CTSim QA Flashcards
how to measure MTF from CATPHAN (bb image)
the line spread function (LSF) is first obtained by processing the scanned image (Fig. 1) in which the region of interest (ROI) placed on the point source response is integrated in one of the matrix directions. The MTF is then derived from the LSF using a Fourier transform.
what is MTF
determines how much contrast in the original object is maintained by the detector. In other words, it characterizes how faithfully the spatial frequency content of the object gets transferred to the image
how is pixel size evaluated with CATPHAN?
compare number of pixels between 2 opposite holes and the real distance (50 mm)
how is the slice width calculated with CATPHAN?
count beads and multiply by z-axis increment
OR
z-axis length at FWHM of a bead in a sagital or coronal image
pre-patient and post-patient collimation in CT
pre-patient = radiation beam in the longitudinal direction distal to the x-ray source - measured by radiation profile width
post patient = immediately prior to the detector array - measured by sensitivity profile width
pre-patient collimation has effect on patient dose
If the radiation profile width is wider than indicated, unnecessary radiation will be delivered to the patient, thus increasing the total dose from the scan. An excessively narrow radiation profile or sensitivity profile width may cause increased quantum noise due to reduced photon count. Excessive sensitivity profile width can result in some lose of resolution in the longitudinal direction.
max difference in mean HU for ROIs sampled throughout uniform volume
10 HU
Uniformity check throughout the phantom is important because we use that CT data for dose calculations
why do we use bowtie filter
compensate for beam hardening on uneven patient surface (more hardening in center)
why do we QA gantry tilt?
tilt the gantry moreso in diagnostic imaging as doctors like to look at the brain at a slant. For planning purposes, want a straight-on image as this will correspond to the linac. In a hospital with shared resources, might go back and forth with the gantry tilting
when is shielding survey for CT done?
initially, must meet regulatory limits
how often is CTDI checked?
annually or after major CT scanner component replacement
must be within 20% of manufacturer specification
tolerance for alignment of gantry lasers with center of imaging plane
2 mm
daily
tolerance of orientation of gantry lasers wrt imaging plane
2 mm over length of laser projection
monthly and after laser adjustements
spacing of lateral wall lasers wrt lateral gantry lasers and scan plane
this distance is used for patient localization marking
2 mm
monthly and after laser adjustments
oreitnation of wall lasers wrt imaging plane
2 mm over length of laser projection
monthly and after laser adjustments
orientation of ceiling laser wrt imaging plane
2 mm over length of laser projection
monthly and after laser adjustments
orientation of CT scanner tabletop wrt imaging plane
monthly or when daily QA tests reveal rotational problems
2 mm over length and width of tabletop
table vertical and longitudinal motion
1 mm over range of table motion
monthly
table indexing and positioning
anually
1 mm over scan range
gantry tilt accuracy
annually
1 degree over gantry tilt range
also check that is accurately returns to nominal position after tilting (1 degree or 1 mm)
accuracy of scan localization from pilot images
anually
1 mm over scan range
what does CPQR give as treshold for lasers vs AAPM TG-66?
CPQR says 1 mm
AAPM says 2 mm
radiation profile width
annually, meet manufacturer specifications
optional if CTDI passes
sensitivity profile width
semiannually, 1 mm of nominal value
generator tests
after replacement of major generator component
per manufacturer spec
explain CT number accuracy tests
daily- water only
monthly- 4-5 materials
annually - electron density phantom
for water, 0 +/= 5 HU
image noise test
daily
manufcaturer spec