PHYSICS of COMPUTED TOMOGRAPHY

RADIODIAGNOSTIC AND
RADIOTERAPHY
DEAPARTMENT OF
CT Training for Radiographers
Dr. Nursama Heru Apriantoro,
BASIC PHYSICS OF CT Scan

CT Training for Radiographers.
SECTION I
2

• Computerized Axial Tomography (CAT)
• Computer Aided Tomography (CAT)
• Computer Assisted Tomography (CAT)
• Computerized Transverse Axial
Tomography (CTAT)
• Computerized Transaxial Transmission
Reconstructive Tomography (CTT)
• Reconstructive Tomography (RT)
• Computerized Digital Tomography (CDT)
• Computerized Tomography (CT)
POPULAR NAME OF CT
3

8 NOVEMBER 1895 SINAR X DITEMUKAN
FISIKAWAN BERKEBANGSAAN JERMAN
WILHELM CONRAD RONTGEN
March 27, 1845
MERUPAKAN GEM DENGAN PANJANG
GELOMBANG BERKISAR 0,01 nm – 10 nm
(3 x 1016 Hz – 3 x 1019 Hz)
KARAKTERISTIK SINAR X
4

Memiliki daya tembus yang besar.
ƒDapat diserap oleh materi
(tergantung dari nomor atomnya)
ƒƒDapat dihamburkan (difraksi sinar-
x)
ƒMenimbulkan ionisasi.
Memiliki efek fotografi /
menimbulkan fluoresen.
KARAKTERISTIK SINAR X
5

RADIOGRAFI
KONVENSIONAL
.
PEMANFAATAN SINAR X
6
RADIOGRAFI
TOMOGRAPHY

 X-ray based
 Multiple Projections (each from different Angle)
 X-ray tube motions around the patient
 Detector motions around the patient opposite the tube
 In special technique :The patient's body is move
continuously as the x-ray beam is scanned around the
body
 Detector output and angular position fed into a
computer
 Computer perform calculation to estimate density of
tissues in each square of a slice
 Mathematically reconstruct image
 Create Voxel to Hounsfield Unit Value (CTNs)
 CTNs represent linear attenuation coefficient
CT SCAN
7

TAHUN 1971 GENERASI PERTAMA CT SCAN
DIPUBLIKASIKAN
Godfrey Newbold Hounsfield
28 August 1919 – 12 August 2004
Allan M. Cormack
23 Februari 1924 – 7 Mei 1998
CT SCAN
8

CT DEVELOPMENET
9
1970 1972
1976
1978
1990

ELECTRON BEAM OF CT
10
 Electron beam scanner
 Primarily for cardiologist, 50 msec scan times
 Tungsten target and high energy electron beam
https://www.youtube.com/watch?v=E9vLclEQRtc

CT DEVELOPMENT
11
HELICAL, 1990
Slip ring technology
Acquired data while table is
moving
Typical pitch ratio 0.5 - 2
on)(collimatithicknessslice
rotationpermovementtable
pitch 
MULTI DETECTOR, Late1990’s-
2000
Multiple detector array
Cone beam
MSCT

CT DEVELOPMENT
12
DUAL SOURCE, 2005
A Scanner using two x-ray source
and two detectors at the same time
Enables scanning a heart at any
heart rate and at the lowest radiation
dose possible
Dynamic Focal Spot,
Flying focal spot
Double ray density
Double spatial resolution

CT DEVELOPMENT
13
MULTI ENERGY DETECTOR
Consist of upper and bottom
detector
Upper detector to detects lower
energy
Bottom detector to detects higher
energy
Signal from both can be combine as
well

CT SCANNER DEVELOPMENT
14

SECTION II
15

Conventional Radiography Vs Tomography
16
a
t
ta
ba
SBm
)(
)(



)(
1
22
ba
a
Bmt



S
2t

CT Training for Radiographers. 17
BASICS PHYSICS of CT

BASICS PHYSICS of CT

CT Collimator
To decrease
scatter radiation
To reduce patien
dose
To Improve image
quaility
To determine slice
thickness

DETECTORS
• Scintillation Cristal + photodioda+ PMT
Sodium Iodide (Na I + Tl)
Calcium Fluoride (Ca F2)
cadmium tungstate (CdWO)
Bismuth germinate oxide (BGO)
Lutetium Terbium Aluminum Garnet (LuTAG)
Yttrium Gadolinium Oxide (YGO)
• Gas
Xenon Gas
Krypton

