CT Exam 3

Which of the following is FALSE about spatial resolution of the image? A. Resolution is dependent on matrix size B. Resolution is dependent on reconstruction FOV C. Resolution is dependent on scanned FOV A. 1 only B. 2 only C. 3 only D. 1 2 and 3

( C ) - Resolution is dependent on scanned FOV -> The spatial resolution of an image is dependent upon the reconstruction field of view and the image matrix. Image spatial resolution is not dependent upon the scanned field of view. In fact, it is possible to retrospectively reconstruct images with several different resolutions from the same scanned raw data set.

If a thin slice is used, which of the following would help reduce the apparent image noise? A. reduced reconstruction field of view B. increased matrix C. Increased mAs D. Increased kVp

( C ) -> when thin slices are used, there is an increase in the appearance of image noise which is proportional to the reduction in slice thickness. Aside from iterative reconstruction where available, increasing the mAs is the most common way of partially compensating for this increased grainy appearance. Since the mAs affects the dose to the patient, it should only be increased if the result would otherwise not be diagnostic. (You originally picked D -> increasing kVp)

If the slice thickness is changed from 5 mm to 7 mm and all other parameters are held constant, _______% more photons will be used in making the slice A. 30% B. 40% C. 60% D. 80%

(B) -> As the slice thickness is increased, the number of photons used in making the slice is increased in the same proportion. When changing from 5 to 7 mm, that is a difference of 2 mm. Thus, 2 mm / 5 mm x 100% = 40%. (You picked A originally)

True or False Magnification increases the spatial resolution of the image but also increases noise

*** Important to understand Magnification of an image does not increase the spatial resolution of the image nor increase the appearance of noise. It simply enlarges the individual pixels of the reconstructed image for display purposes. Magnification is applied to the image data and not the raw data.

The number of photons measured by detectors is reduced in a ____ fashion as the thickness of the tissue is increased 1. Linear 2. Nonlinear 3. Proportional 4. Expontential

4. Exponential

Which does the Fourier transform calculate? A. Attenuation to detector time B. Patient dose C. Convolution D. Projection ata

A. Attenuation to detector time

The ability to differentiate small differences in density on an image A. Contrast resolution B. Spatial resolution C. Temporal resolution D. Detector efficiency

A. Contrast resolution

Which of the following is a filtering technique in which the reformatted raw data are copied to create what looks like the original raw data set> A. Convolution B. Kernel C. Multiplanar reconstruction D. Sectional reformatting

A. Convolution

Which of the following terms is also referred to as a convolution algorithm? A. Kernel B. Interpolation C. Ring artifact D. Iterative algorithm

A. Kernel

On MDCT, the final reconstructed slice thickness is determined by the: A. Number of rows of detectors and the size of the detectors whose attenuation at a gets combined B. Pitch C. Reconstruction interval D. Collimation of the x-ray beam

A. Number of rows of detectors and the size of the detectors whose attenuation at a gets combined

Which of the following parameters allows adjacent slices to be overlapped without increasing the dose to the patient? A. Reconstruction interval B. Slice thickness C. Tabel increment D. Pitch

A. Reconstruction interval

This artifact occurs when the sampler does not collect enough bits of the signal for the ADC converter. There is not enough information to fill in the gaps. Occurs when the computer adds information in place of missing data

Aliasing Artifacts

This artifact occurs when the sampler does not collect enough bits of the signal for the ADC converter. There is not enough information to fill in the gaps. It appears as streaks in the image

Aliasing artifacts

An image of thicker anatomical slice appears ____ noisy than one from a thin anatomical slice and why?

An image of thicker anatomical slice appears LESS noisy than one from a thin anatomical slice and why? Because there are more photons detected

a signal that is a continuous stream of information

Analog signal

The DAS is made up of 3 components. What are those 3 components

Analog to digital converted (ADC). digital to analog converted (DAC) and 2 types of amplifiers.

takes the continuous stream of analog data and selects bits and pieces to create a new digital stream of data

Analog to digital converter

Which of the following artifacts is a result of faulty detector elements? A. Partial volume averaging B Ring artifact C. Aliasing artifacts D. Streaking

B Ring artifact

Dose modulation is most closely related to which of the following terms? 1. Automatic mA 2. Convolution 3. Automatic exposure control A. 1 and 2 B 1 and 3 C. 2 and 3 D. 1 2 ad 3

B. 1 and 3

A 16 slice helical scanner produces ____ images from a single slice of scan data A. 8 B. 16 C. 32 D. 64

