Download Evolution of CT scanners

Assessment of Effective Dose in
Computed Tomography using an
Anthropomorphic Phantom
Paul Collins
Supervised by
Brendan Tuohy
5th September 2005
Paul Collins
Computed Tomography Dosimetry
Overview
• Motivation and Objectives
• Methodology
• Organ Location in Phantom and Measurement
• Patient ‘Effective Dose’ Calculation
• Results
• Conclusions
5th 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
5th 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
5th September 2005
Paul Collins
Computed Tomography Dosimetry
Why MSCT
• Benefits
• Disadvantages
– Near Isotropic
– Fast Imaging
– Increase Data
• .33sec Rotation Times
– High Quality Images
– Thinner Slices
– Large Volume
Acquisition
• Staff Workload
• Data Storage
– Increase in Patient
Dose
• Technology Changes
• Changes in practise
0.6mm Slice thickness
6sec scan time
Siemens 64 Slice Scanner, 2004
5th 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
5th 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
– mA then has to be increased to provide useful
diagnostic images
– linear relationship between mA and patient dose
5th 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)
5th 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 ImageJ
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
5th 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
–3D 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
5th September 2005
Paul Collins
Computed Tomography Dosimetry
Slice 22 of phantom
Human Slice Server Screen Capture
5th September 2005
Paul Collins
Computed Tomography Dosimetry
Organ Selection
• Organs selected to
allow for effective
dose measurement
– ICRP 60
Measured
Organs/Tissues
Gonads
Bone Marrow (Red)
Colon
• Organs were located
using previous
method
• At least two TLDs
placed within each
organ e.g.
– #2 TLDs in gonads
– #6 TLDs in lung
Lung
Stomach
Bladder
Liver
Oesophagus
Thyroid
Skin
Bone Surface
Brain
Kidney
Eye Lens
5th September 2005
Paul Collins
Computed Tomography Dosimetry
Effective Dose
• Effective Dose Calculation
– Sum of weighted equivalent
doses in tissues and organs
E   wT  H T
T
E:
Effective Dose (Sievert)
WT: Tissue weighting Factor
HT :
Equivalent Dose (Sievert)
T:
Tissue/Organ
HT = Absorbed Dose x Radiation Factor
Tissue or organ
Gonads
0.2
Bone Marrow (Red)
0.12
Colon
0.12
Lung
0.12
Stomach
0.12
Bladder
0.05
Breast
0.05
Liver
0.05
Oesophagus
0.05
Thyroid
0.05
Skin
0.01
Bone Surface
0.01
Remainder
0.05
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
5th September 2005
Tissue Weighting Factor wT
Paul Collins
Computed Tomography Dosimetry
Protocols
• Standard Imaging
Protocols
–
–
–
–
Abdomen/Pelvis
Head
Chest
RT protocols
(Radiotherapy)
5th 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
kV
Effective mAs
Slice thickness (mm)
Scan time
No. of slices
Scan length (mm)
Pitch
Collimation
Tube Rotation (s)
2 Slice
6 Slice
16 Slice
130
63
8
17.09
51
408
2.5
2x4mm
.8
130
95
5
17.35
82
410
1.25
6x2mm
.6
120
200
5
11.61
81
405
1.2
16x1.5mm
.75
 Effective mAs
Effective mAs = mAs/pitch
5th September 2005
 Slice thickness
 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
5th September 2005
2 Slice
Tissue or organ
Absorbed
Dose
[mGy]
Gonads
Bone Marrow (Red)
Colon
Lung
Stomach
Bladder
Liver
Oesophagus
Thyroid
Skin
Bone Surface
Brain
Kidneys
Eye Lens -Left
-Right
Paul Collins
Computed Tomography Dosimetry
8.27
6.22
7.66
4.93
5.67
4.91
6.69
3.31
0.15
7.89
6.22
0.02
5.45
0.04
0.04
6 Slice
16 Slice
Absorbed Absorbed
Dose
Dose
[mGy]
[mGy]
11.08
11.52
12.26
1.21
8.98
8.47
8.96
7.97
0.07
12.69
11.52
0.01
11.52
0.06
0.05
13.02
15.49
14.65
3.97
12.38
10.82
10.55
11.42
0.39
15.39
15.49
0.06
16.95
0.07
0.06
Abdomen/Pelvis Protocol
Effective Dose [mSv]
Effective Dose
12
10
8
6
4
2
0
2 Slice
6 Slice
16 Slice
Scanner Slice Acquisition
Effective Dose [mSv]
% Increase Relative to 2 Slice
5th September 2005
2 Slice
5.20
0.00
Paul Collins
Computed Tomography Dosimetry
6 Slice
7.97
53.47
16 Slice
10.40
100.09
Other Protocols
Head Protocol
2 Slice
1.6131
Effective Dose [mSv]
16 Slice
2.7091
• Axial imaging
• Skin dose – 32.80mGy
• mAs 260mAs to 350mAs
• Brain dose – 30.52mGy
Radiotherapy Protocols
Effective Dose [mSv]
• Axial imaging
5th September 2005
Head Protocol
1.7093
Chest Protocol
6.2943
Abdomen Protocol
5.9474
• 3mm Slices
Paul Collins
Computed Tomography Dosimetry
Radiation Risks
• Risks from Effective dose (ICRP 60)
• 1 mSv equates to a cancer risk of 1 in 20000
Effective Dose [mSv]
% Increase Relative to 2 Slice
2 Slice
5.20
0.00
6 Slice
7.97
53.47
Abdomen/Pelvis Protocol
• 1 in 4000 risk for 5mSv
• 1 in 2500 risk for 8mSv
• 1 in 2000 risk for 10mSv
5th September 2005
Paul Collins
Computed Tomography Dosimetry
16 Slice
10.40
100.09
Conclusions
• Dose is increasing
• 64 slice scanner UCHG
–  0.33sec rotation
 0.6mm slice thickness
• Effective dose of 64 Slice scanner?
5th 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?
5th September 2005
Paul Collins
Computed Tomography Dosimetry
Questions?
Toshiba 256 detector 4D-CT scanner
5th September 2005
Paul Collins
Computed Tomography Dosimetry