7/25/2009
Practical Procedures for Achieving
and Maintaining ACR CT Accreditation
• Requirements for physics testing
• Practical aspects of ACR testing CT
systems
• Performance
P f
off pre-application
li ti testing
t ti
and annual surveillance testing
• Establishment of routine quality
assurance protocols and record
keeping
David R. Pickens, PhD
Vanderbilt University Medical
C t
Center
Nashville, Tennessee
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Outline
1
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Image Requirements
ACR Web Page for CT Accreditation
http://www.acr.org/accreditation/computed.aspx
Source for instructions, forms, dose spread
p
sheets, and FAQ
ACR also has people available by phone to answer questions
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Physics Testing
Clinical Testing
• SMPTE
• Phantom Alignment
• CT Number Calibration
and slice thickness
• Low
L
contrast
t
t resolution
l ti
• Uniformity
• High contrast resolution
• CTDI Images of head,
body, pediatric phantom
• Specific images from
representative studies
done using protocols at
the testing location
• Images from studies
performed with the
system being tested
• Specific instruction for
head/neck, chest,
abdomen for adult and
pediatric examinations
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Video Test Pattern for First Box of Film Sheet
Film Output for Submission
Of Phantom Images
SMPTE test pattern or
similar required in first
box of each of two
submitted sheets
If the facility has a film
printer the submitted
printer,
physics images must be on
film in 3 x 4 format, up to 21
images.
No aliasing of bar
patterns or other artifacts
95% square must be
visible (white)
5% square must be
visible (black)
Facilities with no film
printers can submit physics
images on CD (call ACR)
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Equipment Requirements
Equipment Requirements
• Standard phantom
• Calibrated meter
and CT probes
• CTDI phantom sets
• Miscellaneous
equipment
Inclinometer
ACR CT Accreditation Phantom
Level and Lead Ruler
100 mm ionization
chambers
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Phantom Scanning Form
Equipment Requirements
•
•
•
•
Nested phantom set
15.5 cm long
Body: 32 cm
Head: 16 cm (peds body)
Peds Head: 10 cm
Plus plastic pins to fill holes
Accepts 100 mm ionization
chamber
•
•
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ACR Table 1 For a 16 Slice System Used for Adults
kVp
mA
Brain W/out
Routine
Abd Renal Mass
(Adult head)
(HR Chest)
(Adult Abdomen)
120
120
120
380 ma
133.3 ma
433.3 ma
(570 mAs)
(100mAs)
(325 mAs)
Rotation time (s)
1.5
0.75
0.75
Scan FOV
50
50
50
Display FOV (cm)
22
35
35
Recon Algorithm
UC
L
B
Axial or Helical
Z-axis collimation
A
A
H
4.5
0.6
1.5
(T, in mm)
# data channels used
4
2
16
18
1.2
22.5
(N)
Axial table increment mm
or
Hel table speed mm/rot (I)
N/A
N/A
0.938
Recon scan width (mm)
4.5
1.2
3
Recon scan interval (mm)
4.5
10
3
Pitch
8
Read the instructions
Be sure you understand what the site uses for standard acquisitions
Be sure you know what patient types are done
Be sure you understand the characteristics of the system and how
used for each study
– Detector configuration
– Number of data channels ((N)) used
– Maximum number of data channels (Nmax)
– Z-axis collimation (T)
– Table increment (I) -- axial or for each tube rotation in helical
Fill out Table 1 of the ACR Phantom Scanning Forms
FOV appropriate for the size of the ACR phantom is permitted. Not so
with the dose phantoms: must use FOV for the protocol being tested
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ACR Phantom
• Four sections or modules
24 mm asymmetrical detector
(4 x 1.5, 16 x 0.75, 4 x 1.5)
Allows various combinations
32 detectors, 16 data channels
– Module 1: Alignment, CT #, and slice width
– Module 2: Low contrast resolution
– Module 3: Uniformity and noise, distance accuracy,
and slice sensitivity profile (SSP)
– Module 4: High contrast resolution
Pitch = Table speed/N*T
In mm/rotation
1. Must be filled out correctly
2. Will use this to do scans
3. Will do either axial or helical
scans depending on your
protocols
4. Important to use axial scans
for module 1
