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Radiation dose modulation and dose reduction techniques
Introduction to presentationCT Dose Index CTDI100
CTDIvol
DLP Effective dose CT dose reduction techniques AEC Filter Reconstruction algorithm
CT Dose Index CT dose index (CTDI) is the
standardized measure of the radiation output of a CT system,
Currently used measures are the CTDI100 and CTDIw, modified to CTDIvol for modern helical scanners.
CTDI phantoms(PMMA)
Measurement of CTDI• Ionisation pencil chamber
– Air-filled chamber – X-rays cause ionisations in
chamber, causing current to flow, proportional to exposure
– Instant readings, good accuracy, easy to use
100 mm
Measurement of CTDI• Single slice measurement
(z-axis)
Dose
TNominal beam width
dzzDT1 CTDI
-
dzzDT1 CTDI
50
50-100
-50 +50
50
50
D(z)dzn.T1CTDI100
n = no. slices imaged simultaneously T = nominal imaged width
Davge
T100DavgeCTDI100
CTDI100 • CTDI100 is a measure of radiation dose on a 100mm
long pencil ionization chamber.• Centre • Periphery - 1 cm depth (mean of 4 positions)
C
P1
P2
P3
P4
320 mm
‘Body’
‘Head’
160 mm
140 mm
Measurement of MSAD(Multiple Scan Average Dose)
CTDI ‘Series’ doseMulti-scan
‘Series’ dose
CTDI
I
TN MSAD
T
T is the nominal scan width (mm)
I is the distance between scans (mm)
I
N is the number of scans
N × T is the total nominal scan width
CTDI100 • Typical CTDI100 values (mGy)
40
Head
Body
40
40 40
40
20
20 20
20
10
Periphery : centre 1:1 Periphery : centre 2:1
Derivatives of CTDI100 • CTDIW
• Weighted average CTDI: represents the average dose for contiguous irradiation
• CTDIw = 1/3 CTDI100,C + 2/3 CTDI100,P
CTDIC CTDIP
Derivatives of CTDI100
• CTDI100 & CTDIw refer to a pitch of 1
• Average absorbed dose dependent on pitch
Contiguous Extended Overlapping
CTDIvol
• Volume CTDI: Considers contiguous scanning
• CTDIvol = CTDIw Pitch
Pitch = 1 CTDIvol = CTDIw
Pitch = 2 CTDIvol = CTDIw
2
Pitch = 0.5 CTDIvol = 2 x CTDIw
Radiation risk: DLP• CTDIvol is a measure of absorbed dose, energy
absorbed per unit mass • To measure radiation risk from stochastic effects
the total energy absorbed must be considered • In CT this can be estimated using the Dose Length
Product, DLP
DLP = CTDIvol . L (mGy.cm)
DLP1 DLP2
where L = scan length
Dose length product
L1
T
Pitch 1 8 rotations
Pitch 2 8 rotations
L2
T
as CTDIvol2 = CTDIvol1
2
Estimates of effective dose (E)• Can be obtained from DLP • Effective dose = DLP. CF (mSv) • • • • • • Conversion factors not scanner specific or location
specific
Region of body Conversion factor,EDLP (mSv mGy-1 cm-1)
Head & neck 0.0031 Head 0.0021 Neck 0.0059 Chest 0.014 Abdomen& pelvis 0.015 Trunk 0.015
CT dose reduction techniquesAEC systems have a number of
potential advantages, including better control of patient radiation dose, avoidance of photon starvation artifacts, reduced load on the x-ray tube, and the maintenance of image quality in spite of different attenuation values on CT scans
Types of AECPatient-Size AEC
Z-axis AEC
Rotational or Angular AEC
Operation of AEC Systems on Different Multidetector CT
Scanners Exposure 3D CARE Dose 4D
Patient-Size AEC Patient size AEC: the tube current is adjusted based on the overall size of the patient to reduce the variation in image quality between small patients and large patients. For a given patient size, the appropriate milliamperage is selected and is used for the entire examination or scan series
Hi-mA
Lo-mA
Z-axis AEC: tube current is modulated according to patient attenuation along the z-axis.The goal is to reduce the variation in image quality of images from the same series.
