Image Production CT
Acquisition thickness refers to: A. The number of active detector rows B. The thickness of active detector rows C. The thickness of reconstructed slices D. The number of data collection channels
. The thickness of active detector rows The acquisition thickness is the same as the thickness of active detector rows. If the detector rows are 1.25 mm thick, we would say the acquisition thickness is 1.25 mm.The acquisition thickness can be changed by combining detector rows. Realize that the acquisition thickness is not necessarily the same as the reconstruction slice thickness.
Calculate the pitch given these variables: Acquisition thickness = 0.5 mm Detector rows = 20 Rotation time = 0.8 seconds Tube current = 250 mA Table movement = 10 mm A. 1 B. 0.5 C. 2 D. 10
A. 1 Pitch is calculated as the table movement / the beam width. The beam width is calculated as the detector rows multiplied by the acquisition thickness. In this question the table movement equals 10 mm, and the beam width equals 10 millimeters. 10 divided by 10 equals 1.
What CT data acquisition mode collects a series of adjacent axial sections? 1. Axial scanning 2. Helical Scanning 3. Volume scanning A. 1 only B. 2 only C. 1 and 2 only D. 1,2, and 3
A. 1 only Axial scanning collects a series of adjacent axial slices, one section at a time. While the table is stationary, the scanner collects one section of data. The patient table slides the patient to the next position, and the next section is acquired. This continues until the full scan is complete.
During what CT acquisition mode does the patient table move between x-ray exposures? 1. Axial scanning 2. Helical Scanning 3. Volume scanning A. 1 only B. 2 only C. 1 and 2 only D. 1,2, and 3
A. 1 only Axial scanning collects a series of adjacent axial slices, one section at a time. While the table is stationary, the the x-ray tube exposes and the scanner collects one section of data. The patient table slides the patient to the next position, and the next section is acquired. This continues until the full scan is complete.
If the pitch is equal to 1.1, how far is the patient table moving during each rotation of the tube? A. 1.1 times the beam width B. 1.1 divided by the number of data channels C. 1.1 times the number of detector rows D. 1.1 divided by the acquisition thickness
A. 1.1 times the beam width If the pitch is greater than 1, this means the table is moving a greater distance in one tube rotation than the width of the X-ray beam. A pitch of 1.1 tells us to table is moving 1.1 times the width of the beam. A pitch greater than 1 can be imagined as a spiral stretched out. If the pitch is less than 1, the spiral is overlapping on itself.
The signal produced by a detector is a(n): A. Analog electrical signal B. Processed image data C. Raw digital data D. Unprocessed binary data
A. Analog electrical signal When detectors capture remnant x-ray photons they produce an electrical signal that is proportional to the number of photons they received. More photons means a stronger signal. This electrical signal is sent to other components of the scanner where it is converted to a raw (unprocessed) digital signal and later a digital image (processed) signal that is suitable for viewing.
When imaging a patient in CT, what portion of the patient is processed for viewing on the technologist's computer monitor? A. Any part of the patient within the display field-of-view B. Any part of the patient within the scan field-of-view but outside of the display field-of-view C. Any part of the patient outside of the scan field-of-view D. Any part of the patient within the scanner gantry
A. Any part of the patient within the display field-of-view Patient anatomy in the display field of view is the only portion of the patient that is processed for viewing. All anatomy within the scan field of view gets scanned, but only the anatomy in the display field of view gets processed for viewing.
What is the relationship between acquisition thickness and spatial resolution? A. As acquisition thickness increases, spatial resolution decreases B. As acquisition thickness increases, spatial resolution increases C. As acquisition thickness increases, spatial resolution remains constant D. As acquisition thickness increases, spatial resolution remains constant in the inner detector rows
A. As acquisition thickness increases, spatial resolution decreases The acquisition thickness is the same as the thickness of detector rows. Thin detector rows are better for imaging smaller parts, which is the idea of high spatial resolution. If the acquisition thickness is increasing, the spatial resolution decreases. This means the scan will have a decreased ability to visualize small parts.
Pitch is defined as the ratio of the table movement to the: A. Beam width B. Acquisition thickness C. Tube rotation time D. Radiation output
A. Beam width Pitch is a ratio that describes the relationship between the movement of the table and the width of the beam. When the table movement per tube rotation is the same as the beam width, the pitch is equal to 1. If the table moves more than the beam width, the pitch is greater than 1. If the table moves less than the beam width, the pitch is less than 1.
