Assessment of Effective Dose in Computed Tomography using

Assessment of Effective Dose in Computed Tomography using an Anthropomorphic Phantom Paul Collins Supervised by Brendan Tuohy 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Overview • Motivation and Objectives • Methodology • Organ Location in Phantom and Measurement • Patient ‘Effective Dose’ Calculation • Results • Conclusions 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Motivation Investigate the rise of patient dose in CT • Indications are that patient dose is rising in CT • Due to evolution of CT technology and subsequent changes in practise • Conventional CT has now evolved to Multi-Slice CT (MSCT) which has to potential to vastly increase dose Objectives • Evaluate changes in patient dose due to advancement of MSCT scanners 5 th September 2005 Paul Collins Computed Tomography Dosimetry

What is MSCT Siemens, 2004 SSCT MSCT (Single Slice CT) (Multi-Slice CT) • Evolved from development of the Detector Array • 64 Slice CT : 1 rotation = up to 64 Slice acquisition 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Why MSCT • Benefits • Disadvantages – Near Isotropic – Fast Imaging – Increase Data • . 33 sec Rotation Times – High Quality Images – Thinner Slices – Large Volume Acquisition • Staff Workload • Data Storage – Increase in Patient Dose • Technology Changes • Changes in practise 0. 6 mm Slice thickness 6 sec scan time Siemens 64 Slice Scanner, 2004 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Why Patient Dose is increasing • Technology Changes – Extra volume scanned – Extra helical rotations – Interpolation for axial reconstruction – Z-axis over beaming • Protocol Changes –May be tendency to image more volume –Thinner Slices –Higher quality – Penumbra region not utilised by detectors 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Why slices thickness affects patient dose? – Image noise is random fluctuations of pixels values – Finite number of x-ray photons are transmitted (i. e. in a slice) – Thinner slices Less x-ray photons larger variation in pixel values more noise – m. A then has to be increased to provide useful diagnostic images – linear relationship between m. A and patient dose 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Evaluation of patient dose 1. Randoman aka ‘Séamus’ – Tissue Equivalent Humanoid Phantom – Verified for measurement of absorbed dose in CT – 35 axial slices – Plugs to hold TLDs 2. Diagnostic TLDs – Measure Absorbed dose to Organs – ~45 TLD were placed at specific organ locations for each protocol – Organ selection and patient dose calculation guided by ICRP publication 60 (International Commission on Radiation Protection) 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Organ Location • TLDs needed to be placed accurately within phantom to measure organ dose • Methodology – Whole Body CT of Phantom – Calibrated Image using Image. J software tool • Any point to point distance know – Labelled Vertebral Column • Could now relate each slice of phantom to specific vertebra – Human Slice Server used to locate organs according to vertebral column 5 th September 2005 Human Slice Server Sagittal view Paul Collins Computed Tomography Dosimetry Whole Body Phantom CT

Organ Location • Human Slice Server –virtual reconstruction of human anatomy in any orientation or location – 3 D datasets from a human body Human Slice Image ( T-11 ) • frozen & digitised into 1 mm slices –Labelled images of organs/tissues/vertebrae etc. –Organs were then located in phantom using the vertebrae as a guide 5 th September 2005 Paul Collins Computed Tomography Dosimetry Slice 22 of phantom

Human Slice Server Screen Capture 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Organ Selection • Organs selected to allow for effective dose measurement – ICRP 60 • Organs were located using previous method • At least two TLDs placed within each organ e. g. – #2 TLDs in gonads – #6 TLDs in lung 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Effective Dose • Effective Dose Calculation – Sum of weighted equivalent doses in tissues and organs E: Effective Dose (Sievert) WT: Tissue weighting Factor HT : Equivalent Dose (Sievert) T: Tissue/Organ HT = Absorbed Dose x Radiation Factor The remainder is composed of the following tissues and organs: adrenal, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus. Radiation factor for photons 1 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Protocols • Standard Imaging Protocols – – Abdomen/Pelvis Head Chest RT protocols (Radiotherapy) 5 th September 2005 • CT Scanners – Philips ACQSim (RT scanner) – Siemens Sensation Emotion Duo – Siemens Sensation Emotion 6 – Philips Brilliance 16 Paul Collins Computed Tomography Dosimetry

Abdomen/Pelvis Protocol Effective m. As = m. As/pitch 5 th September 2005 ß Slice thickness ß Scan Time Paul Collins Computed Tomography Dosimetry

Organ Absorbed Dose • Increases in average dose to organs located in primary radiation beam • Highest absorbed doses to – Skin – Bone Marrow – Colon – Gonads – Oesophagus 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Abdomen/Pelvis Protocol 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Other Protocols Head Protocol • Axial imaging • Skin dose – 32. 80 m. Gy • m. As 260 m. As to 350 m. As • Brain dose – 30. 52 m. Gy Radiotherapy Protocols • Axial imaging 5 th September 2005 • 3 mm Slices Paul Collins Computed Tomography Dosimetry

Radiation Risks • Risks from Effective dose (ICRP 60) • 1 m. Sv equates to a cancer risk of 1 in 20000 Abdomen/Pelvis Protocol • 1 in 4000 risk for 5 m. Sv • 1 in 2500 risk for 8 m. Sv • 1 in 2000 risk for 10 m. Sv 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Conclusions • Dose is increasing • 64 slice scanner UCHG – 0. 33 sec rotation 0. 6 mm slice thickness • Effective dose of 64 Slice scanner? 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Conclusions Image Quality Patient Dose • Patient dose generally increases with better image quality • Diagnosis is the goal of CT • Is patient diagnosis improving with increasing patient dose? • Does greater image quality result in better diagnosis? • ALARA principle –Does diagnosis improve from 2 Slice to 6 Slice to 16 Slice? 5 th September 2005 Paul Collins Computed Tomography Dosimetry

Questions? Toshiba 256 detector 4 D-CT scanner 5 th September 2005 Paul Collins Computed Tomography Dosimetry