CT Production 50 QA

An acquisition is made on a 4-slice MSCT system with a detector array of 16 x 1.25-mm detector elements along the z-axis. With a selected beam width of 20 mm, what beam pitch would result in the table moving 35.00 mm for each rotation of the gantry?

1.75 -The beam pitch for a given acquisition is equal to the table feed per rotation divided by the total collimation. TFPR/TC The Total Collimation for this acquisition is equal to the total number of sections (detectors) multiplied by the detector dimension. TFPR X TC or 16 multiplied by 1.25 mm. = 20 = TC The beam pitch may be therefore calculated by TFPR/TC 35 / 20 = 1.75 Beam Pitch

Which of the following section widths would cause the greatest amount of partial volume averaging?

10mm -Large section widths result in long voxel lengths with possibly several tissue types accounted for within the voxel. When a CT number is assigned to the representative pixel, the density of all of the tissue types is averaged, yielding one attenuation coefficient. Narrow section widths tend to decrease the partial volume effect.

Which of the following mathematical techniques is used for the reconstruction of volumetric MDCT images?

180-degree interpolation (180LI) -The 180-degree linear interpolation technique (180LI) is commonly used for current MDCT image reconstruction. This method interpolates data acquired at a distance only 180 degrees away from the location of the reconstructed slice

Which of the following statements concerning the translate-rotate mode of CT data acquisition is/are correct? 1. 360-degree circular detector arrays are used. 2. Data are collected only during translation. 3. Was used in first- and second-generation CT scanners.

2 and 3 only -The first- and second-generation CT scanners used a translate-rotate. The x-ray tube and detector(s) translated across the patient's head, recording transmission measurements. The entire system then rotated 1 degree. This process of translation-rotation then repeated itself for a total of 180 degrees. First- and second-generation CT scanners used anywhere from 2 to 30 detectors.

Which of the following steps could be taken to reduce the artifact? See Fig. 1. Reduce section thickness .2. Angle gantry around fillings. 3. Decrease kVp

2 only -The only effective method of reducing streak artifact because of metal is to remove the metal object(s) from the scanning field. In this case, the dental fillings cannot be removed, but the gantry could be angled to avoid their interference.

Which of the following acquisitions may be characterized as overlapping?

2.5-mm sections reconstructed every 1.25 mm -Overlapping images are reconstructed with a section interval that is less than the section width. A percentage value may be assigned to the degree of overlap. For example, 2.5-mm-thick images reconstructed every 1.25 mm are said to possess a 50% overlap.

A 64-slice MSCT system employs an array of 64 detectors, each with a dimension of 0.625 mm. What beam collimation is required to expose the middle 32 detectors of the array to transmitted x-radiation?

20mm -The selected detector configuration for a given acquisition determines the necessary beam collimation. The selected 32 detectors, each with an individual dimension of 0.625 mm, would require a beam collimation of 20 mm to expose each to transmitted radiation (32 ✕ 0.625 = 20 mm).

The approximate inherent filtration of the CT x-ray amounts to an aluminum equivalent of:

3 mm -The tube housing, cooling oil, and so on constitute the inherent filtration, which amounts to approximately 3.0 mm of aluminum-equivalent filtration.

For a single-slice CT (SSCT) system, which of the following statements regarding pre-patient collimation is true? 1. An increase in pre-patient collimation increases patient radiation dose. 2. Pre-patient collimation is used to focus radiation through the section of interest. 3. Pre-patient collimation directly controls slice thickness.

3 only -Pre-patient collimation influences the slice thickness by reducing the size of the primary beam. This is accomplished with the use of lead shutters that absorb the outer margins of the primary beam. Increases in pre-patient collimation diminish patient radiation dose by reducing the number of x-ray photons that reach the patient.

The patient scanned in the image measured 38 cm across. An appropriate DFOV for the display of this image would be:

40 cm

Which of the following acquisitions may be characterized as noncontiguous?

5.0-mm sections reconstructed every 7.5 mm -Noncontiguous images are images acquired with an interval greater than the section width. For example, 5.0-mm sections reconstructed every 7.5 mm fail to evaluate the intermittent 2.5 mm of tissue.

Medical and dental radiation accounts for how much humanmade radiation

90%

What manipulation involves the use of image data?