Which the following statements correctly describes the flow of electrons through an x-ray tube? A. Cathode to anode B. Anode to cathode C. Positive to negative D. target to a filament
A. Cathode to anode The cathode of the x-ray tube is the same as the filament, the source of electrons, which is negatively charged. The anode is the target, which is positively charged. When a voltage (kVp) is applied, the negatively charged electrons accelerate to the positively charged anode. Electron interactions in the anode are responsible for x-ray production.
Increasing the acquisition thickness of a CT scan means: A. Combining detector rows together to function as one detector row B. Increasing the displayed slice thickness C. Combining reconstructed slices together to form thicker slices D. Increasing the space between spirals in the helix
A. Combining detector rows together to function as one detector row The acquisition thickness refers to the thickness of detector rows. The only way to change this thickness is to combine detector rows together so they function as one detector. For example, two 1.25 mm detector rows can be combined together to form a single 2.5 mm row. This has the advantage of decreasing patient dose, but it also results in decreased spatial resolution.
Reconstructed image data is produced by the: A. Computer system B. Data acquisition system C. Detector array D. Array processor
A. Computer system The computer system is the last step in the data transformation process when the raw digital data is reconstructed into image data that can be viewed on the monitor and sent to PACS for reading and archival.
A CT detector configuration is changed from 0.625 mm x 16 rows to 1.25 mm x 8 rows. The change may result in: A. Decreased patient dose B. Increased spatial resolution C. Increased beam coverage D. Increased scan time
A. Decreased patient dose In this situation, the detector configuration has been changed to use thicker detector rows, but the same total beam coverage. Thicker detector rows require less radiation to produce a quality signal, which allows for a decrease in patient dose. This change will actually decrease the spatial resolution, but the total scan time and beam coverage will stay the same.
During helical scanning, if the table moves a greater distance than the width of the beam the pitch will be: A. Greater than 1 B. Less than 1 C. Equal to 1
A. Greater than 1 When the table movement is greater than the width of the beam, this results in a pitch greater than 1. For example, if the table movement equals 12 mm, and the beam width equals 10 mm, the pitch is equal to 12 divided by 10, which is 1.2.
If the rotation time of a scan is decreased, what other exposure parameter should be changed to maintain the image quality? A. Increase mA B. Increase kVp C. Decrease scan length D. Decrease mA
A. Increase mA Rotation time and mA must be balanced to maintain the same total exposure. If one is increased, the other must decrease. If one is decreased, the other must increase. This is the idea of mAs reciprocity.
What technical change is expected to increase the penetrating ability of an x-ray beam? A. Increased kVp B. Increased mA C. Increased rotation time D. All of the above
A. Increased kVp The only technical change that results in higher energy photons is an increase in kVp. Increasing mA and rotation time increase the number of photons, but not the energy of those photons.
A CT scanning protocol is set to use a pitch of 0.79. This will result in: A. Overlap in the scan data B. Missing views in the scan data C. Overlap and missing views in the scan data D. Scan data without overlap or missing views
A. Overlap in the scan data A pitch less than one means the table is moving a distance less than the width of the X-ray beam. This causes the scanned data to overlap on itself. This is often done intentionally to create high-resolution scans with extra detail. If the pitch was greater than 1, this is like stretching out the spiral. There will be small gaps in the scanned data.
Which of these statements concerning the scan field-of-view (SFOV) are true? Choose two. A. The SFOV is usually larger than the display field-of-view (DFOV) B. Data in the SFOV is sent to the radiologist C. Data in the SFOV is saved on the CT computed system D. The size of the SFOV is set by the technologist using the scout images
A. The SFOV is usually larger than the display field-of-view (DFOV) C. Data in the SFOV is saved on the CT computed system
When using a CT scanner with non-uniform detector rows, which of the following statements is true: A. The inner rows have the highest spatial resolution B. The inner rows require less radiation dose C. The outer rows are thinner D. The inner and outer rows have the same physical thickness
A. The inner rows have the highest spatial resolution Non-uniform detector rows have different detector thicknesses. The inner rows are thinner for high spatial resolution, but the outer rows are thicker for decreased dose and faster scanning. A scan requiring very high spatial resolution will only use the inner detector rows.
