Image Production Flashcards
Which of the following reduces the scatter radiation reaching the detectors?
A. Post-patient collimation
B. Pre-patient collimation
C. Pre-detector collimation
D. Both a and c
D. Both a and c
Pre-detector collimation and post-patient collimation are synonymous. Both terms describe a device designed to remove scatter radiation before it reaches the detector.
Which of the following is the primary interaction between x-ray photons and tissue during computed tomography?
A. Bremsstrahlung
B. Characteristic
C. Compton effect
D. Coherent scatter
C. Compton effect
Owing to the high-energy beam used in CT, the Compton effect is the predominant interaction between x-ray and matter. CT scanners use between 120 and 140 kVp, yielding x-ray energies with averages between 70 and 80 keV.
The assignment of different generations to CT scanners is based on the configuration of the:
A. patient and gantry.
B. tube and detectors.
C. anode and cathode.
D. tube and collimators
B. 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:
A. homogeneous radiation.
B. unattenuated radiation.
C. monochromatic radiation.
D. remnant radiation.
B. 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.
In 1979, the scientists __________ and __________ shared the Nobel Prize for their research in computed tomography.
A. Watson and Crick
B. Olendorf and Hounsfield
C. Hounsfield and Cormack
D. Hounsfield and Ambrose
C. 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 compensating filter used at the x-ray tube of a CT scanner is called a:
A. kernel.
B. water bath.
C. bow-tie filter.
D. wedge filter.
C. 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.
The interaction between x-ray and matter that is responsible for the production of the scatter radiation absorbed by the patient and detectors is:
A. Compton effect.
B. bremsstrahlung.
C. photoelectric.
D. characteristic.
A. 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.
The types of detectors used in computed tomography are:
- Gas ionization
- Stimulable phosphor
- Scintillation
A. 1 only.
B. 3 only.
C. 1 and 3 only.
D. 2 and 3 only.
C. 1 and 3 only.
The types of detectors used in computed tomography are scintillation and gas ionization detectors. 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 is used in gas ionization CT detectors?
A. Neon
B. Xenon
C. Helium
D. Nitrogen
B. 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.
Which of the following statements concerning the translate-rotate mode of CT data acquisition is/are correct?
- 360-degree circular detector arrays are used.
- Data are collected only during translation.
- Was used in first- and second-generation CT scanners.
A. 2 only
B. 3 only
C. 1 and 2 only
D. 2 and 3 only
D. 2 and 3 only
The first- and second-generation CT scanners used a translate-rotate method of data acquisition. 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 terms accurately describes the type of x-ray beam used in a third-generation CT scanner?
A. Pencil beam
B. Fan beam
C. Nutating beam
D. Electron beam
B. 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 technologic advances has led to the development of spiral/helical CT scanning?
- Slip-ring technology
- Electron beam technology
- High-efficiency x-ray tubes
A. 1 only
B. 1 and 2 only
C. 1 and 3 only
D. 1, 2, and 3
C. 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.
For a given CT acquisition, the calculated mAs applied to each reconstructed slice may be referred to as the:
A. effective mAs.
B. peak mAs.
C. absorbed mAs.
D. constant mAs.
A. 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.
The approximate inherent filtration of the CT x-ray amounts to an aluminum equivalent of:
A. 1.2 mm.
B. 3.0 mm.
C. 5.8 mm.
D. 10.0 mm.
B. 3.0 mm.
The tube housing, cooling oil, and so on constitute the inherent filtration, which amounts to approximately 3.0 mm of aluminum-equivalent filtration.
In multislice CT (MSCT), the beam pitch is equal to the table feed per rotation divided by the:
A. number of detectors.
B. pre-patient collimation.
C. total collimation.
D. number of channels.
C. total collimation.
The beam pitch, which is unique to multislice CT, is equal to the table feed per rotation divided by the total collimation. The total collimation is equal to the combined thickness of all of the sections that are simultaneously acquired with each gantry rotation.
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?
A. 1.0
B. 1.5
C. 1.75
D. 2.0
C. 1.75
The beam pitch for a given acquisition is equal to the table feed per rotation divided by the total collimation. The total collimation for this acquisition is equal to the total number of sections (detectors) multiplied by the detector dimension, or 16 multiplied by 1.25 mm. The beam pitch may be therefore calculated by dividing 35.00 by 20, or 1.75.