XENON GASES
21

SCINTILLATION DETECTOR
22

CT DETECTOR
 High efficiency; Kemampuan untuk
menyerap & mengubah foton x-ray
menjadi sinyal listrik
 Quick response time, waktu yang
dibutukan untuk menghasilkan data dari
setiap scanning
 High dynamic range, Kemampuan
untuk mendeteksi rasio berbagai
intensitas sinar x
 Stability, sinyal detektor dari waktu ke
waktu bersifat konsisten

It = Io exp (-μx)
Absorption and Attenuation
25
Io It
x







t
o
I
I
x
ln
1

μ1 μ2 μ3 μ4 μn
x x
Io
It

















t
o
i
i
t
o
n
xxxx
I
I
x
I
I
x
eIIeIIeIIeII
ln
1
ln
1
...
,,,
1
4321
4
34
3
23
2
12
1
01




BASIC CT RECON’S
26
1 2
3 4
4 6
4 6
5 11
9 10
8 14
16 17
13 16
19 22
3 6
9 12
1 2
3 4
X-RAY

Reconstruction process
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
Gray ScaleDIGITALANALOG
7 5 5
1 3 6
2 0 1

GRAY SCALE IMAGE
28

VOXEL, PIXEL, MATRIX SIZE
• Voxel (volume element) - for each image
slice, there is an x, y and z dimension.
These are coming close to isotropic (the
same in each dimension). A typical
voxel would be created from a 35 cm
FOV, 512 x 512 matrix and 0.6 -10.0
mm thick slice.
10 mm
.68 mm
.68 mm6 mm
.68 mm
.68 mm

Pixels
and
Voxels

31
VOXEL PIXEL
MATRIX SIZE, 16 x 16
PIXEL = SFOV / MATRIX SIZE
µ
(x,y,z)
p
(x,z)

CT NUMBER
32
• Hounsfield Units (HU), normalize to water.
 HU1000x
-)(
),('
water
water

 x,y
yxsCTN 
 HU1000x
-)(
),('
water
water

 x,y
yxsCTN 
• CTN Water = 0 HU (all energies)
• CTN range = -1024 to +3072
• 12 bit, 212
= 4096

CT NUMBER
Find the CTN of some tissues bellow :
• Bone = 0.528
• Blood = 0.208
• Gray Matter = 0.212
• White Matter = 0.213
• CSF = 0.207
• Water = 0.206
• Fat = 0.185
• Air = 0.0004

CT NUMBER

WL sama dan WW berbeda

WL beda dan WW sama

IMAGE FORMATION CT NUMBER
Some Preset Window and Levels
• Tissue Level Window
• Lung -550 HU 1600
• Soft Tissue 100 400
• Bone 800 1000
0
-1000 HU
Output gray levels
256
0 (black)
1000
0 HU
2000
1000 HU

CT RECONSTUCTIONS
39
Io It(y)
x
y
Io
It(d,Ө) x
y
d


dxyxyII
IdxyxyI
t
t
),())(/ln(
)),(exp()(
0
0


  dlldldtII )cossin,sincos()),(/ln( 10 
  dlldldfdg )cossin,sincos(),( 
F(x,y)
g(d,Ө)
Radon Transform

Fourier Transform
)()(
~
~
 

tetfF tj

The fourier transform changes SIGNAL from TIME
DOMAIN to FREQUENCY DOMAIN back again
F(ω) can be represented as :
Real + Imaginer
or
Magnitude + Phase
)()()()()(
~
~
~
~
 
 
dttjSintfdttCostfF 

Reconstruction process
intensityattenuated:)(intensity,incident:)(
[Np/m]ted)reconstrucbe(totcoefficienabsorption:),(
0 xIxI
yxf

















0
0
),(
0
),sincos(),(:projectionBack
),(ofFFT1Dis),(ewher
),(
2
1
),(:dataFiltered
)(
)(
ln),(),(:dataprojection
)()(:intensityattenuated
dyxpyxf
XpUP
dXeUUPXp
XI
XI
dYyxfXp
eXIXI
f
jUX
f
dYyxf

Back Projection
42
Backprojection at 4, 16, and 100 angles
-1
0
1
-1
0
1
-0.5
0
0.5
1
1.5
-1
0
1
-1
0
1
-0.5
0
0.5
1
1.5
-1
0
1
-1
0
1
-0.5
0
0.5
1
One, two, and four angles of backprojection

Attenuation Correction Factor
43
(a) Can be determined from attenuation coefficient measurements
determined from CT scan; (b) Array of attenuation correction
factors; (c) to provide final attenuation-corrected CT scan

Back Projection
44
Sample Back Projection Filtered Back Projection
FILTER

BASIC III
45

SSCT vs MSCT

SSCT MSCT
SSCT vs MSCT

SSCT vs MSCT
48

SSCT vs MSCT
49
SSCT COBRA
(Philips)