B. 16

Which of the following odes NOT occur when reconstruction FOV is decreased? 1. Spatial resolution is increased 2. Pixel size is increased 3. Noise in the image increases A. 1 only B. 2 only C. 3 only D. 1 2 and 3

B. 2 only

Which of the following includes a sampler, quantizer and a coder? A. ADC B. DAC C. DAS D. Detector array

B. ADC

Which of the following includes a sampler, quantizer, and a coder? A. DAC B. ADC C. DAS D. Detector array

B. ADC

CT Numbers _________ would appear black if a window width of 400 and a window level of -100 are used A. above +200 B. below -300 C. Below - 200 D. Above +100

B. Below - 300

The data acquisition system (DAS) is located: A. Between the detectors and the analog to digital converted (ADC) B. Between the detectors and the computer C. Between the detectors and the digital to analog converter (DAC) D. Int he gantry

B. Between the detectors and the computer

Which operation forms the basis for Car reconstruction? A. Lambert Beer Law B. Fourier Transform C. Convolution D. Filtered back projection

B. Fourier Transform

Which operation forms the basis for CT reconstruciton? A. Lambert Beer Law B. Fournier Transform C. Convolution D. Filtered back projection

B. Fournier Transform

Increased pitch results in: A. increased spatial resolution B. increased partial volume averaging C. Increased image contrast D. Ring artifacts

B. Increased partial volume averaging

Compared with volume acquisition, axial acquisition results in: A. Increased noise B. lower partial volume averaging C. Better high contrast resolution D. Thinner slices

B. Lower partial volume averaging

What is one disadvantage with using thicker slices, and why? A. Beam Hardening B. Partial volume Averaging C. Noise D. Artifacts

B. Partial volume averaging Because it allows the averaging of several millimeters of varying density tissues.

A measure of the size of the smallest object that can be visualized or resolved, in an image A. Contrast resolution B. Spatial resolution C. Temporal resolution D. Detector efficiency

B. Spatial resolution

A digital to analog converter is needed to: A. Change the remnant beam to an electric signal B. Transform the electric signal to light C. Display data on the LCD monitor D. Encode transmitted radiation into binary data

B. Transform the electric signal to light

combines the incoming raw data with what a similar scan would look like and creates a data set that represents the anticipated outcome. This makes an educated guess.

Back projection

when the x-ray beam passes through a very dense area and the tissue absorbs the lower-energy, or lower kVp, photons. This action then increases the average power of the remaining photons.

Beam hardening

A digital to analog converted is needed to: A. Change the remnant beam to an electric signal B. Transform the electric signal to light C. Display data on a LCD monitor D. Encode transmitted radiation into binary data

C. Display data on a LCD monitor

The Lambert-Beer Law is most closely associated with a _____ beam. a. polychromatic b. fanned c. homogeneous d. heterogeneous

C. Homogenous

The detectors measure the transmitted x-ray beam. the beam that has been attenuated or changed after interacting with the tissues of the patient is called the ____ beam A. Primary B. incoming C. Secondary D. DOminant

C. Secondary

Which of the following parameters is responsible for partial volume averaging? A. Matrix size B. kVp C. Slice thickness D. Patient dose

C. Slice thickness

Which of the following has the highest patient dose assuming all other parameters are the same? A.mA=200, rotation time= 2 seconds B.mA=200, rotation time= 1 seconds C.mA=150, rotation time= 3 seconds D.mA=400, rotation time= 1 seconds

C.mA=150, rotation time= 3 seconds Doubling either the tube current or the rotation time will double the patient dose. Doubling one of these factors while halving the other will leave the patient dose unchanged. (Confusing topic)

reformatted raw data are assigned different filters and are used to enhance the desired anatomy of interest

Convolution

What is the CT number Calibration test

Ct number of water must measure between -3 and +3 (should be at 0, but if it deviates out of this range then its still good to use)

A set of images is reconstructed with a 150 mm reconstruction FOV. Keeping all other parameters constant, a second set of images is reconstructed with a 300 mm reconstruction FOV, and then magnified so that the anatomical structures in the first and second set appear the same size. Which of the following statements is FALSE? 1. the resolution of the second set of images is better than the first set 2. the apparent noise in the second set of images is more pronounced than the first set 3. the patient dose of the first set of images is higher than the second set A. 1 and 2 B. 1 and 3 C. 2 and 3 D. 1 2 and 3

D. 1 2 and 3

Filament size is directly related to: 1. slice thickness 2. mA selected 3.s spatial resolution A. 1 and 2 B. 1 and 3 C. 2 and 3 D. 1 2 and 3