In an MSCT system, the detector array is composed of multiple rows of individual detector elements along the:
A. x-axis.
B. y-axis.
C. z-axis.
D. entire circumference of the gantry.
C. 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 number, length, and organization of the individual detector elements in an MSCT system are referred to as the:
A. detector configuration.
B. array pitch.
C. CT generation.
D. detector pitch.
A
Detector configuration refers to the number, length, and organization of the individual detector elements in an MSCT system.
The type of MSCT detector array that contains midline narrow elements flanked by wider detectors is called a(n):
A. uniform matrix array.
B. adaptive array.
C. hybrid array.
D. stationary array.
C
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.
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?
A. 10 mm
B. 20 mm
C. 32 mm
D. 64 mm
B
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 component of the DAS responsible for strengthening the signal emitted from a detector is termed the:
A. digital-to-analog converter (DAC).
B. preamplifier.
C. high-resolution comb.
D. bow-tie filter.
B
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.
A. Radon
B. Tsien
C. Bracewell
D. Cormack
A
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 types of image reconstruction was used in the first prototype CT scanner?
A. Convolution method
B. Iterative technique
C. Fourier transform
D. Back-projection
B
The first-generation prototype CT scanner designed by Dr. Godfrey Hounsfield used an iterative form of image reconstruction.
CT images that have been reconstructed from a portion of the data acquisition process in the hopes of reducing patient motion artifacts are called:
A. dynamic images.
B. subtraction images.
C. segmented images.
D. filtered images.
C
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.
Which of the following is not an iterative method of CT image reconstruction?
A. Point-by-point correction
B. Fourier transform
C. Simultaneous reconstruction
D. Ray-by-ray correction
B
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.
As a solid-state CT detector measures transmitted radiation, it emits a proportional response in the form of a(n):
A. digital signal.
B. quantity of gas ions.
C. analog signal.
D. modulation transfer function.
C
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.
During a CT scan, each sample of ray sum measurements made by the data acquisition system (DAS) is called a:
A. signal.
B. view.
C. projection.
D. ray.
B
The term view is used to describe 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.
The ability of an object to attenuate the x-ray beam is assigned a value known as the:
A. linear attenuation coefficient.
B. Hounsfield value.
C. CT number.
D. ray sum.
A
The ability of an object to attenuate the x-ray beam is assigned a value termed the linear attenuation coefficient (μ).
Which of the following mathematical techniques is used for the reconstruction of volumetric MDCT images?
A. 180-degree interpolation (180LI)
B. 360-degree interpolation (360LI)
C. Back-projection
D. Fourier reconstruction
A
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.
The primary advantage of an isotropic MDCT data set is:
A. reduced scan time.
B. increased signal-to-noise ratio (SNR).
C. improved raw data convolution.
D. high-quality multiplanar reformations (MPRs).
D
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.
The technique that allows the user to select the range of pixel values used in a 3-D CT reformation is termed:
A. thresholding.
B. windowing.
C. retrospective reconstruction.
D. targeting.
A
The selected threshold allows the user to select the range of pixel values rendered in the 3-D model. For example, a higher, bone threshold (>300 HU) can be chosen to build a skeletal model of the patient’s skull. Decreasing the threshold to include voxels with lower attenuation values results in a 3-D model with the patient’s soft tissues included.
What is the dimension of each pixel in the matrix of an image with the following parameters?
FOV = 25.6 cm Matrix = 512 mm ✕ 512 mm
A. 0.5 mm
B. 0.5 cm
C. 0.05 cm
D. Both a and c
D
The dimension of a pixel may be calculated by dividing the field of view (FOV) by the matrix size. The DFOV, 25.6 cm, must first be converted into 256 mm. This is then divided by 512 mm for a pixel dimension of 0.5 mm. Keep in mind that the pixel is a two-dimensional item, square in shape, and the measurement of 0.5 mm corresponds to only one side. The numerical lengths of 0.5 mm and 0.05 cm are equal.
The following formula is used to calculate the linear attenuation coefficient:
I = I0e–μx The symbol “x” identifies:
A. Euler’s constant.
B. absorber thickness.
C. the unknown.
D. transmitted photons.
B
The Lambert-Beer law is used to calculate the attenuation coefficient of a volume of material. The symbol “x” represents the thickness of the absorber attenuating the radiation.
Which of the following statements best describes why the image of the chest is displayed at a window whose level is –700 and width is 1500? See Fig.
A. CT images of the lung should always be displayed in predetermined “lung” windows.
B. Soft tissue demonstration is not required during the CT evaluation of the chest.
C. Strict protocols regarding image display should never be altered by the operator.
D. The level is set at the average Hounsfield value for the tissue of interest within a range determined by the window width.
