFOV = 25 b. Slice thickness = 2 mm, DFOV = 35 c. Slice thickness = 5 mm, DFOV = 42 d. Slice thickness = 7 mm, DFOV = 45

a. When the voxels from the source data are isotropic, or near isotropic, overlapping reconstructions provide little benefit and are not generally worth the inconvenience. To determine whether a voxel is isotropic, first calculate pixel size (DFOV in mm/512), then compare the pixel size to the slice thickness. When slice thickness is considerably greater than the pixel size, overlapping reconstructions are likely to be beneficial. In choice (a) the pixel size is 0.48— nearly the same as the slice thickness.

What can be done to improve the linearity of a CT system? a. Daily calibrations b. Increasing mAs c. Decreasing kVp d. Widening the window width when viewing the images

a. With time a system's linearity can be degraded by small changes in detector channel variation and responses. Daily calibration helps to avoid fluctuation in linearity by compensating for these tiny changes.

What reconstruction algorithm was used by Hounsfield in his original experiments?

algebraic reconstruction technique

Artifacts that result from undersampling are called?

aliasing

Which type of artifact will appear on a digital image as a result of poor sampling of the analog signal?

aliasing

What are artifacts that result from undersampling are called?

aliasing artifacts.

Which of the following factors affects in-plane spatial resolution? I. focal spot size II. detector cell size III. sampling frequency

all 3

Which of the following factor(s) is/are the purpose of collimation in CT?: I. to protect the patient II. to ensure a constant beam width at the detector III. to reduce scatter radiation

all of them

The window width and window length settings for a given examination of the abdomen are 400 and 0, respectively. Which of the following is true?

all structures between -200 and 200 will spread through the gray scale

What is"a precise set of steps to be performed in a specific order to solve a problem"?

an algorithm.

What converts an analog signal (voltage) into digital data (numbers)?

an analog-to-digital converter

What can be done to reduce the effect of dental fillings on a scan of the sinus? This is also effective for reducing dose to the lens of the eye during a brain scan.

angle the gantry

What is the reduction of the intensity of a beam of x rays as it passes through the patient known as?

attenuation

Raw data that results from an MDCT scan acquisition is used so that the 1- mm slices are combined to produce thicker slices for viewing. This is called what? a. 3D reformation. b. image reconstruction. c. segmentation. d. multiplanar reformation.

b. Any process that reuses the raw data is called reconstruction. Reformation, whether it is multiplanar or 3D, uses only image data.

For which application are the raw data necessary? a. To magnify the image b. To decrease the display field size c. To create a histogram d. To obtain a Hounsfield measurement of a specific structure

b. Changing the display field requires the raw data. In doing this, the pixel size decreases and the spatial resolution increases.

Basic ingredients defining a helical scan include all of the following EXCEPT: a. a continually rotating x- ray tube. b. multiple parallel rows of detectors. c. constant x- ray output. d. uninterrupted table movement.

b. Helical scan systems can contain either a single row of detectors or multiple parallel rows.

Which is a disadvantage to workstation- created MPRs? a. They require that the raw data be sent to the workstation, which is difficult to transmit via the PACS. b. To produce the highest quality MPRs the thinnest slices must be sent to the workstation. This amount of data can slow down the PACS. c. Most radiologists have not been trained to

b. In many institutions, MDCT data are combined to create thicker slices for viewing. These slices tend to be better for radiologist interpretation and easier to transmit via the PACS, and consume less computer memory to store. However, to create the highest quality reformations on the workstation, the thinnest possible slices must be available. Data sets from MDCT are frequently huge, potentially slowing down the PACS network. Only image data are used to create MPRs, not raw data.

All of the following are key aspects of a quality assurance program EXCEPT a. the tests that make up the program must be performed on a regular basis. b. all tests must be completed by a medical physicist. c. the results from all tests must be recorded using a consistent format. d. documentation should indicate whether the tested parameter is within specified guidelines.

b. Performing the tests in a quality assurance program is most often a responsibility shared by technologists and medical physicists.

Assume the raw data are still available. In what scenario would it be impossible to create an MPR? a. Source images vary in slice thickness. b. Source images vary in gantry tilt. c. Source images vary in image center. d. Source images vary in DFOV.

b. To create a reformation, source images must possess the same DFOV, image center, and gantry tilt, and they must be contiguous. If the raw data are available, new source images can be created that possess the same center (c) and DFOV (d). Reformations can be made from images with variable slice thicknesses. However, nothing can be done retrospectively that will change the gantry angle of the original acquisition.