Adjusting the width and level of a window setting -The adjustment of the window width or level is a computer manipulation of image data. The image has already been reconstructed from scan or "raw" data. The window changes only the range of pixel values that will be assigned a shade of gray. Adjustments in the algorithm, DFOV, or matrix require the raw data so that the image may be retrospectively reconstructed.

Retrospectively Reconstructed is what

Adjustments in the algorithm, DFOV, or matrix requiring the raw data

The ROI measures -987 HU. This phantom insert most likely represents:

Air

As a solid-state CT detector measures transmitted radiation, it emits a proportional response in the form of a(n):

Analog Signal -The detector emits an electronic analog signal in proportionate response to the transmitted radiation it absorbs. As the detector array rotates about the patient, the signal varies according to the measured x-ray energy flux

Errors during the measurement of transmitted radiation by the detectors can result in a form of noise on the image referred to as a(n):

Artifact -An artifact is a form of noise that occurs from an error during data acquisition or CT image display.

Phantom measurement of the uniformity of a CT system is performed primarily to assess the negative effects of:

Beam Hardening

Which of the following reduces the scatter radiation reaching the detectors?

Both A and C

The type of compensating filter used at the x-ray tube of a CT scanner is called a:

Bow-Tie Filter -The bow-tie filter used at the x-ray tube of a CT scanner absorbs a larger amount of radiation at the periphery of the patient, where the part thickness is reduced. The center of the patient is placed at the center of the filter, where the largest amount of radiation is allowed to pass through. The use of this type of filter attempts to compensate for the differences in thickness of the often oval-shaped patient.

Cupping artifacts most commonly occur in the:

Brain -Cupping artifacts occur when the periphery of an image is much higher in density than the center. This difference in density causes beam hardening to negatively affect the accuracy of the image. The dense bone that surrounds the soft tissue of the brain is a common site for cupping artifacts to occur

The process of scanning a phantom device of known density to improve the accuracy of CT attenuation measurement may be referred to as:

Calibration -During the process of calibration, CT acquisitions are made of a phantom of a known density (i.e., water) or an empty gantry field (air). The CT system makes corrections of the data acquired, ensuring that the calculations of linear attenuation and Hounsfield values during clinical procedures are accurate.

Which of the following is the primary interaction between x-ray photons and tissue during computed tomography?

Compton Effect -CT scanners use between 120 and 140 kVp, yielding x-ray energies with averages between 70 and 80 keV

The interaction between x-ray and matter that is responsible for the production of the scatter radiation absorbed by the patient and detectors is:

Compton Effect -In the Compton interaction, an x-ray photon ejects an outer-shell electron of an atom. The photon loses some of its energy in the collision and then continues on in a different, scattered direction. This interaction is the major source of the scatter radiation involved in the formation of the CT image.

CT images that have been reconstructed from a portion of the data acquisition process in the hopes of reducing patient motion artifacts are called:

Segmented Images -CT images are usually constructed from transmission data acquired during a 360-degree rotation of the x-ray tube. A CT image can also be constructed from a portion of the data acquisition phase. For example, if a patient moves during the last third of a 2-second scan, an image can be constructed from the first 240 degrees of tube rotation, yielding an image free of motion. This process, referred to as segmentation, is a software capability of many modern CT scanners

DAS

Data Aquisition Signal

Beam Pitch equation

Table feed per rotation / Total collimation TFPR / TC

The ability of the CT system to reduce involuntary motion artifacts and provide CT images free of motion artifact is determined by which of the following components of image quality?

Temporal Resolution -The stop-motion capability of a CT system is referred to as temporal resolution. Temporal resolution quantifies the CT system's ability to freeze motion and to provide an image free of blurring.

The term beam hardening is used to describe which of the following physical phenomena?

The increase in average photon energy of a heterogeneous x-ray beam

Which of the following will serve to decrease the noise of a CT image?

Decrease in matrix size -A decrease in matrix size will cause a subsequent increase in pixel dimension. This larger size causes an increase in the number of x-rays passing through each pixel, thereby increasing the signal-to-noise ratio

Which of the following technical factor adjustments will result in a reduction in patient radiation dose?

Decrease milliamperage (mA) -Reducing the mAs lowers the overall quantity of x-radiation produced and will in turn reduce the radiation dose to the patient

Which of the following is capable of causing an edge gradient artifact?

Dense Bones -The edge gradient effect occurs at areas of abrupt change in density, which are represented by high spatial frequency signal. The computer may have difficulty interpreting this type of rapidly changing information, and streaks may appear as a result. The interface between dense bone and soft tissue is a common site for the edge gradient effect to occur

Which of the following components of the CT system is the most common cause of ring artifacts on the reconstructed CT image?

