AIM: Ch 10: CT Flashcards
It is the moving part of the scanner apparatus
Gantry
The data that were collected in this scanning pattern correspond to attenuation measurements that are parallel to each other, and this geometry is called a ____. This is a more general geometrical concept, and it has utility in some nuclear and x-ray tomography geometries till today
Parallel beam projection
In the first years of CT, the x-ray tube and detectors used a linear scanning trajectory, as illustrated in Figure 10-8.
It refers to the data collected at a specific angle of interrogation of the object, and this term is synonymous with the terms profile or view
Projection
Rays are individual attenuation measurements that correspond to a line through the object defined at one end by the x-ray source and at the other end by a detector.
A projection is a collection of rays.
The individual rays in this geometry each correspond to a line integral that spans between the x-ray source and the x-ray detector; however, there are a lot more detectors in this geometry compared to the parallel beam geometry
True fan beam geometry
For typical 64 to 128 detector array CT scanners, the fan angle is approximately ____ degrees, while the full cone angle is about ____ degrees
60
2.4
Figure 10-10 shows a more accurate depiction of a modern multislice CT scanner, where the fan angle defines the “fan beam geometry,” but there is nonnegligible x-ray beam divergence in the orthogonal direction, which gives rise to the concept of narrow cone beam geometry.
Some niche CT scanners make use of flat panel detectors for the detector array. This geometry represents a ____, where the cone angle is almost as great as the fan angle. The use of the planar flat panel detector system leads to a 2D bank of detectors, which has no curvature, and so correction methods are needed for the different distances from source to detector, giving rise to inverse square law and heel effect–based differences in fluence to each detector element in the 2D array.
Full cone beam geometry
Flat panel detectors are used for cone beam systems in dental and maxillofacial CT systems, breast CT systems, and in radiation therapy imaging applications where the cone beam CT system is mounted orthogonally to the high-energy linear accelerator treatment beam.
The ____ is the center of rotation of the CT gantry, and in most cases (but not all), it is also the center of the reconstructed CT image—that is, pixel (256, 256) on the 512 x 512 reconstructed CT image
Isocenter
The ____ is defined by the physical extent of the curved detector arrays (the fan angle)
Maximum FOV
It is defined by the ratio of B and A where A is the source-to-isocenter distance and B is the source-to-detector distance. This factor helps in defining the actual width of the detector by getting the product of this factor and the minimum CT slice thickness.
Magnification factor, M
Almost all modern CT scanners have a design where the x-ray tube and detector are attached rigidly to the rotating gantry, and this leads to a so-called third-generation or ____
“Rotate-rotate” geometry
Thus, as the gantry rotates, the x-ray tube and detector arrays stay in a rigid alignment with each other. This fixed geometry allows the use of an antiscatter grid in the detector array.
The grid septa are aligned with the dead space between individual detectors, in an effort to preserve the geometrical detection efficiency of the system.
As the width of the x-ray beam has increased with multidetector array CT (to 40, 80 mm, and larger), there is greater need for more aggressive scatter suppression. At least one manufacturer uses a 2D grid on its large multidetector array CT system.
The x-ray tube is mounted onto the gantry such that the plane of the anode disk is parallel to the plane of gantry rotation (Fig. 10-14A), which is necessary to reduce ____ that would add significant torque to the rotating anode if mounted otherwise.
Gyroscopic effects
Furthermore, this configuration means that the anode-cathode axis and thus the heel effect run parallel to the z-axis of the scanner. This eliminates heel effect–induced spectral changes along the fan angle, which is important. The angular x-ray output from an x-ray tube can be very wide in the dimension parallel to the anode disk but is quite limited in the anode-cathode dimension (Fig. 10-15), and so this x-ray tube orientation is necessary given the approximately 60-degree fan beam of current scanners.
CT gantry rotation speeds are approaching ____ (rotation/s), and these high angular velocities create enormous g-forces on the components that rotate
5 rotations per second (0.20 s rotation time)
While the x-ray detectors are moving in unison with the x-ray source, the patient is stationary, and thus the 1.5-mm circumferential displacement of the x-ray source over the time it takes to acquire one projection can lead to THIS ARTIFACT and a loss of spatial resolution. To compensate for this, some scanners use a magnetic steering system for the electrons as they leave the cathode and strike the anode.
Motion artifacts
One CT manufacturer also uses a ____ to provide oversampling in the z-dimension of the scan. This x-ray tube design combines high power with the ability to stagger the position of the x-ray focal spot inside the x-ray tube, as illustrated in Figure 10-17A. The electron beam is steered using magnetic fields provided by steering coils, and due to the steep anode angle, modulating the electron beam landing location on the focal track causes an apparent shift in the source position along the z-axis of the scanner.
