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
in plane spatial integrity test
daily- x or y direction
monthly- both directions
1 mm
field uniformity test
within 5 HU
monthly- most commonly used kVp
anually- other used kVp settings
electron density to CT number conversion test
annually or after scanner calibration
-commissioning resilts and manufacture specs
spatial resolution
annually
manufacturer specs
contrast resolution
annually
manufacturer specs
CPQR daily laser alignment action level
1 mm
CPQR daily CT number for water
0 +/- 4 HU action level
CPQR daily CT number for water noise - action level
10 % or 0.2 HU from baseline, whichver is greater
CPQR daily CT number for water uniformoty action level
2 HU
CPQR monthly couch table-top level
2 mm over length and width of tabletop action
CPQR monthly lasers orthogonality/orientation
1 mm over length of laser projection action
CPQR monthly couch displacement
1 mm action
CPQR quaterly tests for reproducibility
CT number for other materials
3D low contrast resolution
3D high contrast sptiatial resolution (at 10 and 50% MTF) - within +/-0.5 lp/cm or +/-15% of baseline, whichever is greater
slice thickness (sensitivity profile)- 0.5 mm from baseline for slice > 2 mm, 50% from baseline for slices 1-2 mm, 0.5 mm from baseline for slices < 1 mm
CPQR quaterly tests for amplitude and periodicity of surrogate with monitoring software
1 mm, 0.1 s action
CPQR quaterly test for amplitude of moving target measured with 4DCT
< 2 mm action
CPQR quaterly tests for intergrity and positioning of moving targets at each 4D respiratory phase
2 mm FWHM difference from baseline measurement (increased for amplitudes > 2 cm)
CPQR annual CTDIw test
10% from baseline
CPQR annual kVp, HVL, mAs linearity tests
+/- 2 kVp, +/-10 % difference from baseline measurements
CPQR annual gantry tilt test
within 0.5 degrees
CPQR annual tests for reproducibility
4D low contrsst resolution at each respiratory phase
4D high contrast spatial resolution at each respiratory phase
4D slice thickness (sensitivity profile) at each respiratory phase
CPQR annual test simulated planning
+/- 2 mm
why are there so many laser and couch tests for CT?
important that patient setup is accurate so it is reproducible. If bed is not straight or level, could have image distortions which wouldn’t represent the patient anatomy (and then you’re planning on bed anatomy). With 6DOF beds, it is easier to correct for differences from reference image
why are door interlocks avoided with CTSim?
If the scan is interrupted
during image acquisition, the entire scan may have
to be repeated. This would expose the patient to unnecessary
radiation. A more troublesome situation would be interruption
of a scan while the patient is being injected with a contrast
material. Exposure to a person accidentally entering a
CT-scanner room during image acquisition is minimal and
well below regulatory limits. The interruption of a scan acquisition
therefore has a potential to be much more harmful
to the patient than beneficial for a person entering the scanner
room.
how does laser QA device indicate if gantry lasers are aligned with imaging plane?
If the gantry lasers are
aligned with the imaging plane then the image should show a
well-defined inverted letter ‘‘T’’ in each peg
If
there is a partial image of the inverted letter ‘‘T,’’ or no
image at all, then the gantry lasers are not aligned with the
imaging plane
If the images inside two pegs
are not the same then the QA device is rotated with respect to
the imaging plane. Most frequently, this indicates that the tabletop is rotated with respect to the imaging plane. The measuring cursor option on the scanner can be used
to evaluate if the gantry lasers intersect in the center of the
imaging plane. The measuring cursor usually forms a cross.
If the horizontal line of the measuring cursor is positioned
through horizontal holes on both pegs in image in Fig. 8~a!,
and the vertical line of the cross hair through the hole in the
center of the base plate, then the locator indicator for the
cursor can be used to assess alignment accuracy. The location
indicator (x,y) for the cross-hair position should read
~0, 0!. If there is a different y value, then the horizontal
gantry lasers are not aligned with the center of the imaging
plane and should be adjusted if out of tolerance. If the x
value is different, then the overhead laser is not properly
aligned or, more importantly, the tabletop itself may be improperly
installed.
how can the laser QA device be used to assess spatial integrity?
The separation between the vertical
holes in two pegs in Fig. 8~a! should measure 25061 mm
using the scanner measuring tool.
how can one test if the CT couch is rotated (base to top) and also level and orthogonal to imaging plane?
To verify that the base is not rotated with respect to the
imaging plane, two small pieces of wire ~1 to 29 long! are
taped in the center of the couch top, one at the gantry side
and one at the foot side ~similar to the test above!. The lateral
coordinates of two wires in their respective images should be
identical.
To verify that the couch base is level in the axial
direction, the couch top can be scanned in several places and
scanner cursor tool can be used to evaluate if the couch base
is level.
To verify that the longitudinal couch axis is orthogonal
to the imaging plane, two small pieces of wire can be
taped to the couch top ~in the same longitudinal position but
laterally spaced as far as possible!. Each wire should be oriented
at 90° with respect to the other wire and at 45° with
respect to the imaging plane. The wires are first scanned with
the couch in the lowest vertical position and then in the
highest achievable position. The separation between the
wires in two images should be identical. Variation in wire
separation in two images indicates that the couch base is not
orthogonal with the imaging plane. This can be due to gantry
or couch base tilt.
CPQR quaterly 4DCT amplitude and periodicity of surrgoate with monitoring software
1 mm, 0.1 s
CPQR quaterly 4DCT reconstruction
functional!
CPQR quaterly 4DCT amplitude of moving target merasured with 4DCT
< 2 mm
CPQR quaterly 4DCT spatial integrity and positioning of moving target at each respiratory phase
2 mm (FWHM) difference from baseline measurement (increased for amplitudes > 2 cm)
CPQR quaterly 4DCT mean CT number and standrd deviation of moving targets at each respiratory phase
10 HU and 10% from baseline measurements
increased for amplitudes > 2 cm
CPQR quaterly 4DCT MIP and avg, min image reconstruction
2 mm FWHM difference from baseline measurement (increased for amplitudes > 2 cm)
for what protocols do we do 4DCT QA?
4D-CT image performance is highly dependent on the protocol used. These tests should be conducted for each kVp and mAs used clinically, as well as for each 4D-CT reconstruction technique used clinically (time-based, phase-based, or amplitude-based). Ideally, this can be accomplished by using CT-QA phantoms, such as the CATPHAN® (The Phantom Laboratory, Salem, USA), that can be motion driven (e.g., CATPHAN Shaker, Modus Medical Devices, London, Canada).
We use QUASAR
annual OBI QA- kVp treshold
2 % of nominal
annual OBI QA- HVL treshold
5% of nominal
annual OBI QA- CTDIw treshold
should be within 5 % of original. Wee measure CTDI over 150 mm so have to compensate to make it CTDI 100 (multiply by 100/150)
what meter do we use for annual OBI QA at Halifax
Ray Safe meter
annual OBI QA measured time vs nominal
within 2 %
annual OBI QA orientation check and rad field
do orientation check (ant/post etc) and rad field (passes if within 0.25 of 5x5)
annual OBI QA dose vs mAs plot
should be linear
annual OBI QA CTDI
1-10 mGy depending on body part