Detector -Ring artifacts are typically caused by faulty detectors. An error in a detector results in the back-projection of an incorrect ring of density on the reconstructed CT image

The number, length, and organization of the individual detector elements in an MSCT system are referred to as the:

Detector Configuration

A straight line appearing vertically on the scanogram (pilot) of a fourth-generation CT scanner is an artifact most likely caused by:

Detector Malfunction

Which of the following terms best describes the artifact present in the image?

Edge Gradient -The edge gradient artifact manifests as a streaking effect at areas of extremely-high-density interfaces.

For a given CT acquisition, the calculated mAs applied to each reconstructed slice may be referred to as the:

Effective mAs -Effective mAs describes the calculated mAs per acquired slice. The primary influencing factor for effective mAs is table speed, which is an important component controlled by the selected detector pitch. The detector pitch chosen by the CT operator directly affects the speed at which the patient travels through the gantry. The mAs divided by the pitch yields the effective mAs for each slice in a given CT acquisition

Which of the following terms accurately describes the type of x-ray beam used in a third-generation CT scanner?

Fan Beam -The third-generation CT scanner uses a fan beam projected onto a wide detector array. The third-generation CT x-ray beam is commonly mistaken for the "pencil beam" used with only two detectors in the first-generation CT scanner.

Which of the following is not an iterative method of CT image reconstruction?

Fourier Transform -The iterative methods of CT image reconstruction include simultaneous reconstruction, ray-by-ray correction, and point-by-point correction. The Fourier transform method is an analytic method of CT image reconstruction

In 1979, the scientists __________ and __________ shared the Nobel Prize for their research in computed tomography.

Hounsfield and Cormack -Dr. Godfrey Hounsfield and Dr. Allan Cormack shared the Nobel Prize for their work on the development of computed tomography in 1979.

The type of MSCT detector array that contains midline narrow elements flanked by wider detectors is called a(n):

Hybrid Array -A hybrid array consists of two detector sizes. The narrower detectors are positioned midline, flanked by the wider detectors. For example, the central rows of a 16-channel MDCT system are 0.625 mm with the remaining peripheral rows at 1.25 mm

Which of the following technical changes may increase the partial volume effect present on a spiral CT scan?

Increase in Pitch -When the pitch is increased during a spiral CT scan, either the section width or the table speed has been increased. The effects of increased section width on the partial volume effect are obvious. The increased table speed also plays a role in increasing partial volume averaging. Because the table moves through the gantry at a faster rate, each rotation of the tube and detectors is responsible for recording more information. This causes a broadening of the section sensitivity profile (SSP), which manifests as partial volume averaging

Which of the following types of image reconstruction was used in the first prototype CT scanner?

Iterative Technique -The first-generation prototype CT scanner designed by Dr. Godfrey Hounsfield used an iterative form of image reconstruction.

The ability of an object to attenuate the x-ray beam is assigned a value known as the:

Linear attenuation coefficient -The ability of an object to attenuate the x-ray beam is assigned a value termed the linear attenuation coefficient (μ)

During CT angiography, images may be reconstructed using only the greatest density encountered along each ray. This type of specialized CT image is called a(n):

MIP Image -Maximum intensity projections (MIPs) are commonly used during specialized studies such as CT angiography. A MIP image is constructed by displaying only the maximum intensity pixel found along each ray. Remember that a ray is the path from the focal spot of the x-ray tube to a detector. This technique is valuable in displaying contrast-enhanced blood vessels that are surrounded by various types of tissue.

MIP

Maximum intensity projections -A MIP image is constructed by displaying only the maximum intensity pixel found along each ray

A CT image of a homogeneous material contains variations in CT number from pixel to pixel. This image is said to have:

Noise -A CT image of a homogeneous material should have pixels with the same CT number. Any variation in CT number between pixels indicates that noise has entered the system, causing a loss in accuracy.

Which of the following archival media is capable of storing the largest number of CT images?

Optical Disk -Magnetic optical disks vary in storage capacity, with some capable of storing several thousand 5122 matrix CT images. This is considerably more than magnetic tape, which had previously been the standard choice for CT image archival.

Which of the following technical parameters would greatly improve the quality of CT studies requiring multiplanar reformation (MPR) images?