D
The window chosen for image display should always be tailored to meet the needs of the individual anatomy. The level should be set at an average value for the tissue of interest with a width wide enough to include all variations within the area of interest.
A voxel may be defined as which of the following?
A. The portion of the CRT displaying the image
B. A miniature image
C. A volume element
D. An arrangement of pixels
C
A voxel may be defined as a volume element. It is represented within a matrix by a pixel.
The portion of the primary beam interacting with a single detector is known as a:
A. ray.
B. view.
C. profile.
D. sample.
A
The term ray is used to describe the portion of the x-ray beam that falls upon a single detector.
What is the display field of view used for a 3202 matrix image with a pixel dimension of 0.75 mm ✕ 0.75 mm?
A. 12 cm
B. 24 cm
C. 36 cm
D. 48 cm
B
The display field of view may be calculated by multiplying the pixel dimension by the matrix size. In this example, one side of the pixel measures 0.75 mm and the matrix used is 3202. The DFOV is 240 mm or 24 cm.
The Hounsfield value of a pixel is directly related to which of the following?
A. Window width
B. Field of view size
C. μ of H2O
D. Window level
C
The value of a pixel in Hounsfield units is calculated through a comparison of the linear attenuation coefficient (μ) of a voxel of tissue to that of water.
The dimensions of a voxel may be calculated as the product of which of the following?
A. Matrix size and pixel size
B. Pixel size and section width
C. DFOV and matrix size
D. DFOV and pixel size
B
The pixel is a two-dimensional representation of a voxel. The section width is equal to the length of the voxel. To calculate the dimensions of a voxel, the pixel dimension must be multiplied by the section width.
A voxel whose attenuation coefficient is less than that of water is assigned a pixel value with a(n) __________ CT number.
A. positive
B. extremely large
C. negative
D. invalid
C
The attenuation coefficient of a tissue describes the tissue’s ability to attenuate x-radiation. CT numbers are assigned to pixels based on the basis of the attenuation of the tissue within the voxel. The assignment of a CT number in Hounsfield units arises from a comparison of the attenuation coefficient of the tissue with that of water. Materials with attenuation coefficients less than that of water are assigned negative CT numbers.
Which of the following formulas may be used to calculate the dimensions of a pixel?
A. Pixel size = matrix size/DFOV
B. Pixel size = DFOV ✕ matrix size
C. Pixel size = slice thickness/matrix size
D. Pixel size = DFOV/matrix size
D
The dimensions of a pixel may be calculated by dividing the DFOV by the matrix size. Keep in mind that the pixel is a square with four equal sides and the dimension of one side is usually given in millimeters (mm). The area of the pixel is calculated by squaring the dimension, and the units are then adjusted to square millimeters (mm2).
A CT scanner measures the linear attenuation coefficient of a voxel of tissue as 0.40. The linear attenuation coefficient of water for this scanner equals 0.20. The CT number assigned to the pixel representing this voxel of tissue equals:
A. –1000 HU.
B. 0 HU.
C. 1 HU.
D. +1000 HU.
D. +1000 HU.
0.4-0.2/0.2*1000=1000
The CT number of a pixel may be calculated by subtracting the linear attenuation coefficient of water from the linear attenuation coefficient of the tissue within the voxel. This number is divided by the linear attenuation coefficient of water. The quotient is multiplied by a contrast factor of 1000 to yield the value of the pixel in Hounsfield units.
A voxel’s dimension may be decreased by which of the following?
A. Decreasing the section width
B. Decreasing the matrix size
C. Increasing the section width
D. Increasing the DFOV
A
The section width of a CT image controls the length of the voxel. The dimensions of a voxel may also be reduced through decreases in the size of the pixel. Increases in matrix size and decreases in DFOV serve to decrease the dimensions of the pixel and voxel.
Two adjacent pixels are measured to have a difference of 1 HU. This amounts to a tissue density difference of approximately:
A. 0.1%.
B. 1.0%.
C. 10%.
D. 25%.
A
Pixels that differ by only 1 Hounsfield unit represent tissue whose attenuation coefficients differ by only 0.1%.
When one is choosing a window to display a CT image, the width defines the:
A. midpoint of the range of pixels displayed.
B. range of CT numbers (pixels) to be displayed.
C. range of pixel values included in an ROI.
D. average CT number of the tissue of interest.
B
The width of a CT window controls the range of pixel values that are assigned a shade of gray. The width is centered on a level that is equal to the value of the tissue of interest.
Which of the following corresponds to the longitudinal dimension of the CT image voxel?