VR techniques assign each voxel an opacity value based on its ___. This value determines the degree the voxel will contribute to the final image. a. location b. Hounsfield units c. depth d. width

b. VR displays are built by collecting and manipulating data along an imaginary line from the viewer's eye through the data set. VR techniques assign each voxel an opacity value based on its Hounsfield unit, then they use this value to determine how much the voxel will contribute (along with other voxels along the same line) to the final image.

What is the process of converting the data from the attenuation profile to a matrix is known as?

back projection

What term is used to refer to the size and shape of the beam emanating from the x-ray tube and passing through the patient?

beam geometry

What refers to an increase in the mean energy of the x-ray beam as it passes through the patient?

beam hardening

Cupping artifacts most commonly occur in what part of the anatomy?

brain

If the main tissue of interest is liver, which of the following is the best approximate window level setting?: a. −600 b. 0 c. 50 d. 600

c. 50 HU

Which term describes an isotropic voxel? a. A rectangular solid b. A square c. A cube d. A slab

c. An isotropic voxel is a cube, measuring the same in the x, y, and z directions.

What quality control test must be performed by a medical physicist? a. Cross- field uniformity b. Spatial resolution c. Radiation dose measurements d. Laser light accuracy

c. Dose measurement must be performed for each of a facility's CT scanners and must be performed by a medical physicist.

A limitation of scanner-created MPRs is what?: a. Only one examination protocol per scanner can be programmed to automatically create MPRs. b. They take more time to create than manually produced MPRs. c. In most cases, only straight sagittal and coronal planes can be automatically generated. d. They are not consistently produced, so technologists must remember to check to see whether they were created.

c. On most scanners, if oblique or curved reformations are needed the technologist must create them manually.

All of the following are scan parameters EXCEPT: a. mAs. b. slice thickness. c. matrix. d. pitch.

c. Scan parameters are those factors that can be adjusted by the operator. Modern scanners do not allow a choice of matrix. Other scan parameters are DFOV, reconstruction algorithm, and kVp.

The process of selectively removing or isolating information from the 3D data set is referred to as a. clipping. b. cutting. c. segmentation. d. dissociation.

c. Segmentation is done to remove obscuring structures from the 3D image. (Comprehensive Text Chapter 8 ; Heading: Image Reformation/Region- of- Interest Editing)

The interscan delay inherent in an axial scan method is caused by which factor? a. Tube cooling b. Detector realignment c. Table movement between data acquisitions d. Serial image reconstruction algorithms that must reconstruct data from each slice before data for the next slice can be acquired

c. Since the table remains stationary during data acquisition, there must be a slight pause between acquisitions while the table moves to the next location. Tube cooling may also contribute to the interscan delay, but only after many slices have been acquired. Detectors are not realigned between data acquisitions , and all current systems perform image reconstruction in a parallel, not a serial, fashion.

Which of the following is NOT a reconstruction possibility when an MDCT system produces four 1- mm slices with each gantry rotation? a. Slices can be combined to create two 2- mm slices. b. Slices can be combined to create one 4- mm slice. c. Slices can be divided to produce 16 0.25- mm slices. d. Slices can be reconstructed to create images that overlap by 0.5 mm.

c. The thinnest images that can be reconstructed for a data set are often predetermined by the slice thickness used for the data acquisition. Images can be added together to create thicker slices for viewing, but in many instances data cannot be divided to produce thinner slices. It is never possible to create an image that is thinner than the size of the individual detector cell. Any helical data allow the slice incrementation to be changed retrospectively.

What advanced tool in computed tomography allows users to view a large set of images very quickly?

cine visualization

What determines the slice thickness in computed tomography?

collimator width

What term describes the ability of a CT scanner to differentiate objects with minimal differences in attenuation coefficients?

contrast resolution

To sharpen the image, a(n) ___________ process is applied to modify the frequency contents of the projection before back-projection

convolution

What is the process of applying a filter function to an attenuation profile is known as ?

convolution

What is another term for filtered back projection?

convolution method

A water phantom is scanned and several ROIs are placed in the resulting image. The ROIs placed at the perimeter of the image measure differently from the ROIs near the center. This indicates a problem with what?

cross- field uniformity.

What quality assurance test uses ROI's placed at the center and edges of a water phantom?

cross-field uniformity

How often is the accuracy of a scanner's slice thickness tested? a. Daily b. Weekly c. Monthly d. Semiannually

d.

Which of the following is a TRUE statement regarding MPRs? a. They are created from raw data. b. They are 3D in nature. c. All MPR images have the same image quality as the source image. d. They can be created either at the operator's console or at a separate workstation

d. An MPR uses only image data and it is 2D in nature (meaning it displays the original CT attenuation values). The image quality of an MPR matches that of the source image only when the source data contains isotropic voxels. Modern software allows MPRs to be created quickly and easily from either the operator's console or a workstation.