5. Modules 2-4 done either axial
or helical according to your
protocol
6. Dose phantoms must be done
axial
• 20 cm diameter,, 4 cm spacing
p
g between modules,,
total length 16 cm
• Alignment beads located in center of module 1 and
4 (1 mm steel)
• Markings at center of each section plus marking
for head, foot, top
• Phantom holder to aid in positioning
No dose reduction used for phantom imaging
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Module 1: Alignment, Slice
Thickness, HU Values
Phantom Support
and Positioning
• Phantom alignment
– This must be correct
– Beads (1 mm) must be seen uniformly or phantom will fail
– Longer bar must be located in center of slice measurement
section on 2 mm slice. This is more sensitive to alignment
than beads
• Water value must be less than or equal to +// 7 HU (+/( / 5)
using abdominal protocol and ROI of ~200 mm2
• Images required for water values at all kVps system uses
• Polyethylene (-107 to-87 HU), acrylic (110 to 130 HU),
bone (850 to 970 HU), air (-1005 to 970 HU)
• Slice thickness determined to nearest 0.5 mm by
counting visible bars over approximate 3,5,and 7 mm
slice thicknesses. Use next larger value system will do
Nylon standoffs
can cause artifacts;
Position carefully
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Positioning of ACR Phantom
Notes on Phantom Alignment
• Centering of slices depends on how system
specifies start
– Our 16 slice system specifies at center of array
– Our 64 slice system specifies at beginning of
array
• External lasers can be out of alignment a
significant amount (one of ours ~1 cm)
• Inner laser usually properly aligned
• For axials from a helical protocol, be sure that
the image of interest will fall in the center of
the array to avoid cone beam artifacts (40 mm
arrays and wider)
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Proper Alignment of the ACR Phantom
Phantom Alignment
Careful alignment with a level
assumes that the external
lasers are properly aligned and
the gantry is properly leveled.
Alignment without cradle is often very difficult
Window width = 1000, Window level = 0
Hi resolution chest protocol
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Module 1 Alignment Problem
Module 2: Low Contrast Detestability
•
Alignment incorrect:
1. BBs not shown at all
four locations
2. Long bar on slice
thickness ramp not
properly visible on
t off iimage
top
•
Adult abdomen protocol
•
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Low contrast performance
– Large rod 25 mm
– 100 mm2 ROI over 25 mm rod and next to it
– Series of rods of 6 mm, 5 mm, 4 mm, 3 mm and 2 mm
– For adult head and adult abdomen protocols, must see clearly all 4
rods 6 mm diameter at window width of 100 and widow level of 100
This section interacts with dose measurements
– Must be able to see rods
– Must keep dose below ACR recommendations
– Must keep dose as low as possible
– Measurements made without system dose reduction turned on
For systems with established protocols, may be more efficient during
testing to use specified protocol that is known to be below the ACR
dose requirements to make sure the rods are visible.
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Low Contrast Section of ACR Phantom
Pediatric Abdomen Protocol
Low Contrast Section of ACR Phantom
345 mAs
~23 mGy
Adult Head Protocol: 120 kVp, 540 mAs, 4.5 mm
WW = 100, WL =100
300 mAs
~20 mGy
200 mAs
~13.3 mGy
At the required window settings of 100, 100:
ACR requirements not to exceed 25 mGy with
all four 6 mm rods visible
How you view the image can make a difference:
Viewing distance, size on monitor, room lighting
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Module 3 Uniformity and Noise Measurements
Module 3: Uniformity
• Uniform section of water-equivalent material
• Two small bbs spaced at 100 mm
Each ROI about 400 mm2
WW = 100, WL = 0
– Distance measurements
– SSP
Edge to edge variation no
more than 5 HU
• Use for uniformity testing
– Edge and section HU differences of > 5 HU
– ROIs must be placed properly and be of proper size
for this
– Center number using adult abdomen protocol must be
-7 to 7 HU or better (+/- 5 HU better)
– Image must not show artifacts with ww = 100 and wl =
0.
