Manufacturer Specific Considerations for CT Accreditation

Transcription

Manufacturer Specific
Considerations for CT
Accreditation
Melissa C. Martin, M.S., FAAPM, FACR
AAPM Spring Clinical Meeting
March 20, 2012
Dallas, TX
Phantom Submission
Slide provided by Doug Pfeiffer
Phantom Submission
Phantom Submission
Phantom Site Scanning Form Page 1
Phantom Site Scanning Data
Form - Page 2
Phantom Images
Phantom Images
Phantom Images
Phantom Scoring
CT # Accuracy
Contrast-Noise Ratio
Contrast-Noise Ratio
CT # Uniformity
Dosimetry
Table 1 - Common Mistakes
For mA row – entering mAs, effective
mAs or mAs/slice
Help site understand difference between
these
And that they are not all equivalent
mA ≠ mAs
mAs ≠ eff. mAs
mAs ≠ mAs/slice

Dosimetry
Siemens – eff. mAs (effective mAs)
mA * rot _ time
Eff . _ mAs =
Pitch
mA =
Eff . _ mAs * Pitch
rot _ time
Philips – mAs/Slice (similar definition to eff.
(mAs / Slice) * Pitch
mAs)
mA * rot _ time
mA =
mAs / slice =
Pitch
rot _ time
Toshiba and GE use mA, time , Pitch as
separate values
Dosimetry
Common Mistakes include:
Reporting mAs or eff. mAs or mAs/slice in Table
1
Then using mAs or eff. mAs when performing
CTDI measurements

Example: 200 eff. mAs, pitch .9, rot. time = 0.5 sec
In this case, mA = 360
Should perform CTDI measurement with 180 mAs
Spreadsheet will use pitch 0.9 and correct for values of
effective mAs

Dosimetry
Common Mistakes include:
If site does this incorrectly, spreadsheet will have
incorrect values
If they perform acquisition with 200 mAs
And then use N,T and I such that a pitch of 0.9 results,
then CTDIvol reported will be too high

If Pitch < 1, CTDIvol reported will be too high
If Pitch > 1, CTDIvol reported will be too low

Dosimetry
Common Mistakes - Reporting collimation incorrectly
Admittedly this can be confusing for some scanners
Example: Siemens Sensation 64

Scanner user interface says 64 x 0.6 mm
Scanner uses z-flying focal spot, which double samples on zaxis of anode to obtain 2X images.
Actual beam width is N=32, T = 0.6 mm
For Pitch 1.0, table travel will be 19.2 mm/rotation
Site sometimes list N=64, T=0.6mm, I= 38.4mm (-> Pitch 1)
In spreadsheet, this yields CTDIvol that is half what it should
be
Dosimetry
Common Mistakes - Reporting collimation
incorrectly

Consult ACR CT Accreditation website for FAQs and
clarifications
http://www.acr.org/accreditation/computed/ct_faq.asp
x
Dosimetry
Dosimetry
Dosimetry
Dosimetry
Dose Levels
Dosimetry
Dosimetry
Dosimetry
Pass/Fail Criteria
Phantom Deficiencies
Toshiba CT Scanners
Aquilion
Scanners (32, 64
or 320)
Slide provided by Erin Angel, Toshiba Medical Systems
Toshiba Aquilion 32 or 64 slice
CT Scanners

The maximum number of axial images able
to be acquired simultaneously in a single
rotation (Nmax) is 4.
The minimum rotation time for a full 360
rotation can be found in the product data
sheet or via loading any helical protocol and
choosing the lowest available 360 rotation
time (note: grayed-out times are not
available).
Toshiba Aquilion 32 or 64 slice
CT Scanners

Slice thicknesses available in helical mode include:
0.5mm, 1.0mm, 2.0mm, 3.0mm, 5.0mm, 7.0mm,
and 10.0mm.
Slice thicknesses available in axial mode include:
0.5mm, 1.0mm, 2.0mm, 3.0mm, 4.0mm, 6.0mm,
8.0mm, 12mm (16mm, 24mm, and 32mm are
also available for non-clinical applications).