Which of the following combinations of mA and scan time will result in 350 mAs and be the best choice for a cardiac scan? a. mA = 300, scan time = 0.50 seconds b. mA = 200, scan time = 2 seconds c. mA = 350, scan time = 1 second d. mA = 700, scan time = 0.5 seconds

d. Choice (a) is 150 mAs. Choice (b) is 400 mAs. Both (c) and (d) equal 350 mAs; however, choice (d) is preferable because the shortest possible scan time should be used for cardiac imaging to reduce the effect of cardiac motion. (Comprehensive Text Chapter 6 ; Heading: Scanning Parameters/Milliampere- Second Setting)

All of the following are synonyms for helical scanning EXCEPT: a. volumetric scanning. b. spiral scanning. c. continuous acquisition scanning. d. dynamic scanning.

d. Dynamic scanning refers to repeating data acquisitions in the same location. Hence, it is also called nonincremental scanning. This is most frequently done to watch how a structure fills with an iodinated contrast agent. Dynamic scans are most often done using a helical scan method, but they can also be performed with axial scans.

A magnification factor of 1.5 is used to enlarge the image data, resulting in which of the following? a decrease in the pixel size. b. an increase in the pixel size. c. an inaccuracy in any subsequent distance measurement. d. an image that is larger and may allow a more accurate cursor placement for measurements .

d. Image magnification is not synonymous with changing the display field size. Because magnification uses only image data and not raw data, pixel size is not affected. A magnified image retains accuracy in all image measurements.

An MIP is a good method to display all of the following EXCEPT a. pulmonary nodules. b. fracture extent. c. contrast-filled coronary arteries. d. bronchial tree.

d. MIPs are best used for higher attenuation structures such as bone, calcifications, and contrast- filled structures. Lower attenuation structures, such as the bronchial tree, are not well visualized.

Which of the following is an imaging challenge that depends on contrast resolution? a. A calcified nodule in the lung b. Tiny, contrast- filled arteries that are just 1 mm apart c. Bony erosion in the ossicles of the internal auditory canal d. Distinguishing between the white matter and gray matter of the brain

d. The ability to differentiate on an image a structure that varies only slightly in density from its surrounding is its contrast resolution. Brain imaging requires good low- contrast resolution because of the small density difference between white matter and gray matter. The ability to resolve as separate objects small, high- contrast objects, such as contrast- filled vessels, tiny bones, or a calcified nodule, is a system's spatial resolution.

Decreasing the window width in a CT image decreases which of the following?: a. slice thickness. b. mAs. c. the appearance of quantum mottle (image noise). d. the anatomic diversity displayed.

d. the anatomic diversity displayed

Doing what to slice thickness would increase the noise of the CT image?

decreased slice thickness

What is the disadvantage of CT with respect to conventional radiography?

decreased spatial resolution

Partial volume artifacts can be reduced by doing what?

decreasing slice thickness

Regarding the detectors in the CT system, what is geometric efficiency controlled primarily by?

detector spacing and aperture.

In which of the following are numerical representations of objects subjected to a series of operations to produce a desired result?

digital image processing

What is the pitch ratio expressed as?

distance the table travels during 360 degrees to the slice thickness

What dose descriptor is used to describe a measurement of the total amount of exposure for a series of scans?

dose-length product

What is the ratio of the largest signal to the smallest signal measured by a computed tomography detector known as?

dynamic range

What is the equation for effective mAs?

effective mAs = true mAs/pitch

What characterizes fourth-generation CT scanners?

fan beam geometry and complete rotation of x-ray tube around a stationary ring of detectors

What generation of CT is classified as a high-speed computed tomography scanner?

fifth-generation CT scanner

What generation of scanner uses an electron gun to "shoot" electrons toward a target ring?

fifth-generation scanner

In which scanner generation is the apex of the fan beam located outside the detector?

fourth-generation scanner

Which type of image processing technique is intended to modify the spatial position of the pixels in an image?

geometric processing operation

What is the ability of the tube to withstand by-product heat called?

heat capacity

What term describes the scanner's ability to resolve closely placed objects that are significantly different than the background?

high-contrast resolution

What is a plot of the pixel values as a function of their frequency of occurrence known as?

histogram

What can be done to reduce the amount of data that makes up the image?

image compression

What is the most straightforward way to reduce or eliminate the motion of an object?

increase scan speed

What happens to the noise level when a high spatial resolution kernel is used?

