CT Major Exam 1
Area Covered Formula -
# of slices x table movement (mm measurement) = mm of body scanned Ex- 40 slices with a 5mm table movement = 200 mm body scanned
Calculating Voxel Size Formula
(DFOV x 10) divided by Matrix times Slice Thickness Examples DFOVmm / Matrix X Slice thickness DFOV of 500/matrix of 512= .97mm X 5mm = 4.88mm2 Voxel DFOV of 500/matrix of 512= .97mm X 2mm = 1.95mm2 Voxel DFOV of 500/matrix of 256= 1.9mm X 5mm = 9.76mm2 Voxel DFOV of 250/matrix of 256= .97mm X 2mm = 1.95mm2 Voxel
For less than 5mm slice thickness accuracy testing, you should have a variance of no more than:
+ or - 0.5mm
When doing a slice thickness accuracy test, for a slice thickness of 5mm, the variance should be no more than:
+ or - 1mm
For Uniformity, the ROI readings on the perimeter should not vary more than:
+ or - 2 from the center.
0.1% contrast difference corresponds to ___ HU.
1
MTF Scale is from 0 to 1, and _____ is perfect.
1 MTF curves further to the right have better spatial resolution.
1% contrast difference corresponds to ___ HU.
10
Anything above a pitch of ___ is not used.
2
When doing laser light accuracy, the light should coincide with the radiation field within:
2mm
Where does a string of pearl, or string of beads, artifact come from?
A MIP'd image This is seen on the registry
Interpolation is __________ associated with some loss of image resolution.
ALWAYS
If the tube is at a 0 azimuth, you get an:
AP image
If I am doing a brain scan, what type of image acquisition will I use?
Axial step and shoot
_______ is the reduction in CT number toward the middle of a uniform test object.
Cupping
Matrix size and _______ determine the pixel size.
DFOV
The best detail image is obtained by using a small ____ and a large ______:
DFOV matrix
What is the limiting factor in all scanners?
Heat
What are the 3 Methods of Data Acquisition?
Localizer scanning (scout) Step and Shoot (axial or conventional) Helical (volumetric)
Selecting voxels with the highest value to display is called:
MIP, or MIP'ing used for bone, or contrast filled structures
To select voxels with the lowest value to display is called:
MinIP, or MinIP'ing used for bronchial tree
Are the terms collimation and slice thickness synonymous with the MDCT?
NO
Both the liver and the brain should have a _____ window width.
Narrow
_______ is the increase in CT number toward the middle of a uniform test object.
Peaking
Volume Formula MDCT
Pitch x total acquisition time x (1/rotation time) x (slice thickness x slices per rotation)= amount of anatomy covered Example Pitch of 1.2, 20-s acquisition, 0.5-s rotation, 2.5-mm slice, 4 slices per rotation, 1.2 x 20 s x 1/0.5 s (=2) x (2.5 mm x 4) = 480 mm of anatomy covered
Volume Coverage SDCT Formula
Pitch x total acquisition time x (1/rotation time) x slice thickness = amount of anatomy covered. Example Pitch of 2 x 20s acquisition time x 1/1sec rotation x 5mm slice thickness = 200 mm covered
Calculating Pixel Size Formula
Pixel size (mm)= DFOV/matrix size Examples DFOV of 50cm (10)/matrix of 512= .97 mm pixel DFOV of 25cm (10)/matrix of 512= .48 mm pixel DFOV of 50cm (10)/matrix of 256= 1.9mm pixel DFOV of 25cm (10)/matrix of 256= .97mm pixel
Range Formula
Range= Win Level + (Win Width/2) Range= Win Level - (Win Width/2) Must do both equations, to have the range.
Contrast resolution is improved by:
THICKER slices LARGER pixels LARGER FOV SMALLER matrix If you are improving contrast, you are decreasing resolution.
What width are the pre and post collimators set at?
The same width
What can lateral scouts be used for?
To determine the gantry angulation
T/F. The patient must be centered appropriately in the gantry in both x and y directions.