When the pitch is less than one: A. There will be overlap in the scan data B. There will be missing views in the scan data C. There will be overlap and missing views in the scan data D. There will be no overlap or missing views in the scan data
A. There will be overlap in the scan data
During an axial scan, the total amount of anatomical coverage per tube rotation is dependent on which of these variables? 1 - The number of active detector rows 2 - The thickness of active detector rows 3 - The pitch A. 1 only B. 1 and 2 only C. 2 and 3 only D. 1, 2, and 3
B. 1 and 2 only Total anatomical coverage depends on the total width or collimation of the X-ray beam. The width of the X-ray beam is dependent on the number of activated detector rows and the thickness of those rows. The total beam coverage is calculated as the number of rows multiplied by the row thickness. Pitch applies only to helical scanning, so it would not apply to this question.
Consider two identical scanning protocols, except one protocol uses a pitch of 0.73 and the other uses a pitch of 0.87. The protocol using a pitch of 0.87 will have: 1 - Decreased scan time 2 - Increased spatial resolution 3 - Decreased patient dose A. 1 only B. 1 and 3 only C. 2 only D. 1, 2, and 3
B. 1 and 3 only When pitch is increased, this results in a faster scan and less dose to the patient. Think of increased pitch as stretching out the spiral. More anatomy is scanned with each rotation of the tube. Less time is needed, and less dose is used. Unfortunately, the spatial resolution will decrease.
A common kVp setting used in CT is: A. 20 kVp B. 120 kVp C. 200 kVp D. 20-500 kVp
B. 120 kVp Most CT scanners are equipped with a limited number kVp options, which includes 80, 100, 120, and 140 kVp. Higher and low kVp settings are not possible.
Calculate the total beam width of a CT scan using 32 detector rows, 0.5 mm detectors, 100 mA and 140 kVp: A. 64 mm B. 16 mm C. 50 mm D. 32 mm
B. 16 mm The beam width is calculated as: 32 rows x (0.5 mm)/ row= 16 mm beam width.
What CT scanning mode forms a spiral of scan data during imaging? 1. Axial scanning 2. Helical scanning 3. Volume scanning A. 1 only B. 2 only C. 1 and 2 only D. 1,2, and 3
B. 2 only Helical scanning acquires patient data in the shape of a helix, or spiral, around the patient. This occurs because the tube and detectors rotate around the patient while the patient is in motion. This is not true of axial or volume scanning. Refer to the video on Scanning Modes.
The width of the x-ray beam is controlled by: 1. kVp setting 2. Collimators 3. Pixel dimensions A. 1 only B. 2 only C. 1 and 2 only D. 1,2, and 3
B. 2 only The shape of the x-ray beam as it leaves the x-ray tube is controlled by the pre-patient collimators. Pixel dimensions and other technical settings have no influence on this. Only the collimators change the actually width/geometry of the beam.
An 8-slice scanner has: A. 8 detectors B. 8 detector rows C. An 8mm x-ray beam D. An 8 second rotation time
B. 8 detector rows When a scanner is referred to as an "8 slice scanner", the number refers specifically to the number of detector rows.
What portion of the CT data collected during a scan is saved by the scanner but not reconstructed for viewing? A. Any portion of the patient within the display field-of-view B. Any portion of the patient within the scan field-of-view but outside of the display field-of-view C. Any portion of the patient outside of the scan field-of-view D. Any portion of the patient within the scanner gantry
B. Any portion of the patient within the scan field-of-view but outside of the display field-of-view The display field of view is sometimes the same size as the scan field of view, but not always. If the display field of view is smaller, there will be some portions of the patient with in the scan field of view but not within the display field of view. These portions of the patient are still included in the raw data and saved on the scanner, but they will not be viewed within the image data. Refer to the video on field-of-view.
Which of the following correctly describe the anatomical data within the scan field-of-view (SFOV)? A. Data from the SFOV is used for multiplanar reformations B. Data from the SFOV is saved in the CT computer system C. Data from the SFOV is sent to the radiologist for viewing
B. Data from the SFOV is saved in the CT computer system All data within the scanned field-of-view is saved temporarily on the CT computer system. This data is immediately processed by the computer for image reconstruction. The information from the scan field-of-view is used to create the image data in the display field-of-view. You cannot use unprocessed data in the scan field of view to create reformations (this is done with image data, nor can this data from the scan field-of-view be viewed by the radiologist
If mA is decreased by a factor of 2, how is the radiation dose to the patient changed? Assume all other factors remain unchanged. A. Dose is increased by a factor of 2 B. Dose is decreased by a factor of 2 C. Dose remains unchanged D. Dose is decreased by a factor of 22
B. Dose is decreased by a factor of 2 The relationship between mA and dose is directly proportional and linear. If mA decreases by a certain factor, dose will decrease by the same factor. In this question, dose decreased by a factor of 2 between mA decreased by a factor of 2. In other words,there are half as many photons interacting with the patient and therefore the dose would also be half.