D. 1 2 and 3

For a given distance of coverage, which of the following is TRUE? 1. As reconstruction FOV increases, patient dose increases 2. As pitch increases, patient dose decreases 3. As slice thickness increases, patient dose decreases A. 1 only B. 2 only C. 3 only D. 2 and 3

D. 2 and 3

___ the window ___ will make the image appear darker so that bony structures can be evaluated. a. Increasing, with b. Decreasing , level c. Decreasing, width d. Increasing, level

D. Increasing, level

Which of the following is not an example of a kernel? A. Edge enhancement filter B. Smoothing filter C. Standard convolution algorithm D. Iterative algorithm

D. Iterative algorithm

A smoothing filter is applied to scans that display very: A. High contrast B. High brightness C. Low brightness D. Low contrast

D. Low contrast

______ determines how contrasty the image appears A. kVp B. Window level C. mAs D. Window width ______ Determine how bright the image appears A. kVp B. Window level C. mAs D. Window width

D. Window width B. Window level

changes the data provided by the ADC back to a useable analog signal, ensures the data is free from sampling errors, and can converts the data to a smaller continuous stream of information

Digital to analog converter

Whats another term for saying reconstruction FOV?

Display FOV

The data acquisition system collects, processes, and stores image data True or false

False

Pitch does affect the amount of overlap or gap between slices True or false

False Important: Pitch DOES NOT affect the amount of overlap or gap between slices (i.e. anatomical information being skipped). It merely just determines how information is going to be collected. Once its collected, the pitch has nothing to do with how slices are reconstructed.

The Lambert-Beer law only works with a heterogeneous x-ray beam True or false

False The Lambert-Beer Law stated that for every equal amount of tissue irradiated, an equal amount of x-ray photons will be absorbed. The expected x-ray absorption This is not used in modern day - we use a heterogeneous beam of different x-ray strengths which is based off of the linear attenuation coefficients

True or false Increase the pitch increases the dose to the patient

False Increasing the pitch decreases the dose to the patient

True or false Setting a pitch of .75 means that 75% of each slice overlaps the slice next to it

False Setting a pitch of .75 means that 25% of each slice overlaps the slice next to it

True or false As the pitch is increased, the helix becomes stretched. This only means that the way in which we acquire is stretched out. It also means that anatomy is skipped

False. Anatomy is NOT skipped with pitch. The helical formation is just stretched out.

If temporal resolution is compromised, spatial resolution is not True or false

False. If temporal resolution is compromised, typically spatial resolution is as well Think of CT scanners before helical and multi row CT scanners. A slice was acquired during each rotation. If there was patient motion, the scanner was not capable of imaging faster than that motion. Thus, temporal resolution and spatial resolution were compromised.

calculates the length of time, or attenuation time, it takes for the x-rays to pass through the patient and strike the detectors The new values are called _______ Coefficients. These coefficients are what make the CT image a reality. These are then recalculated. These new calculations are now called linear attenuation coefficients. Then, the data is sent by the log amplifier to the digital-to-analog converter

Fournier transform

This is the final step before the viewer can see the scan a math process that enables spirally-acquired images to appear as individual slices. The image data are then sent to the computer and separated out into a viewable format

Interpolation

a math process that enables spirally-acquired images to appear as individual slices. The image data are then sent to the computer and separated out into a viewable format

Interpolation

This uses what should be the likely outcome of the scan. It compares the actual scans and then averages two together. This process is repeated again and again until the scan data and anticipated data are the same

Iterative reconstruction

How can we test spatial resolution?

Line pairs You use a phantom which has several plastic "holes" inside of a water phantom. The test consists of scanning the phantom and determining the smallest line pair that can be resolved. This is the resolution of the type of scanner. This test is called a resolution test and is able to give the spatial resolution of the scanner

Difference between monochromatic versus polychromatic

Monochromatic - x-ray beam in which all the photons have the same energy Polychromatic - x-ray have a variety of energies

Occurs when there are gaps in helical scanning, creating artifacts. This calculates what the Hounsfield number should be based on pixel values of the adjacent tissue. This leads to misdiagnosis because the actual tissue may have different Hounsfield numbers.

Partial volume averaging

Equation for pitch on single slice CT Equation for pitch for MDCT

Pitch = movement of the patient table in on 360 degree rotation / slice thickness For MDCT, it needs to be refereed as to what equation is being applied: Pitch = movement of the patient table in on 360 degree rotation / Total slice volume or Pitch - movement of the patient table in on 360 degree rotation / thickness of a single slice

How is pitch defined? How is pitch defined differently in multi-row CT detectors?