Available beam energies* are

80 kVp, 100 kVp, 120 kVp, and 135 kVp
Toshiba Aquilion 16 slice CT
Scanners

The maximum number of axial images able
to be acquired simultaneously in a single
rotation (Nmax) is 4.
The minimum rotation time for a full 360
rotation can be found in the product data
sheet or via loading any helical protocol and
choosing the lowest available 360 rotation
time (note: grayed-out times are not
available).
Toshiba Aquilion 16 slice CT
Scanners

Slice thicknesses available in helical mode include:
0.5mm, 1.0mm, 2.0mm, 3.0mm, 5.0mm, 7.0mm,
and 10.0mm.
Slice thicknesses available in axial mode include:
0.5mm, 1.0mm, 2.0mm, 3.0mm, 4.0mm, 6.0mm,
8.0mm, 12mm (16mm, 24mm, and 32mm are
also available for non-clinical applications).

Available beam energies* are

80 kVp, 100 kVp, 120 kVp, and 135 kVp
Toshiba Scanners - Location of
Technical Parameters
Toshiba Scanners - Scan Detail
Tab
Scan Details for Axial Scanning
Recon Details Tab
IEC Pitch Value and Table
Speed

Always use the IEC definition of pitch
when reporting to ACR

IEC pitch can be calculated either by:
Dividing the helical pitch (HP) by the number of
detector channels being used (N)
Clicking on the HP button on the Scan Details tab to
find the IEC pitch value (ex: PF 0.828). The IEC pitch
value is called “Pitch Factor” on your Aquilion
scanner, also denoted PF.

IEC Pitch Value and Table
Speed
IEC Pitch Values and Table Speed

Detailed Pitch
Row Helical Pitch (HP) IEC Pitch Factor
16
11
0.688
32
21
0.656
64
41
0.641

Standard Pitch
16
15
0.9375
32
27
0.84375
64
53
0.828
Fast PitchRowHelical Pitch (HP)IEC Pitch Factor
(PF)16231.437532451.4062564951.484

Fast Pitch
16
23
1.4375
32
45
1.40625
64
95
1.484
ACR Tips for Toshiba CT’s
Have the scanner calibrated for air and water,
at all relevant kVps, prior to testing
Make sure you scan the ACR phantom with
the small (S) Scan FOV for the image
quality tests. Scanning a small size phantom
with a large size FOV can lead to CT number
and uniformity errors.
For best results be sure the phantom is
properly centered, utilizing the lasers and a
level.
ACR Tips for Toshiba CT’s

Have your “Average Clinical Parameters” chart
filled out in advance and be certain to scan the
phantom with the corresponding parameters.
Before proceeding with phantom image quality
testing, ensure your protocols are optimized for
dose while obtaining adequate image quality and
that radiation doses do not exceed ACR guidelines.
For proper landmarking, note where the center of
the slice will be when you prescribe a scan in axial
or helical mode
Finding the Center Slice Location
Axial Mode: If a scan is prescribed, for example,
from 24mm to 26mm in 4 x 0.5mm axial mode, the
center slice location will be as follows:
Helical Mode: If a scan is prescribed, for example,
from 24mm to 26mm in 4 x 0.5mm helical mode,
the center slice location will be as follows (note:
this is different from axial mode):
Objectives
CONCLUSIONS
For all scanners, make sure that the facility has had
service perform preventive maintenance recently.
Review the protocols used with the technologist prior
to performing phantom scans and dosimetry.
Contact the vendor help desk for further clarification
on a particular scanner.
If at all possible, burn the CD’s for ACR Submission
prior to leaving the facility. Do not depend on staff at
the facility to correctly burn the CD’s.
Contact Information:
Melissa C. Martin, M.S., FACR,
FAAPM
Therapy Physics Inc.
879 West 190th St., Ste 400
Gardena, CA 90248
e-mail:
[email protected]
website: www.TherapyPhysics.com
Phone: 310-612-8127