atte
nuat
ion
Patient-Size AEC Rotational AEC: The tube current is decreased and increased rapidly (modulated) during the course of each rotation to compensate for differences in attenuation between lateral (left-right) and A-P (anterior-posterior) projections .
high
atte
nuat
ion
low attenuation
Combination of AEC functions • Tube current is adjusted during scanning to compensate for
attenuation differences – dose applied to patient only where needed, avoiding dose where it isn’t
mA
position
On line’ modulation–uses attenuation data from previous rotation–adapts tube current to patient attenuation ‘on the fly’
Scan projection radiographs (SPRs, known as scout, scanogram or topogram views) are the main way that AEC systems assess the attenuation of the patient in order to set the tube current
To obtain correct attenuation data from SPR always centre the patient carefully
Patient is positioned in the isocenter – optimal dose and image quality
Patient is positioned too high - increased mAsPatient is positioned too low - reduced mAs and increased noise
Toshiba:defining image quality requirements
Specify s.d. level (or ‘image quality level)–patient mAcalculated to achieve this noise level at any scan parameter settings
Set min & max mA
Benefits of CT scanner AEC Consistent image quality
Potential for dose reduction through
exposure optimisation
Reduced tube loading
Extended scan runs(OLP)
Reduction in photon starvation artifact
30
• Images along length of phantom (no AEC)
• • • • • • • •
Constant mA •
Testing the AEC
31
Testing the AEC
• Measure noise with AEC off and on • Monitor mA, CTDIvol
0 4 8
12 16 20 24 28
-150 -100 -50 0 50 100 150 Z-position (mm)
Noi
se (%
)
automA off
Noise Index 12
Increased mA
Decreased mA
Constant mA
Same mA
Increased mA
Decreased mA
32
Coronal view Sagittal view
z-axisAEC off
z-axis AEC on
Noise increases
Constant noise
Testing the AEC – Viewing with MPR
photon starvation artifact
without angular mA modulation with angular mA modulation
Bowtie FilterLonger bowtie path lines up with shorter
patient path
Reduce x-ray scatter (noise and artifact)
Maintain uniform x-ray at detectorReduce surface dose by 50%
• Retrospectively gated CCTA(dose modulation )
~15 mSv 100%
20% ~30% reduction
Image obtained in middiastole (75% of R-R interval)
reconstructed using 1.5-mm slice thickness at high tube current
shows low-noise
Image obtained in midsystole (30% of R-R interval)
reconstructed using 1.5-mm slice thickness shows higher-noise
image obtained at low radiation dose
• Prospectively gated CCTA
~ 1 mSv
Prospective Gating of CTA(snapshot plus)
40mm
5mm overlap
Scan range
Conventional technology without Dose Shield
SOMATOM Definition AS+ with Adaptive Dose Shield
Scan range
Dynamic Collimation• In helical scanning extra rotations are needed at end of imaged
volume – Significant extra dose: wide beam widths and short scans
• Dynamic collimation - collimator blades open and close asymetrically at start and end of scan
(a) conventional and (b) adaptive section collimation CT scanning protocols. Foradaptive section collimation, shape of x-ray cone beam at beginning and end of spiral acquisition is controlled by two collimators made of absorbent material.
Image reconstruction(FBP)
attenuationdetector position
90°2 projections4 projections8 projections16 projectionsat
tenu
atio
ndetector position
0°
0(3)
0(9)
0(7)
0(5)
6 6 6 6
10 14
1212
12 12
5 7 5 7
3 9 7 5
3 5
9 7
816
12
12
+1-1
-2+2
Iterative reconstruction
Iterative reconstruction
120 kVp and 300 mA FBP
120 kVp and 300 mA ASiR 100%
120 kVp and 300 mA ASiR 50%