Center should be 0 +/- 5
HU
• Adult abdomen protocol
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An Example of a Problem Phantom Scan
Four Slice Scanner
Abdomen protocol
Problems
HU high for water
HU not uniform in field
Low contrast artifact
Module 4: High Contrast Resolution
• Use with two protocols
– Adult abdomen
– High resolution chest
• For abdomen, must see 5 lp/cm bars
• For HRC, must see 6 lp/cm bars
• Window width = 100, window level ~ 1100;
there is some flexibility with setting the level
• Note that the four beads must be
visible for proper alignment
Cause – temperature
variations in room
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Highest resolution for
system
7-8 lp/cm visible here
2 mm slice thickness
Alignment correct
No significant artifacts
7 lp/cm
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ACR Phantom Notes
High resolution Chest Protocol, Module 4
4 lp/cm
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ACR requires 5 lp/cm
for adult abdomen, 6
lp/cm for adult HRC
• Machine should be calibrated before you get
there; doesn’t always happen
• If you get low contrast, wide, ring artifacts,
y help
an air cal may
• If there are calibration or artifact issues,
check for contrast or other material on x-ray
transparent window of system
Acquired as axial or
helical, depending on
your site protocol
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ACR CT Dose Requirements
• Adult head CTDIvol
– Pass/fail -- 80 mGy
– Reference level – 75 mGy
• Adult abdomen CTDIvol
– Pass/fail – 30 mGy
– Reference level – 25 mGy
• Pediatric abdomen CTDIvol
– Pass/fail – 25 mGy
– Reference level – 20 mGy
Fill in the Forms and Check as You Go
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Phantom in Position for Head Dosimetry
Dose Measurements (CTDI)
• All require axial scans of dose phantoms
• All require use of adult head, adult abdomen,
pediatric abdomen protocols (if performed)
• Head dose phantom in head holder
• Body dose phantom on bare table top
• Pediatric abdomen – use head phantom on table
top
• For helical protocols, must use axial scan with the
detector configuration of the helical protocol (kVp,
mA, time, N, T)
• Use spread sheets provided by ACR to report
CTDIvol (exposure or air kerma, depending on
meter)
Note positioning in head holder and tape to anchor chamber
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Adult Body Configuration for Dose Measurements
Make Sure the Dose Phantom has all pins in place
Can lead to an error in dose
Likely to cause a fail of test
Axial Slice at center of phantom
Three measurements with 100 mm probe at both 12:00
and 3:00
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Annual Testing
System Tests
ACR Dose Phantom Measurements
• If your dose measurements vary when
doing the three averages, check to see if
the system is doing a 420 degree over
scan rather than a 360 degree scan
• Be
B sure FOV is
i sett properly
l for
f the
th dose
d
scan and phantom centered properly
• May be a good idea to check dose first so
that you know your protocol does not
exceed ACR dose limits
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•
•
•
•
•
•
Alignment light accuracy
Alignment of table to gantry
Table/gantry tilt
Table increment accuracy
Slice thickness
Image quality
– High contrast resolution
– Low contrast resolution
– Image uniformity
– Noise
– Artifact evaluation
• CT number accuracy and linearity
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Annual Testing
Other Tests
Laser Light – Bed Positioning Test
• Display devices
– Monitors
– Film-printers
• Dosimetry
– CTDI measurements
– Patient radiation doses for representative exams
– Many newer systems estimate CTDIvol and DLP
for exams
• Visual safety inspection
• State and local inspection and testing as required
• ACR phantom or other suitable phantom can be
used
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Ruler with lead scale
Can check bed index
Can check scout position
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Recommended Quality Control
Verification of Tilt Readout Position and Repeatability
•
•
•
•
Established by medical physicist
Performed by technologist on regular basis
Frequency based on facility and usage
Image quality
–
–
–
–
•
•
•
•
High and low contrast resolution
Image uniformity
N i
Noise
Artifact evaluation
Alignment light accuracy
Slice thickness
CT number accuracy
Display and output devices
This device gives a check, but is not very accurate
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Technologist Quality Assurance Form
Manufacturer-Provided Water Phantom
CT Daily Quality Assurance Data Sheet
Room # __________________________
12:00 ---- 5.1 HU, SD=14.2
3:00 ---- 2.7 HU, SD=14.2
Center -- -1.2 HU, SD=17.8
WW=100, WL=0, ROI=400 mm2
___/___/_____ to ___/___/_____
Date
Technologist Initials
Visual Inspection of Equipment
Scan Type Head/Body
Artifacts? (Y/N)
Action
Taken?
(Y,N. Log below)
Center H.U.
Standard
deviation
12 o'clock H.U.
3 o'clock H.U.
6 o'clock H.U.
16 slice scanner, lung
screening protocol (120 kVp,
25 mAs, 2 mm)
9 o'clock H.U.
Date
Action
Requires scanning water phantom daily at standard acquisition
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Summary
• Qualified medical physicist must be present for annual survey
and ACR testing when dose measurements are made
• Read the materials from ACR including FAQ
• Use the ACR Phantom Site Scanning Data Form
• Understand the operating modes and detector configuration of
the scanner
• Be sure Table 1 properly reflects the standard protocols for the
site
• Remember that clinical images with the site protocols will also
be submitted
• For dosimetry, be sure to use the appropriate spread sheet for
the dose measurements
• Be sure the system is calibrated and the periodic QC program
is ongoing
• Be prepared to perform annual surveillance; does not need to
be submitted to ACR but needs to be available for inspection
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