A. x-axis
B. y-axis
C. z-axis
D. Orthogonal axis
C
The longitudinal dimension of the voxel corresponds to the z-axis of scanning, with a voxel depth equal to the reconstructed section width.
The volume of a voxel may be calculated by multiplying the pixel dimension (mm2) by the:
A. display field of view (DFOV).
B. section width.
C. pitch.
D. scan field of view (SFOV).
B
As a geometric cuboid, the volume of a voxel may be calculated as the product of the pixel area dimension (mm2) and the section width (mm). The pixel dimension (d) must be first calculated by dividing the display field of view (DFOV) by the matrix size. The area of the pixel may then be found by squaring the value of d (mm). The unit of measurement for voxel volume is mm3.
In a modern CT system, the total number of possible Hounsfield values that may be assigned to any one pixel is approximately:
A. 512.
B. 1024.
C. 2000.
D. 4096.
D
The CT computer system is typically capable of displaying 12 bits of data per pixel and is therefore able to display any of up to 4096 Hounsfield values for each pixel (212 = 4096).
The typical range for possible pixel values in a modern CT system is between:
A. –512 HU and +512 HU.
B. –1000 HU and +1000 HU.
C. –1024 HU and +3071 HU.
D. –4096 HU and +4096 HU.
C
The typical range of Hounsfield scale CT numbers is –1024 to +3071 HU. This range equals the 4096 different values possible when one is using a 12-bit system (212 = 4096).
The grainy appearance of the pelvic image shown is commonly referred to as: See Fig.
A. tube arcing.
B. noise.
C. edge gradient.
D. partial volume artifact.
B
Noise degrades the CT image, giving it a grainy, mottled appearance. The terms quantum mottle and noise are often used interchangeably.
Which of the following is the most likely cause of the inferior quality of the image shown? See Fig.
A. Beam hardening
B. Detector malfunction
C. Insufficient patient radiation dose
D. Incorrect SFOV
C
Noise or quantum mottle may be caused when an insufficient amount of x-ray photons is absorbed by the detectors. In this example, the selected mA was most likely too low for an adequate signal-to-noise ratio.
The phantom imaged is used to test which of the following image quality factors of a CT scanner? See Fig.
A. Spatial resolution
B. Noise
C. Linearity
D. Contrast resolution
A
The resolution test pattern embedded within this type of phantom is used to measure the spatial resolution of the CT scanner. The effects of different technical factors on spatial resolution may be examined by calculating the maximum number of visible line pairs.
The graph is used to evaluate which of the following components of CT image quality? See Fig.
A. Spatial resolution
B. Signal-to-noise ratio (SNR)
C. Temporal resolution
D. Contrast resolution
A
The MTF of a CT system is an objective measurement of a system’s in-plane spatial resolution. By evaluating an MTF curve of a particular image, the operator can assess the spatial resolution of a CT system and appraise changes in spatial resolution because of adjustments in technical factors.
What is the maximum number of line pairs per centimeter (lp/cm) the smooth algorithm can demonstrate? See Fig.
A. 1.0
B. 2.0
C. 8.1
D. 9.0
C
The limiting resolution of a particular CT scan is determined at a point on the graph where the signal frequency corresponding to a particular object has reached 10%. When the MTF is lower than 10% (0.1), the object is no longer resolved.
The graph represents which of the following? See Fig.
A. Modulation transfer function (MTF)
B. Signal-to-noise ratio (SNR)
C. Views per rotation (VPR)
D. Slice sensitivity profile (SSP)
D
The section width (slice) for a volumetric acquisition may be graphically displayed as a slice sensitivity profile (SSP). The SSP represents the degree of broadening that occurs along the z-axis during volumetric data acquisition.
Which of the following corresponds to the full width at half maximum of the graph? See Fig.
A. A
B. B
C. C
D. D
C
The full width at half maximum (FWHM) of the SSP curve demonstrates the effective section width of a volumetric CT acquisition. It is measured by examining the slice sensitivity profile at half of its maximum height.
Which of the following components of CT image quality is/are being evaluated? See Fig.
- Linearity
- Section width
- Spatial resolution
A. 1 only
B. 1 and 2 only
C. 2 and 3 only
D. 1, 2, and 3
B
A linearity phantom contains four inserts of various materials surrounded by water. Known ranges of CT values are compared with those obtained by CT acquisition to ensure system accuracy and linearity. This American College of Radiology (ACR) phantom also contains two ramps consisting of a series of wires spaced 0.5 mm apart along the phantom’s z-axis. The accuracy of the selected section width is evaluated by counting the number of wires visible in cross section.