Focal spot steering approach
Another manufacturer uses a different magnetic electron beam steering approach (Figure 10-17B) to achieve a similar effect.
For helical (spiral) scans, this approach leads to an oversampling in the z-dimension, which leads to Nyquist—appropriate sampling in z
Lowest interaction probability in the range of effective x-ray energies in typical CT spectra overlaid on the mass attenuation coefficients for soft tissue
Rayleigh scattering
Highest interaction probability in the range of effective x-ray energies in typical CT spectra overlaid on the mass attenuation coefficients for soft tissue
Compton scattering
At all x-ray tube voltages, give the HU for air and water
-1,000 HU for air
0 for water
When referring to the location in the body, the term ____ is used
Volume element (voxel)
A beam shaping filter, also called a ____, is used on all whole body CT scanners and is located in the x-ray tube assembly. The shape of the filter is designed to attenuate more toward the periphery of the field, which tends to make the signal levels at the detector more homogeneous.
Bow tie filter
The standard adult head is about 17 cm in diameter, whereas the standard adult torso ranges from 24 up to 45 cm or greater, with an average diameter ranging between 26 and 30 cm. Because of these large differences, all commercial CT scanners make use of a minimum of two bow tie filters—a head and a body bow tie. The head bow tie filter is also used in pediatric body imaging on most scanners.
T/F: The bow tie filter has consequences both on patient dose and on image quality.
True
The bow tie filter is a very useful tool because it reduces patient dose with no loss of image quality
NOISE - from central rays which have lower x-ray fluence and higher statistical variance vs peripheral rays which have higher x-ray fluence and lower statistical variance (lower noise)
To improve the detection efficiency of the scintillator material for CT imaging, the scintillation crystals (Gd2S2O and other materials as well) are ____ to increase physical density and light output. It involves heating up the phosphor crystals to just below their melting point for relatively long periods of time (h), and there are many other details to the process. In the end, densification occurs, and the initial scintillating powder is converted into a high-density ceramic
Sintered
They provide power to the detector array and receive the electronic signals from each photodiode
Electronic modules
The electronics module has gain channels for each detector in the module and also contains the analog-to-digital converter, which converts the amplified electronic signal to a digital number
In this scanner, the detectors were 13 to 16 mm wide, and the x-ray beam collimation was used to determine the slice thickness. Collimator settings ranged from 1 mm (thin slice) to 10 mm (thick slice) or larger. This integrated the x-ray signal over the entire width of each detector, and so changing slice thickness was solely a function of the collimation
Single detector array scanner
In this scanner, the slice thickness and x-ray beam width are decoupled. The slice thickness is determined by the detector configuration, and the x-ray beam width is determined by the collimator.
MDCT
Because the beam shape is so different in the penumbra region compared to the center of the beam, the penumbra region is to be avoided, and hence the penumbra is positioned outside of the active detector arrays—those x-rays essentially strike lead shielding on the sides of the detector assembly or inactive detectors. This is called ____.
Overbeaming
The consequence of overbeaming is that for MDCT scanners, the geometrical efficiency is less than the 100% geometrical efficiency of a single detector array scanner.
Once the patient is on the table and the table is moved into the gantry bore, the technologist performs a preliminary scan called the ____
CT radiograph
This image is also called the scout view, topogram, scanogram, or localizer; however, some of these terms are copyrighted to specific vendors.
The PA CT radiograph is preferred over the AP to reduce breast dose in women and girls.
It is at this point in the scanning procedure that all the CT scan parameters are set, usually using preset protocols. For a basic CT scan, these parameters include kV, mA, gantry rotation time(s), type of scan (helical or axial), direction of scan, pitch, detector configuration, reconstruction kernels(s), mA modulation parameters, if used, and so on
It is the basic step-and-shoot mode of a CT scanner
Axial (also called sequential) CT scan
With the advent of ____, it is common to acquire contiguous CT images during an axial acquisition. This implies that between each axial acquisition, the table moves a distance D, which is essentially equal to the width of the detector array at isocenter (nT). This results in a series of images in the CT volume data set, which are contiguous and evenly spaced along the z-direction.
MDCT
In practice, the x-ray beam width is slightly wider than the distance nT, and so the series of axial scans results in some x-ray beam overlap between the axial acquisitions (Fig. 10-33)
With this scanning, the table moves at a constant speed while the gantry rotates around the patient. This geometry results in the x-ray source forming a helix around the patient
Helical or spiraling scanning
Defines the relationship between the width of the active detector arrays used during the scan and the table feed per rotation of the gantry
Pitch
For most CT scanning, the pitch can range between ____
0.75 and 1.5
T/F: The dose is directly proportional to the pitch
False
Pitch settings near 1.5 allow for faster scanning and are used for thoracic or pediatric CT scanning where speed is important. Low pitch values are used in cardiac imaging, or when a very large patient is to be scanned and the other technique factors (kV and mAs) are already maximized.