Overlapping scans with narrow sections

The component of the DAS responsible for strengthening the signal emitted from a detector is termed the:

Preamplifier -The weak electronic signal emitted from the detectors in response to the measurement of transmitted radiation must be strengthened before it is utilized for image reconstruction. The preamplifier is the component of the data acquisition system (DAS) responsible for amplifying the detector signal.

In 1917, Austrian mathematician __________ proved that it was possible to reconstruct a three-dimensional object from the infinite set of all of its projections.

Radon -J. Radon, an Austrian mathematician, was responsible for some of the earliest scientific research for the reconstruction principles used in computed tomography. In 1917 he proved that it was possible to build an image of an object through the use of an extremely large set of its projections.

Which of the following window levels was most likely used to display the image shown?

+250 HU -The window level used to display an image should equal the average Hounsfield value of the tissue of interest. In this example, the demonstration of bone tissue is most important. The type of bone making up the shoulder joint would have an average density of +250 HU

Which of the following azimuth settings was used to produce the localizer image?

0 degrees -The azimuth setting refers to the relationship between the x-ray tube and detectors during scout or localizer production. A 0-degree azimuth describes the situation in which the x-ray tube and detectors do not rotate but remain above and below the patient, providing a frontal projection

The image was produced with a 1.0-mm aperture size and was displayed using a 5122 matrix and a 15-cm DFOV. The voxel dimension for this image would be:

0.29 mm × 0.29 mm × 1.0 mm -The pixel dimension may be calculated by dividing the DFOV by the matrix size. This two-dimensional pixel size is then multiplied by the length of the voxel

Which of the following components of CT image quality is/are being evaluated? See Fig. 1. Linearity 2. Section width 3. Spatial resolution

1 & 2

Soft copies of CT images are stored on which of the following media? 1. Hard drives 2. Magnetic optical disk 3. Laser film

1 & 2 -Storage of CT images in digital file format is referred to as soft copies. Hard copies of CT images are those stored on laser film

The types of detectors used in computed tomography are: 1. Gas ionization 2. Stimulable phosphor 3. Scintillation

1 and 3 only -Both operate by measuring the amount of transmitted radiation passed through the patient and transmitting this information to the computer for image reconstruction. Newer MDCT systems utilize solid-state scintillation detectors exclusively.

Which of the following technologic advances has led to the development of spiral/helical CT scanning? 1. Slip-ring technology 2. Electron beam technology 3. High-efficiency x-ray tubes

1 and 3 only -Spiral/helical CT scanners are a recent development in computed tomography technology. The advents of slip-ring technology and improvements in x-ray tube design have made this innovation possible. Slip-rings have taken the place of the cumbersome cables previously used to transmit the CT signal and supply power to the tube and detectors. This development enables the tube and detectors to rotate continuously around the patient, acquiring data in the form of a helix. The longer exposure times of up to 60 seconds require extremely efficient x-ray tubes with enormous heat capacities.

Which of the following technical adjustments may be employed to improve the temporal resolution of an MDCT system? 1. Decreased scan time 2. Decreased section width 3. Decreased DFOV

1 only

Important characteristics of a quality control program in CT include: 1. Consistent testing frequency 2. Prompt corrective actions when necessary 3. Accurate record-keeping

1, 2, & 3 -The hallmarks of a superior QC program are routine frequency of testing, prompt corrective actions as needed, and accurate record-keeping

For single-slice CT (SSCT) systems, which of the following statements regarding retrospective image reconstruction is FALSE?

The slice thickness and SFOV may be changed. -Retrospective reconstruction uses scan or "raw" data to change the matrix, DFOV, center, and/or algorithm used for a CT image. The slice thickness and SFOV are specifically used for data acquisition and cannot be altered retrospectively with a single-slice CT (SSCT) system. The advent of multidetector CT (MDCT) technology has made retrospective adjustments in section width possible. The scan field of view (SFOV) remains an item that may not be altered retrospectively

The process by which electrons are produced at the cathode of a CT x-ray tube is known as:

Thermionic Emission -When the cathode of an x-ray tube is heated, electrons are released through the process of thermionic emission. It is these electrons that are then accelerated toward the anode for x-ray production

In multislice CT (MSCT), the beam pitch is equal to the table feed per rotation divided by the:

Total Collimation -The beam pitch, which is unique to multislice CT. The total collimation is equal to the combined thickness of all of the sections that are simultaneously acquired with each gantry rotation.

