Unit 4: CT Fundamentals Flashcards
What generation scanners are used for medical imaging purposes in Canada today?
3rd generation
T/F
3rd generation CT scanners have a rotate/rotate motion
True
T/F
3rd generation CT scanners have pencil beam geometry
False; they have fan beam geometry
What type of detector array does 3rd generation CT scanners have?
A curvilinear detector array
What is the primary difference between SDCT and MDCT
SDCT only takes an image of one slice per rotation, where as MDCT can take up to 64 slices in 1 rotation.
T/F
In SDCT the largest allowable slice thickness is less than the detector width.
True
T/F
SDCT is the most widely used compared to MDCT.
False; MDCT
Is it possible to change slice thickness with MDCT?
Yes; depends to the manfacterer
(can also change thickness so that thinner rows are ceen centrally and wider rows peripherally)
What is slice thickness determed by in MDCT?
Determined by a combination of the x ray beam width and the detector configuration
What is the smallest slice thickness seen in MDCT?
.5mm
What the 2 types of scan modes in CT?
- Axial
- Helical
What is the most common scan mode used?
Helical
(axial not really used anymore)
What is the axial scan mode?
Refers to when the CT table moves to the desired location and remains stationary while the x ray tube rotates within the gantry, collecting data. The CT table is then moved further after data is collected.
What is interscan delay?
The slight pause in between data acquisition as the table moves to the next location in axial scan mode
When would axial scan mode be used?
Used in studies where acquisition speed is not of a major concern.
What are the two ways in which axial slices can be obtained?
- Contiguous
- Non Contiguous
What is Cine mode?
With Axial slicing, scanners can be programmed to repeat scans at the same slice location (i.e. no table incremation)
Why are axial protocols that use overlapping slices rare?
They increase the radiation dose to the patient and do not provide additional diagnostic information
What is the primary disadvantage of axial scanning?
The cumulative effect of the pauses (interscan delay) between each data acquisition adds to the total examination time
(time is the disadvantage)
T/F
Data acquired in axial mode are more limited in how they can be reformatted or reconstructed.
True
What is the purpose of interpolation in helical scan mode?
To take the slant and blur out of the helical image
What is pitch?
The ratio of the patient couch movement (during one x ray tube revolution) to the width of the x-ray beam.
T/F
A pitch less than 1 results in overlapping images and higher patient radiation dose.
True
If the distance translated by the table equals the slice thickness, what is the pitch ratio?
1:1
If the table feed is 6 mm per roation, and the beam width is 4 x 1.25, what is the pitch?
1.2
Calculate the anatomical coverage of a helical scan with a 20 second totatl acquisitioin time, a 0.5 rotation time, a 2.5 mm slice thickness and a pitch of 1.2 if 4 slices are acquired with each tube rotation.
480 mm
T/F
An increase in anatomy coverage in helical scanning=increase in time
True
Label 1-3
- Y axis
- X axis
- Z axis
Label 1-4
- Z axis
- x axis
- Y axis
- X axis
Why is it important that the patient be centered at the isocenter?
To ensure proper functioning of the bow tie filters.
What is the purpose of a bow tie filter?
To convey maximum x-ray photons to the thicker part of the body and to reduce x-ray intensity to the thinner part of the body.
T/F
In practice, off-center positioning occurs even though laser indicators identify the isocenter of a CT scanner
True
(thicker part of the filter aligns with the thinner anatomy)
What is each segmented portion of volume of a slice in CT called?
Voxel
What is the voxel projected on the display screen as?
A pixel
Calculate the tissue volume contained within the shown voxel:
117mm^3
What is volume averaging?
All the various tissue attenuation values within a voxel are averaged and represented as a shade of grey (or brightness).
What projections are taken in a CT Reference Image?
AP & Lateral
T/F
The center point in CT is not at the center of the FOV.
True
What is Image Reconstruction (Prospective)?
Process whereby the computer manipulates data collected from the detectors to create a CT image, based on scan parameters.
Label 1
Aperture
What are the 3 selections a technologist can make when selecting Scan Field of View?
Small
Medium
Large
What does SFOV determine?
SFOV determines the number of detector cells collecting data of a certain diameter through the icocenter of the gantry
ahhhhhhh basically its just saying the area we collimated is the area that we expose..this took me 10 min to figure out.
T/F
Anything outside the SFOV is not imaged, as data is not collected outside this circle.
True
T/F
To produce the highest quality image, the operator should select the SFOV that just encompasses the anatomy of interest.
True
What is DFOV?
DFOV is the reconstructed area seen on the display monitor, following the scan.
What does DFOV determine?
Determines how much of the acquired raw data is used to create the image
CT image acquisition:
1. Scout of chest (SFOV)
2. Select DFOV
3. Scan anatomy slice
T/F
DFOV can be equal to or less than the SFOV.
True
What is the benefit of using DFOV?
You dont loose quality compared to zooming into an image and the image is more localized to the anatomy of interest
As pixel size goes down, what happens to SR?
SR increases
T/F
Using DFOV decreases dose
FALSE; still exposes the anatomy in the SFOV
What is the range of HU?
-1000 to +3000
What is the HU for water
0
What is the HU for fat?
-100
What is the HU for bone/metal?
+1000
What is the HU of air?
-1000