What is the primary reason for decreasing the detector thickness for CT angiography examinations? A. Decreased patient dose B. Increased spatial resolution C. Increased beam coverage D. Decreased scan time
B. Increased spatial resolution Angiography exams commonly use a decreased detector thickness. Decreasing the detector thickness (acquisition thickness) is helpful for increasing spatial resolution. Spatial resolution is the ability to see small parts. Angiography exams need to visualize small blood vessels, so smaller detector rows are helpful.
If a routine helical scan of the chest is pre-programmed to use a 16 detector rows and a pitch of 1.13, this will result in: A. Additional views of the same positions in the scan data B. Missing views in the scan data C. Overlap and missing views in the scan data D. Scan data without overlap or missing views
B. Missing views in the scan data A pitch greater than 1 means the table movement per rotation of the tube is greater than the thickness of the X-ray beam. This results in faster scanning, but there are missing views in the scanned data. This is simply noticed as a small decrease in image quality. A pitch greater than one is like stretching out the spiral.
What portion of the patient can be visualized through the process of retrospective reconstruction? A. Only parts of the patient within the display field view B. Only parts of the patient within the scan field of view C. Only parts of the patient outside the scan field of view D. Only parts of the patient within the scanner gantry
B. Only parts of the patient within the scan field of view Any portion of the patient with in the scan field of view can be visualized with retrospective reconstruction. This is true even if those portions of the patient were not represented in the original images. For example, it is common to accidentally clip portions of the patient from the display field of view. This can be corrected through retrospective reconstruction as long as those portions of the patient are inside of the scan field of view.
A single "view" acquired during a CT scan is also called a: A. Slice B. Projection C. Detector D. Attenuation reading
B. Projection A CT scanner collects several projections as the tube and detectors spin around the patient. Each of these projections represent a unique view of the anatomy that is eventually compiled together to form the 3D volume of data.
Physical components located inside the CT gantry typical includes 1. X-ray tube 2. High-voltage generator 3. Computer system A. 1 only B. 2 only C. 1 and 2 only D. 1, 2 and 3
C. 1 and 2 only Both the x-ray tube and generator are often housed within the gantry itself. The computer system, which is responsible for reconstructing raw data into image data, is usually located somewhere outside of the gantry, like with the control panel.
Increasing the mA and kVp of a CT scan will usually result in: 1. Increased patient dose 2. Increased image quality 3. Increased processing time A. 1 only B. 2 only C. 1 and 2 only D. 1, 2, and 3
C. 1 and 2 only Increasing the mA and kVp of CT scan will usually improve the image quality, but this is at the expense of increased patient dose. The ideal mA and kVp should be the lowest possible dose that still results in a diagnostic scan. Changes in mA and kVp have no effect on the processing time.
Based on feedback from an ACR site-visit, your imaging department needs to design a new low-dose CT protocol for patient's less than 30 pounds. Which of these detector configurations would be most suited for this protocol? A. 1 mm x 16 rows B. 2 mm x 8 rows C. 4 mm x 4 rows D. 0.5 x 8 rows
C. 4 mm x 4 rows Thicker detector rows allow for a decrease in radiation and patient dose. This is because the detector rows have a larger surface area to absorb radiation. Less radiation is required, which results in a decrease in patient dose. In this question, 4 mm detector rows is the largest option and therefore allows for the lowest dose.
The specific process that accelerates electron in an x-ray tube is: A. Evacuation of air in the envelope B. Themionic emission C. Application of a voltage D. Bremsstrahlung electron interactions
C. Application of a voltage All options listed are valuable in the x-ray production process, but only the voltage actually causes electrons to accelerate through the tube. A "voltage" is the same as the kilovoltage potential (kVp).
