Quality control in cardiac nuclear medicine and cardiac CT

Klaus BacherGhent University

Department of Basic Medical SciencesDivision of Medical Physics and Radiation Protection

IMIC 2013, IAEA, Vienna

Quality control in medical imaging

• Medical imaging instrumentation: increasing complexity (e.g. hybrid systems)

• Quality in medical imaging relies on proper function and calibration of the instrumentation

• When using ionizing radiation, balancing image quality and patient radiation dose is very important

Need of objective and standardized procedures to evaluate the performance of the instrumentation

Quality control in cardiac nuclear medicine and cardiac CTKlaus Bacher

Quality management and quality control (IAEA)Quality control in medical imaging

• Medical imaging instrumentation = hardware

AND software

Checking the correct functioning of analysis tools is needed (also after updates/upgrades!)

• Key person = medical physics expert • At least once a year a detailed quality control of the

instrumentation by the medical physics expert • More frequent tests to follow up calibration/stability of

equipment: mostly delegated to technologists• Analysis of image quality and software performance: help of

physician is needed

Close collaboration between medical physics expert and physician and technologist is needed!

How?Quality control in medical imaging

• Nuclear medicine: Activity measurement chamber Gamma camera, SPECT PET

• X-ray imaging: Cone-beam CT or CT

Specific co-registration test

Activity measurement chamber

• “Dose calibrator”• Detection of ionisation of

radiopharmaceutical in well geometry by means of an ionization chamber or scintillation detector

• Accurate activity measurement important for radiation dose

• Accuracy of the build-in clock, if applicable• Battery voltage• Detection of contaminations: background activity < 100kBq• Drift of zero adjustment of activity• Measurement of stability with reference source (e.g. 57Co,

137Cs,…): < 5%

Quality control – daily checks

• Analysis of energy response Measurement of reference source (e.g. 57Co) Using all isotope selections Check calibration factors of different isotopes over time <5%

Quality control – energy response

• Analysis of linearity Measurement of different levels of activities of 99mTc or 18F,

starting with high activity Measurements at different time points (decay) Response should be linear, within 5%

Quality control – linearity

• Analysis of reproducibility Perform 1O measurements of reference and calculate standard

deviation Reproducibility <1%

Quality control – reproducibility

• Analysis of geometry/volume dependence Dilution of activity of 99mTc from 0.5 to 5ml and register

readings Variation in readings should be <2%

Quality control – geometry dependence

Gamma camera

• Detection of counts• Reconstruction into image format• Stability of instrumentation may be an issue

Gamma cameraPerformance parameters

• Daily check by NM imager (position of the peak)

Gamma cameraQuality control – energy peak accuracy

• Yearly by MPE (energy resolution) <15%

Gamma cameraQuality control – energy resolution

Gamma cameraQuality control - homogeneity

• Non-uniformity = problems with calibration and/or PMT

• Using reference flat source (extrinsic, with collimators)

• Using point source at distance > 5 x FOV (intrinsic)

• More than 2500 counts/pixelQuality control in cardiac nuclear medicine and cardiac CT

Klaus Bacher

• Build-in software• Uniformity <7%

Gamma cameraQuality control - resolution

Gamma cameraQuality control – spatial distortion

Gamma cameraQuality control - sensitivity

• Deviation between detectors < 10%

Gamma cameraQuality control – system alignment

• 360° acquisition of point source

• Test if mechanical and electronical rotation point is the same

• 2-monthly by the NM imager

• Yearly by MPE• Build-in software

Gamma cameraQuality control - COR

• Build-in software• <1 pixel

Gamma cameraQuality control - COR

• Build-in software• <1 pixel

Gamma camera

• SPECT uniformity• SPECT resolution < 15mm

Quality control – other tests for SPECT

• Coincidence measurement

• Energy window (daily by NM imager)• Sinogram test (daily by NM imager)• PET uniformity (4-monthly by NM imager)• SUV reproducibility (4-monthly by NM imager)

• Build-in software

Quality control

PET-SPECT/CT

• Hybrid imaging systems: increasing importance• Combining anatomical and functional information

CT hardware

• PET in combination with ‘conventional’ multi-slice CT• SPECT in combination with:

‘conventional’ multi-slice CT GE, Siemens, (Philips)

‘low-dose’ CT systems GE (Hawkeye), Philips (BrightView XCT)

PET-SPECT/CT

• Co-registration QC• CT quality control issues• CT patient dose issues

Quality control

• As CT doses are high, CT procedures should be optimized taking into account the required image quality: In-depth study of image quality and dose parameters (at

acceptance/yearly) by a medical physicist Daily/Weekly/Monthly checks by medical imaging staff

To detect minor changes in image quality performance before clinical performance erodes

PET-SPECT/CTQuality control

• Daily/weekly/monthly checks are mostly automated• Most of them are performed at start-up of system:

X-ray tube warm-up and sometimes automated measurement of some X-ray tube characteristics

Image quality tests using a manufacturer-specific phantom

• Exact measurement methods depend on the manufacturer

Philips Medical

PET-SPECT/CTQuality control

CT image quality

• In most situations following parameters are measured CT number of water in ROI in water phantom Noise level: standard deviation of CT numbers in ROI

Routine image quality tests

CT image quality

• In most situations following parameters are measured CT number of water in ROI in water phantom Noise level: standard deviation of CT numbers in ROI CT number uniformity: measurement of CT numbers in 5 ROIs

CT image quality

• Sometimes following parameters are also measured High-contrast resolution (visual inspection or calculation)

CT image quality

• Sometimes following parameters are also measured High-contrast resolution (visual inspection or calculation) Low-contrast performance (visual inspection or calculation)

• CBCT systems do have an inferior low-contrast detectability compared to conventional CT

Image quality in function of CT system

CT CTDIvol = 6.5 mGyCBCT CTDIvol = 6.5 mGy

CT image quality

CTDIvol

• CTDIvol takes into account effect of helical pitch• CTDIvol measured within

• 16cm PMMA phantom (adult head/pediatric scans)• 32cm PMMA phantoms (adult body)

pCTDICTDI w

Measuring CT dose

CT centre

CT periphery

pcw CTDICTDICTDI ,100,100 32

• In CT, CTDIvol is considered as being the “mean radiation dose within one CT slice”

• In chest region, CTDIvol is measured in a 32 cm PMMA phantom• CTDIvol will significantly underestimate the mean radiation dose

in a reference person Example:

CTDIvol=5.2mGy Mean simulated dose reference person=11.4mGy

Interpretation of dose quantities

Measuring CT dose

• Effective dose is often used risk-related dose-indicator • Sometimes indicated on CT systems• Methodological errors are being made when calculating the

effective dose: ICRP 60 vs ICRP 103 Effective dose conversion factors:

Example: in cardiac CT a factor of 0.017 mSv/mGycm is often used, whereas a factor of 0.024-0.027 mSv/mGycm is more appropriate

Interpretation of dose quantitiesMeasuring CT dose

• Quality control is needed within quality assurance program• Quality control ≠ a single yearly test• Quality control needs multi-disciplinary approach

Quality control in cardiac nuclear medicine and cardiac CT · 2016. 8. 9. · Quality control in...

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