It could image the heart with a scan time of about 50 ms. This scanner had a series of semicircular tungsten rings below the patient.
Electron beam CT (EBCT) scanner
An electron beam accelerated at approximately 120 kV was magnetically steered, such that it rotated around the tungsten rings very rapidly, forming a dynamically moving x-ray source for the EBCT scanner. An annular array of detectors was mounted above the patient
For true CT reconstruction, data only need to be acquired ____ degrees plus the fan angle. For the heart, the fan angle is small, and so the requirements are essentially 180 to 190 degrees for a properly centered patient. This unconventional EBCT scanner set the standards for high-speed CT imaging in an era when contemporary CT technology required 3- to 5-s CT scans per image.
180
T/F: While the dynamic information achieved from retrospective cardiac imaging is visually dramatic, it represents a high-dose procedure
True
With prospective gating, the ECG triggers the x-ray tube such that projection data are acquired only during the ____ period of the cardiac cycle—this is the point in the cardiac cycle where the heart is the most stationary.
End-diastolic
The benefit of prospective cardiac CT is that the radiation dose to the patient is significantly less than with retrospective gating.
The downside of prospective gating is that a full temporal record of the beating heart is not acquired, only one snapshot during end-diastole, but as mentioned previously, this image is usually sufficient for evaluation of coronary artery disease
VERSUS RETROSPECTIVE GATING: With retrospective reconstruction, projection data are acquired over the entire cardiac cycle. Therefore, CT images can be synthesized using the ECG gating information over the entire cardiac cycle, and a rendered “beating heart” image can be produced
In general, ____ procedures produce excellent CTA images. Some image rendering techniques use pseudocolor to add realism to the images
Maximum intensity projection
T/F: CT perfusion takes advantage of the wide collimated beam width (20 to 160 mm) of top of the line scanners (at least 64 data channels) to image a volume of tissue in the same region of the body repeatedly
True
It allows a CT scanner to repeatedly image a volume of tissue that is wider than the detector array (nT).
Shuttle mode
This mode allows CT perfusion analysis to be performed over a larger section of tissue
The temporal resolution is lower in shuttle mode
T/F: When mA modulation is performed, lower radiation dose levels can be used to achieve the same image quality.
True
The angular modulation as a function of gantry angle is quasi-sinusoidal in elliptical body parts and constant in round body regions, and these fluctuations occur over relatively shorter distances as highlighted in Figure 10-45
During routine calibration of the CT scanner, the influence of the bow tie filter is characterized by performing ____. These also characterize differences in individual detector response, which may be due to differences in photodiode or amplifier gain
Air scans
Creating the projections from the central data is called ____, and that is what the CT scanner hardware does physically. It is used in iterative reconstruction techniques and many other reconstruction processes as well
Forward projection
We saw that to produce the projection values, knowing the contents of the image, the straightforward process of forward projection is used. The reverse problem, computing the image matrix from the projection values, can be solved (almost) by ____. With this process, the measured projection values are simply smeared back into the image matrix to compute the backprojected image
Backprojection
Using simple backprojection as described above, the reconstructed image has a characteristic 1/r ____ that results from the geometry of backprojection
Blurring
It is possible to correct for the impact of the 1/r function using image processing procedures. The mathematical operation of ____ describes such a procedure.
Convolution
When a convolution kernel has negative values such as that shown in Figure 10-50B, in general it “sharpens” an image.
When “undoing” an effect caused by convolution, deconvolution is used. Deconvolution is mathematically identical to convolution, except that the deconvolution kernel is (by definition) designed to undo a specific effect
The convolution operation described here is a form of mathematical filtering, and this is why the procedure is called filtered backprojection.
It reduces the high frequency component of the reconstructed image, and tends to abate the noise as a result.
Roll-off
The specific roll-off function varies for the specific application in CT. In the mathematical reconstruction literature, the names of specific roll-off filters include Shepp-Logan, Hamming, Cosine, etc.
The basic cone beam reconstruction process, often referred to as the ____, reconstructs the entire volume data set (a series of CT images of a given thickness) simultaneously.
Feldkampt algorithm or FDK
The difference between the forward projection and the measured projection is the ____, which is computed for all projections measured around 360 degrees.