The ROI measures –987 HU. This phantom insert most likely represents: See Fig.
A. bone.
B. acrylic.
C. polyethylene.
D. air.
D
Linearity may be periodically evaluated by scanning a specialized phantom containing samples of an assortment of materials—acrylic, polyethylene, water, air, artificial bone, and so on. A CT number measurement of –987 HU would indicate that the particular phantom insert represents air.
The artifact present was most likely caused by: See Fig.
A. patient motion.
B. surgical staples.
C. gallstones.
D. detector malfunction.
The streaking artifact in the right anterior portion of the image was most likely caused by surgical staples. This type of metallic streaking artifact is caused by the edge gradient effect.
The high-density objects labeled as Number 2 most likely represent: See Fig.
A. gunshot fragments.
B. loose change in the patient’s clothing.
C. surgical staples.
D. ingested metallic material.
C
The dense objects located in the abdomen of this patient most likely represent metallic surgical staples, which often appear in the post-surgical patient.
Which of the following azimuth settings was used to produce the localizer image? See Fig.
A. 0 degrees
B. 90 degrees
C. 180 degrees
D. 270 degrees
A. 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 streaking artifact present is most likely because of: See Fig.
A. detector malfunction.
B. tube arcing.
C. beam hardening.
D. out-of-field errors.
C
Streaking artifacts such as the one in the figure commonly occur during CT scanning of thick body parts like the shoulder. The x-ray beam changes in quality as it passes through different densities. These beam energy changes sometimes manifest as streak artifacts.
Which of the following types of algorithms would be best suited to demonstrate the bony details of the shoulder joint in the figure? See Fig.
A. High spatial frequency
B. Soft tissue
C. Standard
D. Low spatial frequency
A. High spatial frequency
High spatial frequency algorithms demonstrate the greatest spatial resolution and are best suited for imaging sharp density changes, such as those occurring with bone tissue. Bone, edge, and detailare names commonly given to high spatial frequency algorithms.
Which of the following window levels was most likely used to display the image shown? See Fig.
A. –700 HU
B. 0 HU
C. +50 HU
D. +250 HU
D. +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 terms best describes the artifact present in the image? See Fig.
A. Edge gradient
B. Gibbs phenomenon
C. Density gradient
D. Partial volume
A
The edge gradient artifact manifests as a streaking effect at areas of extremely-high-density interfaces.
The artifact present is most likely caused by: See Fig.
A. involuntary motion.
B. aliasing.
C. surgical staples.
D. tube arcing.
C. surgical staples.
High-density objects such as surgical clips and other types of prosthetic devices are often the cause of streaking artifacts because of the edge gradient effect.
The patient scanned in the image measured 38 cm across. An appropriate DFOV for the display of this image would be: See Fig.
A. 34 cm.
B. 40 cm.
C. 44 cm.
D. 48 cm.
B. 40 cm.
The display field of view chosen for an image should be large enough to include the entire area of interest. A DFOV that is significantly larger than the anatomic area of interest will result in a minified image with reduced spatial resolution.
The streaking artifacts present were most likely caused by: See Fig.
A. tube arcing.
B. dental fillings.
C. detector malfunction.
D. insufficient technique.
B. dental fillings.
Metallic items such as dental fillings are a common cause of streaking or “star” artifact.
Which of the following steps could be taken to reduce the artifact? See Fig.
- Reduce section thickness.
- Angle gantry around fillings.
- Decrease kVp.
A. 1 only
B. 2 only
C. 3 only
D. 2 and 3 only
B. 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.
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: See Fig.
A. 0.29 mm × 0.29 mm × 1.0 mm.
B. 2.9 mm × 2.9 mm × 1.0 mm.
C. 3.4 mm × 3.4 mm × 1.0 mm.
D. 0.29 mm × 0.29 mm × 1.0 cm.
A. 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.
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: See Fig.
A. maximum intensity projection (MIP).
B. surface rendering.
C. summed projection.
D. voxel gradient.
B. surface rendering.
Computer programs are capable of constructing 3-D models of anatomy with several different types of rendering techniques. A surface-rendered 3-D model provides excellent surface anatomy while maintaining the normal cross-sectional anatomy appearance of the volume inside the model.
Which of the following anatomic quadrants has been removed from the 3-D model? See Fig.
A. Left posterior inferior
B. Right anterior inferior
C. Left anterior superior
D. Right anterior superior
D
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.”
