Chapter 10 - Computed Tomography Flashcards
Moving part of the scanner apparatus
Gantry
The craniocaudal axis of the patient is parallel to the ____-axis
Z-axis
A circle in the x-y plane but can extend considerably along the z-axis
Scanner field of view (FOV)
A component in all modern CT scanners that allows the rotating gantry to have electrical connections to the stationary components
Slip ring
The data that were collected in this scanning pattern correspond to attenuation measurements that are parallel to each other, this geometry is called _____.
Parallel beam projection
Refers to the data collected at a specific angle of interrogation of the object; a collection of X-rays
Projection
*synonymous with the terms PROFILE or VIEW
For typical 64 to 128 detector array CT scanners, the FAN ANGLE is approx. _______, while the full cone angle is about ______
60 degrees; 2.4 degrees
Refers to a fan of data that converges on a vertex, and using rotate-rotate geometry
Fan-beam projection
*xray tube (apex of the fan)
As modern scanners use more detector arrays, their width in the z-axis dimension give rise to a _______
Narrow cone beam geometry
Some commercial cone beam CT scanners use flat panel detectors, and these systems may be considered as ______.; cone angle is almost as great as the fan angle
True cone beam scanners
Center of rotation of the CT gantry
Isocenter
Defined by the physical extent of the curve detector arrays (the fan angle)
Maximum FOV
CT scan design where the X-ray tube and detector tube are attached rigidly to the rotating gantry
Third generation OR rotate-rotate geometry
Product of the mass attenuation coefficient and density
Linear attenuation coefficient
*bone and pure iodine have high linear attenuation coefficient values than soft tissue, water or adipose
Gray scale values in CT; average linear attenuation coefficient for a volume element (voxel) of tissue in the patient at location(x,y,z)
Hounsfield units (HU)
HU of: A. Air B. Adipose tissue C. Most organ parenchyma D. Tissue with iodinated contrast and bone can run much higher
A. -1000
B. -80 to -30
C. +30 to +220
D. Can run much higher, to a maximum of +3,095 for most 12-bit CT scanners
Term used when referring to a location in the body
Volume element (voxel)
Picture element
Pixel
Beam shaping filter
Bow tie filter
*the shape of the filter is designed to attenuate more toward the periphery of the field(where the attenuation path thru the patient is generally thinner), which tends to make the signal levels at the detector more homogeneous
Using thicker CT slices for interpretation will _____ noise while keeping dose levels _____.
Reduce ; low
The consequences of a finite focal spot and the highly magnified collimation gives rise to a ______ at the edge of the beam
Penumbra
Process where X-rays essentially strike lead shielding on the sides of the detector assembly or inactive detectors
Overbeaming
Preliminary scan in CT
CT radiograph(also known as TOPOGRAM/SCANOGRAM/LOCALIZER)
The basic step-and-shoot mode of a CT scanner; the table is stationary during the axial data acquisition sequences
Axial (also called SEQUENTIAL) CT scan
The table moves at a constant speed while the gantry rotates around the patient
Helical (also called SPIRAL) scanning
Describes the relative advancement of the CT table per rotation of the gantry
Pitch
Collimates the waste dose at the outer edges of a helical scan
Adaptive beam collimation
CT scanner acquires data while it also records the ECG info
Retrospective cardiac gating
The ECG is used to trigger the X-ray tube such that projection data are only acquired during the most quiescent part of the cardiac cycle, end-diastole
Prospective cardiac gating
Allows a CT scanner to repeatedly image a volume of tissue that is wider than the detector array; the table rocks back and forth a prescribed distance, during the temporal acquisition procedure
Shuttle mode
*allows CT perfusion analysis to be performed over a large section of tissue
The angular modulation as a function of gantry angle is _____ in ELLIPTICAL body parts and _______ in ROUND body regions
Quasi-sinusoidal; Constant
Characterizes the influence of the bow tie filter; also characterize differences in individual detector response
Air scans
Creating the projections from the central data is called ____.
Forward projection
Computing the image matrix from the projection values
Backprojection
Th blurring caused by the geometry of the Backprojection can be corrected by deconvolving the measured projection data prior to back projection, resulting in the process of _______.
Filtered backprojection
Starting point in designing the shape of the filter in the filtered backprojection
Ramp function
Reduces the high frequency component of the reconstructed image, and tends to abate the noise as a result
Roll-off
Basic cone beam reconstruction process; reconstructs the entire volume data set simultaneously
Feldkampt algorithm OR FDK
Projection data is _____ , and the CT image is the _____
Known; unknown
***page 357
Iterative reconstruction
A more scientific measure of the spatial resolution characteristics of an imaging system
Modulation transfer function (MTF)
Defined as the shape of the system response to a point input in the z-dimension
Slice sensitivity profile (SSP)
The traditional method for quantifying contrast resolution in CT is to visually assess a _____.
Contrast detail phantom
The direct measurement of “noise”
Standard deviation
Describes how the noise propagates though an imaging system; a spatial-frequency dependent function
Noise power spectrum (NPS)
A term referring to the frequency dependence of the noise in an image
Noise texture
Smaller detector dimensions and oversampling methods can _____ spatial resolution
Improve
Primary factors that affect contrast resolution (noise) in CT (4):
- Technique factors
- Slice thickness
- Reconstruction filter (filtered backprojection)
- Reconstruction method
Fundamental determinants of the dose levels used for CT scanning
- kV (does not have a linear relationship with NOISE)
2. mA and 3. Time (have a linear relationship with DOSE)
Thicker images are less noisy than thinner images acquired at the same technique levels because _______.
Combine signals from more detected X-ray quanta
Refers to one with higher average X-ray energies
“Hard” X-ray spectrum
Has lower average X-ray energies
“Soft” spectrum
The presence of dense (and higher Z) structures in the X-ray path cause the lower energies in the X-ray spectrum to be preferentially attenuated compared to the higher X-ray energy photons; results in artifacts that appear as WEBBING BETWEEN DENSE REGIONS IN THE IMAGE
Beam hardening
Occur when the attenuation levels of a region in the patient are excessive
Streak artifacts
Refers to the use of too few projection images acquired to reconstruct high-frequency objects in the image
View aliasing
Occur when the CT voxels are large enough to encompass several types of tissues, such as bone or tissues from different organs
Partial volume
Cone beam acquisition strategies can lead to undersampling in the cone beam angle dimension
Cone-beam artifact
*remedy: acquire a more complete data set
A stack of attenuating disks separated by low density material; can be used to evaluate cone beam artifacts
Defrise phantom
ROTATE-TRANSLATE geometry using the pencil beam is referred to as the _____.
First-generation CT
Rotate-translate geometry with more detectors added forming a narrow fan beam geometry
Second-generation CT
Rotate-rotate geometry; the X-ray tube and detector array are mounted in a fixed position with respect to each other on a rotating gantry
Third-generation CT
Uses an entire 360-degree ring of detectors mounting in the stationary frame, with a rotating X-ray tube; rotate-stationary geometry
Fourth-generation CT scan
Slows the CT gantry to rotate freely and therefore eliminates the need to deal with gantry inertia between scans
Slip ring
Occurs in third-generation CTs when CT detectors are not properly calibrated with respect to each other
Ring artifact
