Implementation of an Audit and Dose Reduction Program for CT

Y. Matyagin and P.J. Collins

Department of Nuclear Medicine, PET and Bone Densitometry

Royal Adelaide Hospital

CT exposure

Increasing concern worldwide regarding the relatively high level of CT radiation exposure

National Council on Radiation Protection and Measurements: US average dose from medical procedures increased 6x between 1980 and 2007

60% due to CT

F.A Mettler Jr., et al. Radiology 2009 Nov;253(2):520-31.

CT exposure

CT now constitutes:

15% of total number of radiological imaging procedures

50% of population’s medical radiation exposure

25% of total radiation exposure

F.A. Mettler Jr., et al Health Phys. 95(5), 502-507 (2008).

Radiation induced health risk

A 2009 study estimates (using LNT model) 29,000 cases of cancer could be due to CT scans that were performed in USA during 2007

There is potential for significant dose reduction in many studies as part of an image/dose optimization process

B.A. de Gonzales, et al. Arch Intern Med 2009 Dec 14;169(22):2071-7.

Spiral CTCT machine

Motorized table

Rotating X-ray detectors

X-ray beam

Rotating X-ray source

Weighted Computed Tomography Dose Index (CTDIw )

http://www.impactscan.org/slides/impactcourse/principles%20of%20ct%20dosimetry/index.html

• Weighted CTDI represents the average dose in scan plane of Perspex phantom

• CTDIW = 1/3 CTDICentre + 2/3 CTDIPeriphery

• Phantom diameter: 32 cm or 16 cm

• Units: mGy

Volume CTDI (CTDIvol )

CTDIvol : average dose over scanned volume

CTDIvol : accounts for non-contiguous exposure along z-axis

CTDIvol = CTDIw / pitch

Pitch 1.5

Pitch 1.0

Pitch 0.75

Dose-length product (DLP)

Total dose: CTDIvol integrated along the scan length, ie.

DLP = CTDIvol • L (where L = scan length)

Units: mGy • cm

Different tissues of the body have different radiosensitivity

Unit of Effective dose (E) is the Sievert

where wT = weighting factor for organ, or tissue T

HT = equivalent dose to organ/tissue

Effective dose

E w HT TT

Automatic Exposure Control (AEC)

Aim

Compare CT doses (adults) at RAH with an International benchmark

Modify protocol settings to optimize dose/image quality

Dose reference levels

RANZCR have adopted EC1999 values

European Guidelines (EC 2004) were used in this study

Address multi-slice scanners

Includes data from 8 countries

EC 2004. G. Bongartz, et al. European Guidelines for Multislice Computed Tomography. FIGM-CT2000-20078-CT-TIP. March 2004

Methods

CT configured to display relevant dose information (CTDIvol, DLP etc.) in PACS

Baseline audit performed

CT scanner exposure settings were reduced in step-wise fashion so as not to compromise image quality

ResultsCT Doses (% of EC2004)

Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

Brain perfusion

Major International concern regarding brain perfusion CT

Cedars-Sinai Medical Center in USA:

excessive CT doses in 250 patients

Some studies 8 times expected level

FDA reported significant CT overexposure at several other medical centers in 2009

Brain perfusion overdose

Hair loss in patients who received radiation overdoses

W. Bogdanich, New York Times, July 31, 2010

DiscussionCT Doses (% of EC2004)

Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

CT Doses (% of EC2004)Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

Discussion

CT Doses (% of EC2004)Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

Discussion

CT Doses (% of EC2004)Siemens "Somatom Definition AS+" and Toshiba "Aquilion 16"

Discussion

Dose parameters - PACS

Store in separate dose information page:

CTDIvol

DLP

Dose/image quality reference settings

Scan time

Patient’s size information

Toshiba dose page 1

Toshiba dose page 2

Siemens dose page

Automatic exposure control (AEC)

AEC switched on

Dose references level set

Effect of other CT parameters on functioning of AEC evaluated

Min and max tube current

Tube voltage

Beam collimation and reconstructed slice thickness

Image reconstruction filters and algorithms

Large/small patients

A separate protocol should be employed:

Higher tube voltages for large and (possibly) lower tube voltages for small patients

Rotation time and pitch adjusted to ensure tube current is within acceptable limits

Somewhat higher image noise is acceptable for large patients

Future work

More detailed analysis of image quality

eg. using phantoms

Investigate other key parameters

Slice thickness

Inter-slice intervals

Streak artefacts

Acknowledgement

Radiology Department

George Kourlis (CT senior)