Error matrix
It results from the fundamental resolution properties of the image acquisition, as well as the resolution characteristics of the reconstruction filter that is used
Spatial resolution
All of the following determine the ulitimate resolution, except:
a. Focal spot size and distribution
b. Detector dimensions
c. Magnification factor
d. All of the above are included
d. AOTA
It is a more scientific measure of the spatial resolution characteristics of an imaging system
Modulation transfer function (MTF)
It defined as the shape of the system response to a point input in the z-dimension. In standard image science vernacular, this is the LSF along the z-dimension in the CT scanner
Slice sensitivity profile, SSP
The traditional method for quantifying contrast resolution in CT is to visually assess a ____
Contrast detail phantom
While the standard deviation (sigma) measures the overall noise level in a region of the image, the ____ describes both the overall noise level and the noise texture
NPS(f)
Noise power spectrum
The NPS describes how the noise at one point in the image is correlated to the noise in the surrounding points of the image
A term referring to the frequency dependence of the noise in an image
Noise texture
Name the 4 primary factors that affect spatial resolution in CT
- X-ray tube focal spot distribution
- Gantry motion
- Detector size and sampling
- Reconstruction filter
Name the 4 primary factors that affect contrast resolution (noise) in CT
- Technique factors: time and mA (linear), kV and pitch (non-linear)
- Slice thickness
- Reconstruction filter (filtered backprojection)
- Reconstruction method
Iterative CT recon: less noise than filtered backprojection reconstruction
CT systems run at very high mA typically, and this can increase the size of the x-ray focus called ____
Focal spot “blooming”
This artifact results in the presence of dense (and higher z) structures such as bones in the x-ray path that cause the lower energies in the x-ray spectrum to be preferentially attenuated compared to the higher x-ray energy photons, such that the beam undergoes a upward shift in average x-ray energy as it passes through greater thicknesses of bone.
Beam hardening
A “hard” x-ray spectrum refers to one with higher average x-ray energies, and a “soft” spectrum has lower average x-ray energies.
These artifacts occur when the attenuation levels of a region in the patient are excessive, which can exceed the dynamic range of the detector systems or the effective linear range of the detectors
Streak artifacts
Streak artifacts are exacerbated by any motion of the high-density object
It refers to the use of too few projection images acquired to reconstruct high-frequency objects in the image
View aliasing
It occurs when the CT voxels are large enough to encompass several types of tissue, such as bone and tissue or tissues from different organs
Partial volume artifacts
Cone beam acquisition strategies can lead to undersampling in the cone angle dimension, and this can cause a well-known ____
Cone beam artifact
Cone beam artifacts are a result of fundamental deficits in the acquired data, and the most obvious solution for these artifacts is to acquire a more complete data set
It is a stack of attenuating disks separated by low density material which can be used to evaluate cone beam artifacts
Defrise phantom
The system rotated 1 degree, and then the x-ray tube/detector assembly translated in the other direction. This scanning pattern continued until 180 degrees of data were acquired. This rotate-translate geometry using the pencil beam is referred to as ____
1st generation CT
Rotate-translate: pencil geometry
It used the same rotate-translate motion as before; however, more detectors were added and so the initial pencil beam geometry became a narrow fan beam. A detector array using 30 detectors meant that more data could be acquired in a given scan.
2nd generation CT
Rotate-translate: narrow fan beam geometry
X-ray tube and detector array are mounted in a fixed position with respect to each other on a rotating gantry; however, the detector array is now long enough to allow angular sampling over the entire FOV. What CT generation is this?
3rd generation CT
It is still the most widely used geometry on modern scanners
Rotate-rotate geometry of the third-generation CT
For a practical body CT scanner, the FOV should be about 50 cm in diameter, and with a source to isocenter distance of 50 cm, the fan angle a needs to be about 60 degrees. Most modern CT scanners have geometries that roughly (620%) adhere to these values.
The third-generation CT geometry leads to the potential for this artifact production, due to the fixed angular relationship between the detectors and the x-ray source on the gantry
Ring artifact
Third generation scanners (Fig. 10-73A) use a source fan geometry where the signal from two different detectors is used to make the two x-ray intensity measurements (Ir and Ij) necessary to compute a projection value, as described in Equation 10-8. If one detector drifts in terms of its calibration with respect to the other detector, the error in the projection measurement Pj will be propagated throughout the backprojection process, leading to a ring artifact
3RD: SOURCE FAN GEOMETRY
4TH: DETECTOR FAN GEOMETRY
It has a rotating x-ray tube but the entire detector array is stationary; hence, these systems have a rotate-stationary geometry
4th generation CT
The major observation with fourth-generation scanners is that an entire 360-degree array of detectors is needed, instead of the 60-degree detector array of a third-generation scanner. Thus, a factor of six more detector elements is required, adding significant cost.
With the fourth-generation CT, a detector fan geometry is used (Fig. 10-73C). The same detector is used to make both measurements necessary to compute the projection value, and therefore any drift in the detector will be intrinsically factored out since the Ir and Ij measurements are made by the exact same detector element. This fundamentally eliminates the production of ring artifacts in the reconstructed CT image.
