Computed Tomography Flashcards
This is the moving part of the scanner apparatus (CT).
Gantry
In the first years of CT, X-ray tube and detectors used linear scanning trajectory.
This data that were collected in this scanning pattern corresponds to attenuation measurements that are parallel to each other, and this geometry is called what?.
Parallel beam projection
This refers to the data collected at a specific angle of interrogation of the object, and this term is synonymous with the term profile or view.
Projection
Rays are individually attenuation measurements that corresponds 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.
This refers to a fan of data that converges on a vertex , and using rotate-rotate geometry, the apex of the fan is the X-ray tube.
The individual rays corresponds to each detector measurement.
Fan beam projection
There are more detectors.
Minimal divergence of x-ray beam trajectory in and out of the plane figure.
What are the different beam geometries used in CT scan?
Parallel beam geometry
Fan beam geometry
Cone beam geometry
This is the center of the rotation gantry, and in most cases (but not all), it also the center of the reconstructed CT image -that is, pixel on the 512 x 512 reconstructed CT image.
Isocenter
Once the patient is on the table and the table is moved into the gantry technologist performs a preliminary scan called what?
CT radiograph.
This image is also called the scout view, topogram, scanogram, or localizer.
This is the basic step-and-shoot mode of a CT scanner.
The axial (also called sequential) CT scan
The gantry rotates at typical rotation speeds of 0.5 s or so, but the X-ray tube is not turned on all the time.
With the advent of MDCT, it is common to acquire contiguous CT images during axial acquisition.
With this kind of scanning, the table moves at a constant speed while the gantry rotates around the patient.
Helical (also called spiral) scanning
The advantage of helical scanning is speed - by eliminating the start/stop motion of the table as in axial CT, there are no inertial constraints to the procedure.
This describes the relative advancement of the CT table per rotation.
Pitch
For most CT scanning, the pitch can range between ___.
0.75 and 1.5
A pitch lower than 1.0 results in overscanning the patient and hence higher radiation dose to the patient than a pitch of 1.0, all other factors being equal.
A pitch greater than 1.0 represents underscanning, and results in lower radiation dose to the patient.
Pitch settings near _____ allow for faster scanning and are used for thoracic or paediatric CT scanning where speed is important.
1.5
____ 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.
High or low value?
Low pitch values
This is used to evaluate vascular perfusion and other physiological parameters related to blood flow to a specific organ.
CT perfusion imaging
This allows a CT scanner to repeatedly image a volume of tissue that is wider than the detector array.
Shuttle mode
Shuttle mode imaging is a hybrid between CT perfusion and helical CT imaging - the table shuffles back and forth over a relatively small (100 to 200 mm) predefined region in the patient, allowing temporal imaging and hence CT perfusion assessment to be made over a wider length of anatomy than the width of a MDCT.
What are the primary factors that affect spatial resolution in CT?
X-ray Tube Focal Spot Distribution
Gantry Motion
Detector Size and Sampling
Reconstruction Filter
Primary Factors That Affect Contrast Resolution:
Technique Factors
Slice Thickness
Reconstruction Filter (Filtered Backprojecion)
Reconstruction Method
This occur when the attenuation levels of a region in the patient is excessive, which can exceed the dynamic range of the detector systems or the effective linear range of the detectors.
Streak artifacts
Metallic fillings in the teeth are a common source for streak artifacts,as are most implanted devices that have significant metal components.
This refers to the use of too few projection images acquired to reconstruct high-frequency objects in the image.
View aliasing
This occur when the CT voxels are large enough to compass several types of tissue, such as bone and tissue or tissues from different organs.
Partial volume artifacts
Partial volume artifacts arise essentially from reconstructing low resolution images, typically thick slice images.
Cone beam acquisition strategies can lead to undersampling in the cone angle dimension, and this can cause a well-known artifact called what?
This are a result of fundamental deficits in the acquired data, and the most common obvious solution for these artifacts is to acquire a more complete data set.
Cone beam artifacts
The Defrise phantom, which is a stack of attenuating disks separated by low density material, can be used to evaluate cone beam artifacts.
Rotate-translate geometry using the pencil beam is referred to as what?
First generation CT
The scan required over 4 minutes and image reconstruction ran over night.
Translate-rotate geometry are what generation of CT?
First and second generation
This generation of CT scan is characterized by a gantry with rigidly mounted x-ray system and detector arrays, in which both systems rotate together around the patient -resulting in the rotate-rotate motion.
Third-generation CT scan
This system used an entire 360-degree ring of detectors mounting in the stationary frame, with a rotating x-ray tube resulting in rotate-stationary acquisition.
Fourth-generation CT systems
Third-generation scanners use a ______ geometry where the signal from two different detectors is used to make the two x-ray intensity measures necessary to compute a projection value.
Source fan
If one detector drifts in terms of its calibration with respect to the other detector, the error in the projection measurement will be propagated throughout the backprojection process, leading to a ring artifact.
A detector fan geometry is used in what generation of CT scan?
Fourth-generation CT
In rays that pass through a great deal of dense tissue, such as the petrous bone, the high degree of attenuation causes the x-ray spectrum to become what?
To become hardened.
A “hard” x-ray spectrum refers to one with higher average x-ray energies, and a “soft” x-ray spectrum has lower average x-ray energies.
Beam hardening artifacts occur, especially when dense objects such as bone or metal implants exists.
