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CT, Simulators, and CT Simulation

Residents' Meeting


AAPM TG-66 Report

AAPM TG-2 Report #39

AAPM TG-53 Report

NCRP Report 99



X-ray tube Detector / film cassette holder: capable of fluoroscopic imaging Gantry: allows replication of linac geometry Collimator


Set isocenter and creation of patient positioning devices Create films for verification of patient position Set field boundaries

CT Scanners

3rd generation: tube and arc of detectors that rotate 4th generation: tube and full ring of detectors (only tube rotates) Helical acquisition: rotate and translate simultaneously Multi-slice: increase axial length of detector array to image more than one slice at a time

LW Goldman, J Nucl Med Tech, 35(3):115128, 2007

Virtual / CT Simulation

Use CT images to create DRR's to replicate films from simulators. Differences from a conventional CT scanner:

External lasers for patient marking Flat table (to reproduce treatment position) Virtual simulation software

Other differences

May require more robust x-ray tube Large bore may be needed

Multi-slice CT

Use x-ray tube output more efficiently Acquire data faster than single slice scanners and allows thinner slices Number of slices not equal to the actual number of detectors Consider axial length of detectors as well

M Prokop, General Principles of MDCT, Eur. J. Radiology, 2003, 45(S1): S4-S10. AAPM Summer School 2006

300 250 200 Slices 150 100 50 0 1970


1990 Year




Tests for CT

Mechanical alignment X-ray output Safety Image quality Virtual simulation software (may be part of treatment planning system)

Tests for CT


Acceptance / Commissioning Daily Monthly Annually Other

Mechanical Alignment


Level and orthogonal to imaging plane Position indicators should be accurate Table movement should be accurate Minimal couch sag Should not produce imaging artifacts

Mechanical Alignment


Properly aligned with center slice and center Sagittal lasers orthogonal to image plane

Gantry tilt: should be within 1° of the nominal imaging plane Scout image should be accurate Collimation: slice thickness should be correct

X-ray output

kV and half value layer mA-s linearity and reproducibility Scan time accuracy


X-ray dose

Measured in terms of CT dose index (CTDI)

T is the slice thickness, n is the number of slices/scan

CTDI100 is the same as CTDIFDA but integrated over 100 mm. Can be measured with ion chamber. DLP = dose-length product



Emergency stop buttons Door interlocks not recommended Shielding


Primary radiation beam is stopped by detectors Shielding to account for scatter distribution Survey or use exposure data from manufacturer

Image Quality

Uniformity Noise CT number calibration Spatial resolution Contrast resolution Spatial distortions Check that extended FOV does not introduce errors


Scan water phantom for uniformity and noise. Phantoms should be imaged both at center and edge of FOV CT number calibration phantom Image phantom of known dimensions and check for accuracy of image Test influence of reconstruction settings on images

Imaging Tests

Image resolution test pattern Determine MTF using a foil, wire, or edge Contrast resolution

Virtual Simulation Software

Integrity of data transfer from CT scanner

Check orientation of data Perform checks for all patient orientations (e.g., head first supine, feet first prone, etc.) Check that isocenter shifts are correct

Simulate treatment machines correctly

Collimator Gantry rotation Other TG-53 requirements

Virtual Simulation Process

Patient immobilization Imaging protocols

Contrast Scan range Scan protocol

Reference point marked Data output (e.g., DRRs)

TG-66 Recommended Daily QA

Lasers are aligned with center of imaging plane CT number for water In plane spatial integrity Image noise

TG-66 Recommended Monthly QA

Laser alignment WRT to imaging plane Table orientation and motion Image uniformity (at most commonly used kVp) CT number accuracy (4-5 materials)

TG-66 Recommended Annual QA

Gantry tilt Table indexing and positioning Scan localization (scout image accuracy) Radiation profile or dose Electron density / CT # calibration Spatial resolution Contrast resolution

Summary and Discussion

AAPM TG-2 and TG-66 reports cover CT and CT simulation Recommend extensive testing of CT scanners What is necessary and feasible?


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