Pitch, in its basic form, is defined by the movement of the patient table in one gantry rotation divided by the slice thickness. It determines how stretched the x-ray path will be during a helical scan However, this is different with multi-row CT detectors and there are actually two equations. Same on the top (movement of the patient table in on gantry rotation), however the bottom can be divided by total slice volume or thickness of a single slice. These need to be referenced as to understand which one is being made.

What does polychromatic mean?

Polychromatic - x-ray have a variety of energies

Appear as a circular pattern to indicate that data from a detector was missing. The detector must be repaired to correct this issue.

Ring artifacts

The more rows of detectors in a scanner, the _______ the slices A. Thicker B. Thinner C. More signal in

The more rows of detectors in a scanner, the thinner the slices

the more rows of detectors in a scanner, the ______ the slices

The more rows of detectors in a scanner, the thinner the slices

Difference between scan field of view and display field of view -> when do they occur?

The scan field of view occurs before the patient is scanned. This is what we can manipulate to see what we want scanned. The display field of view occur after the matrix has been scanned. This does not affect the patients dose

To increase spatial resolution, the slice thickness needs to be ________ A. thick B. thin C. the same

To increase spatial resolution, the slice thickness needs to be thin

True or False Magnification of an image does not increase the spatial resolution of the image nor increase the appearance of noise. It simply enlarges the individual pixels of the reconstructed image for display purposes.

Treu

A log amplifier calculates the amount of attenuation that occurs during data acquisition True or false

True

Aliasing artifacts occur when the sampler does not collect enough signal bits True or false

True

Most modern CT scanners use scintillation detectors True or false

True

True or false The CT numbers assigned to the pixels may change based on the slice thickness

True

True or false If a patient is placed above or below the SFOV, an aliasing artifact will occur

True

True or false Other than applying iterative reconstruction technique which minimizes image noise, there are no parameters that allow us to decrease the amount of noise but rather there are several ways that we can increase the number of photons that contribute to the pixels of the image (increasing the tube current or scan time, which leads to increase in patient dose)

True

True or false The mAs does not affect the image contrast or spatial resolution

True

True or false When looking at the SFOV, the matrix size never changes, but the pixel size can vary based on the selected SFOV.

True

True or false When magnifying an image, there is no change in spatial resolution since the region of interest and the number of pixels displayed are both decreased

True

True or false While pitch determines how the data is collected, the reconstruction interval determines how the data is calculated into images

True

Xenon is the most common element used in gas ionization detectors True or fals

True

Define the 2 -> either as WL or WW adjusts how "contrasty" the image appears. This determines the range of CT number displayed on an image. Adjusts the brightness of tissues in an image

WW -> adjusts how "contrasty" the image appears. The WW determines the range of CT number displayed on an image. WL -> Adjusts the brightness of tissues in an image

When an x-ray beam passes through the patient, the number of photons in the beam is _______ while the mean energy of the photons is _________

When an x-ray beam passes through the patient, the number of photons in the beam is decreased while the mean energy of the photons is increased

Describe when the x-ray strikes the detector to where the electrical signal ends up and how it is shown on a computer display

When the x-ray photons strike the scintillation detectors, the crystals release small amounts of light. Because the light isn't strong enough, it must go through a photomultiplier tube, or photodiode. The PMT is located just behind the crystals on the detectors. When the crystals release light, the light strikes the photocathode, which turns the light energy back into electrons. The electrons are then focused toward an arrangement of dynodes, which in turn release their own signals. These signals are multiplied over and over again to strengthen the signal. The amount of signal at the end of this process is equivalent to the amount of light given off by the crystals. The electrical signals then travel to the DAS, which processes the signals for computer display

uses selected portions of information rather the entire system

digital signal

If the pitch is increased to more than 2 i.e. in one rotation the patient table moves twice the total slice volume, what type of artifact can occur?

helical artifacts will occur. These artifacts look like swirls or windmills on an image.

Reconstruction intervals: if a helical scan is reconstructed with 5 mm slices and a 3 mm reconstruction interval, there will be a ____ overlap with each reconstructed image

if a helical scan is reconstructed with 5 mm slices and a 3 mm reconstruction interval, there will be a 2 mm overlap with each reconstructed image By decreasing the reconstruction interval, the slice overlap increases and thus more images will be produced that must be archived and evaluated

a sequence of events must be performed correctly and identically time after time. (Ex: As soon as the PMT picks up the light signal, the crystals must be free of any light and be ready to produce the exact same intensity of light from the next x-ray photon interaction.

reproducibility

What is the CT Number Standard Deviation Test

standard deviation of water must not exceed a specified level - given by the manufacturer. This number varies. If it is not within the range, this is an indication of image noise. This could be due to a decrease in the delivered dose or an increase in image noise in the detector amplifier chain