True
T/F. Higher mA settings allow shorter scan times to be used.
True Short can time is critical in avoiding image degradation as a result of patient motion.
Spiral and cone beam effects are more pronounced for the outer detector rows. T/F
True The more the detector channels, the more of a problem
There are two types of multislice CT detectors:
Uniform Non-uniform
Spatial resolution is tested using:
a line pair phantom monthly (tho usually done daily, with the CT number calibration and contrast resolution, as they all share the same phantom)
Contrast resolution is tested using:
a phantom with object of varying sizes with only a small difference in density from the background.
Linearity is measured using a phantom that contains:
a variety of objects with known densities
In retrospective reconstruction, the images are _________ in the same plane.
always
In MDCT, pitch is defines as table movement per rotation divided by:
beam width
The smaller the focal spot, the __________ the image quality.
better
The thinner the slice, the ______ the image quality.
better
The 3 key aspects of Helical scanning are:
constant rotation of x-ray tube constant x-ray output uninterrupted table movement
Anything that will reduce noise will ______ contrast resolution.
contrast
The average CT number of water, or CT number calibration, should be performed:
daily
Temporal resolution is how rapidly data are acquired, and is controlled by:
gantry speed number of detectors speed the system records signals Reported in milliseconds Important for heart or contrast flow studies
What are the two scan types that require a LAT image to be taken?
head and spine
Only _________ ___________ can be used to generate images in a different plane or orientation. This is called image ______________.
image data, reformation
When __________ _____________ are assembled to produce images in different planes, or to produce 3D images, it is called ____________.
image data, reformation
As the number of detector rows _____, the beam becomes _________.
increase, wider
What defines the quality (average energy) of the x-ray beam?
kVp
Increasing slice thickness, pixel size, and patient dose *mAs* results in _____ noise.
less
How do you get the quantity of the x-ray energy?
mA and time together define the quantity (mAs)
mAs formula is:
mA x time = mAs 200 mA x .5 sec = 100 mAs 100 mA x 1 sec = 100 mAs
MIP means
maximum intensity projection
MinIP means:
minimum intensity projection
Both ____ and _______ are measured using a water phantom.
noise, uniformity
Beam width is determined by multiplying the:
number of slices by slice thickness Example- table movement of 6mm slice thickness of 1.25mm 4 slices )4 row detector) Formula: table movement/beam width (#of slices x slice thickness) So 6/(4x1.25) -> 6/5 = 1.2 pitch
Contrast resolution is the ability to differentiate between:
objects with very similar densities as their background
Image quality is linked directly to:
patient dose better quality = higher dose
A small ____ and a thin ____ give the best detail or resolution.
pixel slice thickness
Algorithms can only be applied to _______ data.
raw
When ___________ ___________ is manipulated to create pixels that are used to create an image it is called _____________.
raw data, reconstruction
Two advantages of helical scanning are:
reduces respiratory misregistration reduces motion artifacts from organs
Spatial resolution is the ability to:
resolve (as separate objects) small, high contrast objects AKA detail resolution
Post processing results in
retrospective images
Be able to tell the difference in images of the spatial resolution phantom, and the contrast phantom.
see pictures in image quality ppt
For noise, the standard deviation of an ROI:
should NOT exceed 10.
HIgh pass filers give better __________ but decrease _________.
spatial resolution contrast resolution
A water phantom gets you:
standard deviation CT number of water
Pitch Formula in MDCT
table movement/beam width (# of slices x slice thickness)
Pitch in SDCT is defined as:
the distance in mm that the CT table moves during one revolution of the x-ray tube. Note the difference between SDCT pitch and MDCT pitch is taking into account the number of slices as well as the slice thickness.
Automatic tube current modulation works best when:
the patient is positioned in the isocenter of the tube.
Pitch is:
the relationship between slice thickness and table travel.
If you want the average CT number of water is, you should use a:
water phantom
Uniformity (Cross-field uniformity) is performed:
weekly, with a water phantom