Compared to axial scanning, helical scanning provides for all of the following advantages EXCEPT: A. Decreased scan time B. Improved IV contrast imaging C. Increased spatial resolution D. Improved post-processing
C. Increased spatial resolution For most Imaging procedures, helical scanning is preferred for two reasons: decreased scan time and better post-processing. The scan is faster because the patient moves continuously through the scan. The scan allows for better post-process since one large volume is created, rather than several sequential volumes.
The main difference between image production in CT compared to image production in radiography is: A. X-ray photons are collected by detectors. B. An unprocessed digital signal is converted to image data C. Projections are compiled into a 3D volume of data. D. The x-ray beam is partially attenuation by the patient
C. Projections are compiled into a 3D volume of data. Projections are compiled into a 3D volume of data. Radiographic images are two dimensional, not three dimensional. An x-ray image is one single projection, while CT images are created from numerous projections. Otherwise, all steps in the image production process are very similar.
Changing the slice thickness of an axial reconstruction changes the dimensions of the image data in what axis? A. X- axis B. Y- axis C. Z-axis D. X, Y, and Z- axes
C. Z-axis The z-axis runs longitudinally through the scanner. When the slice thickness is changed, this changes the dimensions of the image data in the z-axis. For example, a thicker slice includes more anatomy in the z-axis.
What CT scanning mode activates the x-ray tube only while the patient table is stationary? 1. Axial scanning 2. Helical Scanning 3. Volume scanning A. 1 only B. 2 only C. 1 and 2 only D. 1 and 3 only
D. 1 and 3 only Unlike helical scanning, axial and volume scanning activate the x-ray tube only when the patient table is stationary. With axial scanning, one section of data is acquired and the patient moves to the next position so the next section can be scanned. With volume scanning, the entire area to be imaged is scanned with just one rotation of the tube.
An increase of which of the following exposure factors would increase the total number of x-ray photons exposing the detectors? 1. mA 2. kVp 3. rotation time A. 1 only B. 2 only C. 1 and 2 only D. 1,2, and 3
D. 1,2, and 3 All options have the effect of increasing the total number of photons exposing the detectors. Increased mA increases the total number of photons being produced. Increased kVp also increases the total number of photons produced and increases the penetrating ability of those photons. Increased rotation time, without changing the mA, results in a longer exposure (per rotation) and more total photons exposing the detectors.
From the list below, identify the CT scanning method that requires a large detector array with numerous data collection channels. 1. Axial scanning 2. Helical Scanning 3. Volume scanning A. 1 only B. 2 only C. 1 and 3 only D. 3 only
D. 3 only Axial and helical scans can be performed even with just one row in the detector array. This is not true of volumes gaining. Volume scanning, by definition, requires a detector array with enough data channels to image in entire organ with just one rotation of the tube. In this context, data collection channels refers to the rows or slices in the detector array.
Increasing mA in CT has the effect of: A. Increasing dose to the patient B. Increasing exposure to the detector elements C. Increasing the intensity of the beam D. All answers listed are true
D. All answers listed are true Increasing the mA results in more photons in the beam (intensity). This results in more dose to the patient and more exposure to the detector elements.
What portions of a patient receive radiation exposure during CT scanning? A. Any portion of the patient within the display field-of-view B. Any portion of the patient within the scan field-of-view but outside of the display field-of-view C. Any portion of the patient outside of the scan field-of-view D. Any portion of the patient within the scanner gantry
D. Any portion of the patient within the scanner gantry A CT scanner exposes a patient to radiation from all directions. Any part of the patient in the gantry is exposed to radiation, even when those parts of the patient are not included in DFOV or SFOV.
The principal advantage of thin detector rows is: A. Faster scan time B. Lower patient dose C. Lower x-ray output D. Higher spatial resolution
D. Higher spatial resolution Thinner detector rows can image smaller parts, which is essentially the definition of high spatial resolution. The down is side is that this usually requires a higher tube output and therefore a higher patient dose. The width of the detector rows may or may not affect the scan time.
Which of these statements correctly describe the display field-of-view (DFOV) in CT imaging? A. Data in the DFOV is reconstructed for viewing B. Data in the DFOV is sent to the radiologist C. Data in the DFOV is converted to image data D. The size of the DFOV is set by the technologist using the scout images E. All statements are correct
E. All statements are correct All statements correctly describe the display field of view. Also understand that none of these statements apply to the scan field of view. The display field of view becomes the image data, but the scan field of view is the unprocessed raw data. Refer to the video on field-of-view.
