CT Flashcards
Moving part of the scanner apparatus
Gantry
Refers to the 2D geometry where there is only minimal divergence of the xray beam trajectory in and out of the plane of the figure
True fan beam
Power rating of xray tubes in ct
5-7 megaJoule
Rotation of gantry
Clockwise
Electron beam striking anode is steered
Counterclockwise
HU of fat
-80 to -30
Most organ parenchyma HU
+30 to +220
Whole body CT scanners use this type of filter to attenuate more toward the periphery of the field
Bow tie filter
All modern multidetector array CT scanners use
Indirect (scintillating) solid-state detectors
Scintillating crystals
Gadolinium oxysulfide
Involves hearing up the phosphor crystals to just below their melting point for relatively long periods of time
Process of sintering
Penumbra is positioned outside of the active detector arrays— those xrays essentially strike lead shielding on the sides of the detector assemblu or inactive detectors. This is called
Overbeaming
Basic step and shoot mode of the CT scanner—— the xray beam is not “on” while the patient is being translated between the acquisition cycles
Axial or sequential CT acquisition
The xray tube has a helical trajectory around the patient’s body
Helical or spiral CT acquisition
Important relationship between the width of the active detector arrays used during the scan and the table feed per rotation of the gantry
Pitch
Main advantage of helical CT versus axial ct
Speed
For most CT scanning, the pitch can range between
0.75 and 1.5
A pitch lower than 1.0 results in
Overscanning and higher radiation
Pitch greater than 1.0 results to
Underscanning and lower radiation
Pitch settings near 1.5 allow for faster scanning and are used for______ where speed is important
Thoracic or pediatric scanning
In a low pitch study, the helix appears
Tight
In a high pitch helical ct, the helix appears
Loose
Reconstruction algorithm used for converting the raw data into CT images takes into account the
Cone angle
To freeze cardiac motion, an image acquisition time window of _____ ms or less is required
100 ms
Fan angle jn cardiac CT is
180-190
Used to evaluate vascular perfusion and other physiological parameters related to blood flow to a specific organ
CT perfusion
Allows a CT scanner to repeatedly image a volume of tissue that is wider than the detector array
Shuttle mode CT
Angular modulation as a function of gantry angle is ________ in elliptical body parts and _____ in round body regions
Quasi-sinusoidal in eliptical body parts
Constant in round body parts
Reconstruction that is used in commercial scanners more often than the convolution approach
Fourier-based reconstruction
Reconstruction that keeps track of the beam divergence in the z-direction
Cone beam reconstruction
Primary method of reconstruction for many decades
Filtered backprojection
Type of reconstruction that starts with an initial estimate of what the object may look like. This guess is then used to compute forward projection data sets, which are then compared with the measured projection data sets
Iterative reconstruction
Factors that determine the ultimate resolution in ct include
Focal spot size and distribution, the detector dimensions, the magnification factor, whether or not gantry motion is compensated for and patient motion
In CT literature, the resolution is quoted in line pairs per ____ instead of line pairs per mm as is typical in the rest of the xray imaging
Per cm
A more scientific measure of the spatial resolution characteristics of an imaging system. It can be measures using a wire or plane of metal foil scanned on a CT scanner is computed from the measured line spread function
Modulation transfer function
Shape of the system response to a point input in the z-dimension
Slice sensitivity profile
Describes how the noise propagates thru an imaging system. It describes both the overall noise level and the noise texture
Noise power spectrum
Fundamental determinants of dose levels used for CT scanning, and they fundamentally impact the noise levels in the CT images
kV, mA, time and pitch
Relationship of mA and time with dose
Linear relationship
Thicker images combine signals from more detected xray quanta and therefore are ____ noisy than thinner images acquired at the same technique levels
Less noisy
Artifacts that 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 detectors
Streak artifacts
High degree of attenuation causes the xray spectrum to become hardened, hence this artifact
Beam hardening
Refers to use of too few projection images acquired to reconstruct high-frequency objects in the image
View aliasing
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
Artifact caused by undersampling in the cone angle dimension, and accentuated by using the defrise phantom
Cone beam artifact
Rotate-translate geometry using pencil beam, is what generation of CT
First
Used the same rotate-translate motion, however, more detectors were added and so the initial pencil beam geometry became a narrow fan beam—- what generation
Second generation
What generation of CT uses rotate-rotate geometry and is still the most widely used geometry on modern scanners today
3rd generation
Generation of CT that used an entire 360 degree ring of detectors mounting in the stationary frame, with a rotating xray tube resulting in rotate-stationary acquisition
4th generation
3rd generation CT geometry leads to potential for what artifact
Ring artifact production
How many degrees is the detector of 3rd gen CT
60 degrees
What CT generation uses a detector fan geometry
4th generation
Value that most closely matches the attenuation profile of a polyenergetic xray spectrum
Effective linear attenuation coefficient
Xray beams are attenuated
Exponentially
Necessary for creating a projection image
Attenuation
Source to skin distance can be estimated by knowing the following
Source to skin distance (SID)
Thickness of the patient
Dead space between the cover of the detector and the actual detector surface
Function of mA and kV, and the mA fluctuates during the procedure depending on the positioning of the fluoroscopic system relative to the patient’s body and on the mode settings of the fluoroscopy system
Kerma rate
Most modern xray dosimetry is based upon what calculation, which used computer stimulation to study the dose deposition during an xray procedure
Monte Carlo
Refers to the beam width as reported by the scanner, not the actual measured beam width, which is generally slightly wider
Nominal beam width
In helical CT scanning, the CT dose is _____ related to the helical pitch
Inversely
Dose length product is approximately _____ to effective dose
Proportional
Measured using either a 16 cm or 32 cm diameter polymethyl methacrylate phantom. The dosimeter is placed serially in the center hole and the peripheral hole, and the measurements are combined
Computed tomography dose index