The assignment of different generations to CT scanners is based on the configuration of the:

Tube and Detectors -CT scanners are often placed into one of several different "generations." The differences in generations are based on the relationship of the tube and detectors and the position of each during data acquisition.

When one is using a third-generation CT scanner, it is important that reference detectors positioned at the peripheral portions of the detector array be exposed to:

Unattenuated Radiation -Reference detectors are used to measure incident radiation intensity. This information is used by the computer during the calculation of the linear attenuation coefficient. If the patient is placed incorrectly within the gantry, the reference detectors may be partially blocked, causing an out-of-field artifact.

During a CT scan, each sample of ray sum measurements made by the data acquisition system (DAS) is called a:

View

Which of the following types of gas is commonly used for gas ionization CT detectors?

Xenon

Which of the following is used in gas ionization CT detectors?

Xenon -Gases with high atomic numbers, such as xenon, are used in ionization-type CT detector systems. The high atomic number of the xenon gas (Z = 54) increases the incidence of interaction with x-ray photons, thus improving the efficiency of the detector.

In an MSCT system, the detector array is composed of multiple rows of individual detector elements along the:

Z-axis -A multislice CT (MSCT) system utilizes a third-generation curvilinear detector array with multiple rows of individual detector elements segmented along the longitudinal axis, or z-axis

The streaking artifact present is most likely because of:

beam hardening

Effective mAs

calculated mAs per acquired slice -primary influencing factor is table speed

Electrons from cathode filament that collide with free k-shell electrons within the tungsten target yield which of the following

charact rays with discrete emission of energy

Primary beam consists of what interactions

combo of both brems and characteristic

What contributes most to occupational exposure

compton scatter

What interaction involves high energy photon and the ejection of an outer shell electron

compton scatter

What interaction involves partial transfer of the incident photon energy to the involved atom

compton scattering

Each time an x-ray photon scatters its intensity

decreases 1000 times

The streaking artifacts present were most likely caused by:

dental fillings

The term View describes what

each data sample made by the DAS. During the process of data acquisition, views are acquired by the DAS hundreds of times per second, according to the system's individual sampling rate

What is associated with compton scattering

high energy incident photons outer shell electrons

Which of the following types of algorithms would be best suited to demonstrate the bony details of the shoulder joint in the figure?

high spatial frequency (bone picture of shoulder)

The primary advantage of an isotropic MDCT data set is:

high-quality multiplanar reformations (MPRs) -An isotropic volumetric data set yields high-quality images with equal resolution in any reconstructed plane. Reconstruction of isotropic, overlapping, thin-section MDCT images greatly reduces the "step" artifact that can negatively affect the quality of MPR and 3-D CT images

What are Noncontiguous Images?

images acquired with an interval greater than the section width. For example, 5.0-mm sections reconstructed every 7.5 mm fail to evaluate the intermittent 2.5 mm of tissue.

Which of the following is used to archive a hard copy of a CT image?

laser film

How do you calculate Effective mAs?

mAs divided by the pitch yields the effective mAs for each slice

Beam Collimation equation

number of selected detectors X dimension of detector = BC

Interaction between photons and tissue that contributes to radiographic contrast and significantly to patient dose is

photoelectric

What interaction contributes most to patient dose

photoelectric

Which interaction results in total absorption of the incident photon

photoelectric effect

During Brems interaction a filament

projectile electron is slowed down

Which of the following anatomic quadrants has been removed from the 3-D model?

right anterior superior -The computer software is able to remove quadrants of information to allow visualization of the inner portions of the model. In this example, the right anterior superior portion has been removed. This information can be ascertained by examining the anatomic position of the model. In the 3-D model, the information is also available on the image from the letters RAS, which signify "right anterior superior."

Types of secondary radiation

scattered leakage

The technique that provides the detail of the outer portions of the 3-D model while maintaining the typical CT detail of the lungs inside is called:

surface rendering

The artifact present is most likely caused by:

surgical staples

The artifact present was most likely caused by:

surgical staples

The high-density objects labeled as Number 2 most likely represent:

surgical staples

The total collimation is equal to

the combined thickness of all of the sections that are simultaneously acquired with each gantry rotation.

Total Collimation equation

total number of sections/detectors X detector dimension TFPR X TC

A free radical is

uncharged molecule with a single unpaired electron in the outer shell

Components of electromagnetic spectrum

visible light uv gamma radiation radio waves