increased

What visualization tool refers to the viewing and evaluation of the images in the axial data set by panning through the set of images?

interactive cine

What is a mathematical technique used to estimate the value of a function from known values on either side of the function?

interpolation

What is the easiest and probably the most accurate method used to report dose?

ionization chamber

What is convolution?

is applying a filter to an attenuation profile

What reconstruction algorithm compares an assumption with the measured values, makes corrections, and then repeats the process?

iterative reconstruction algorithms

What technical factor determines the penetrating power of the photons coming from the x-ray tube?

kVp

Which of the following factors does not affect the dimensions of a voxel

kernel

What units is the power capacity of the generator is listed in

kilowatts (kW)

The spatial resolution of a CT scanner is usually expressed as what?

line pair/cm

A CT test phantom with a variety of known densities is used to test for what?

linearity

What is the term is used to describe the relation between the linear attenuation coefficients of an object and the calculated CT number?

linearity

Which type(s) of image compression results in no loss of information in the compressed image data?

lossless and reversible

What is the ability to differentiate a structure that varies only slightly in density from its surrounding is referred to as?

low- contrast resolution.

What technique factor determines the quantity of photons incident on the patient for the duration of the exposure?

mAs

What algorithm is used only for tissues with the greatest attenuation?

maximum intensity projection

What visualization tool is the most frequently used in computed tomographic angiography examinations to display the structure of vessels?

maximum intensity projection (MIP)

What technical factor exhibits a direct effect on patient dose?

milliampere-seconds (mAs)

What can be used to compare the performance of different computed tomography systems?

modulation transfer function

What is a graphical representation of the CT systems' capability of passing information through it to the observer is called?

modulation transfer function (MTF).

What visualization tool reconstructs faster than any other three-dimensional technique?

multiplanar reconstruction

What was the first dose descriptor to be identified?

multiple-scan average dose

All of the following are major categories of artifacts except:

noise

What is defined as the distance (in millimeters) that the computed tomography table moves during one revolution of the x-ray tube?

pitch

What is the term used to describe a ratio of the difference the table travels per revolution to nominal beam collimation?

pitch

What is the equation for pixel size?

pixel size = DFOV/matrix size

What can the window width in computed tomography be defined as?

range of computed tomography numbers

To what can Filter functions can only be applied?

raw image data

An image that is reconstructed a second time with some changes in technical factors is termed as what?

retrospective reconstruction

What is the logical order in digitizing an image?

scanning, sampling, and quantization

What is another name for surface rendering?

shaded-surface display

What generation of scanners, also called dual-source computed tomography scanners, consists of two x-ray tubes and two sets of detectors?

sixth generation

What term describes how thick a section is imaged and to what extent details within the section contribute to the signal?

slice sensitivity profile

What is cross-plane spatial resolution traditionally described by?

slice-sensitivity profile

Which scanner allows for continuous rotation of the x-ray tube while the patient moves simultaneously through the gantry aperture?

spiral/helical scanner

Metallic materials usually present as what kind of artifacts on the image?

streaking artifacts

What is an indication of a computed tomography system's ability to freeze motions of the scanned object?

temporal resolution

What is Prospective Reconstruction?

that which is auto-matically produced during scanning

What is the total number of gray levels called?

the gray scale

What is Back Projection?

the process of converting the data from the attenuation profile to a matrix

What is Image Reconstruction?

the process of using raw data to create an image.

What is the acronym CTDI is used to describe?

the radiation dose to the patient during a single slice CT scan

What is necessary to reconstruct an image with a larger display field of view?

the raw data

What does changing the reconstruction algorithm change?

the way the raw data are manipulated to reconstruct the image. This contrasts with changing the window setting, which merely changes the way the image is viewed.

What generation computed tomography scanner is based on a fan beam geometry and complete rotation of the x-ray tube and detectors?

third-generation scanner

What is the purpose of image enhancement?

to generate an image more pleasing to the observer

What is Retrospective Reconstruction?

using the same raw data later to create a new image

What visualization technique uses all of the information in the axial data set to display internal structures?

volume rendering

Cupping and peaking artifacts can be observed by the use of which type of phantom?

water

What is the center, or mid point, of the range of computed tomography numbers called?

window level

What controls the range in contrast of the computed tomography image being viewed?

window width

What is the range of CT numbers in the viewed image is called?

window width

What gas is most commonly used in gas ionization computed tomography detectors?

xenon

What Hounsfield values are in the naturally occurring range (i.e., not manmade objects like surgical clips)?

−1,000 (air) to 1,000 (dense bone)