SSCT vs MSCT
50
COBRA
(Philips)
SSCT

SSCT vs MSCT
51

SSCT vs MSCT
52
PENUMBRA EFFECT

SSCT vs MSCT
53

SSCT vs MSCT

MSCT DETECTORS
55

DETECTOR CONFIGURATIONS
X-ray Tube Focal Spot
X-ray Beam Collimator
4 x 2.5 mm Detector Configuration
16-row Mosaic Detector
Diode FET Switching Array
Flex Connector A Flex Connector B

MSCT DETECTORS

Wide Area Coverage
60
Philips Brilliance 40Philips Brilliance 64
Over 525
40-mm
systems
installed
worldwide
as of
December
2005
Philips-Expertise in Detectors
Tomorrow- 256 Nano-panel
Working 256 Nano-panel detector array

MSCT DETECTOR CONFIGURATIONS
61

Flying Focal Spot
62

DFSTM
X-Ray Tube Design
63
Dynamic Focal Spot
X-ray Tube Design
Doubles
Ray Density and thus
Doubles
Spatial Resolution
with the same number
of detectors

PHILIPS : DFSTM
X-Ray Tube Design
64
WithWithout

PITCH
65
• Pitch = table travel per gantry
rotation (mm) / total nominal beam
width
• Pitch = table travel per gantry
rotation (mm) / (slice width ×
number of slices).
• Pitch = 0,13 -1,5, disarankan > 1
• mAs eff = mAs / pitch.

PITCH
66
• 16 Slice MSCT dengan 0,625 mm
slice thickness dan pergerakan
meja 15 mm per rotasi x ray tube.
• Tentukan Beam Pitch ?
pitch1.5
)625.0)(16(
15


mm
mm
BP

MSCT FASTER SCANNING
67
Detector Beam Thick
(mm)
# Rotation Total scan
time (sec)
1 x 1.25 1.25 160 128
4 x 1.25 5 40 32
8 x 1.25 10 20 16
16 x 1.25 20 10 8
64 x 0.625 40 5 4
1.25 mm images and 20 cm scan length at 0.8 sec
rotation and 1.0 pitch

Metode ABC/2 didasarkan pada penghitungan
volume ellipse, dengan menggunakan
persamaan :
V = 4/3 π r3 atau V= 4/3 x π x r1 x r2 x r3
Dengan penyederhanakan diameter d, , maka
diperoleh
V = ½ (d1 x d2 x d3)
Shg dapat ditulis menjadi
VOLUME IN CT SCAN
68
V = ABC / 2

The Total volume (V) was calculated by equation
• HU min adalah range HU minimum pada cairan
yang terdeteksi ROI
• HU max adalah range HU maksimum pada
cairan yang terdeteksi ROI
• from CAVA (Computer Assited Volumetric
Analysis)
VOLUME SUBJEK PADA CT SCAN
69

BASIC IV
70
Multiple-scan average dose
CT DOSIMETRY

CT DOSIMETRY
• MSAD = Multy Sice Average Dose
• CTDIw
• CTDIvol
• DLP
CTDI100
MSAD
Effective
Dose

CT DOSIMETRY
• CTDI menggunakan CTDI phantom dan
Ionization chambar
• CTDIw adalah CTDI weighted average
ditentukan berdasarkan :
CTDIw = (2*CTDIperiphery+CTDIcenter)/3
CTDIvol = CTDIw/Pitch
DLP = CTDIvol * L
L is the Z-length of the scan

DLP DAN DOSE EFEKTIF
DLP = CTDIvol * L
L is the Z-length of the scan
DOSE EFEKTIF = DLP * WT
WT adalah faktor bobot jaringan diperoleh
dari ICRP

CT DOSIMETRY

CTDIw: Dose for a “Procedure”

Present Calculation CT Dose
))((
2
LAT
2
AP
22
DiameterEffective
,
2
;
2
21
2
21
LATviewAPviewDEff
A
DEff
rrrALUASAN
ViewLAT
r
ViewAP
r





















)(DefffCTDISSDE vol 
SSDE (Size Specific Dose Estimate)
f (Deff) = Konversion factor pada diameter efektif
diperoleh dari AAPM Report 204

Doses From CT
BERT*Eff Dose (mSv)Exam
2.4 days0.02Chest x ray (PA film)
8.5 days0.07Skull x ray
158 days1.3Lumbar spine
304 days2.5I.V. urogram
1.0 year3Upper G.I. exam
2.3 years7Barium enema
243 days2CT head
3.3 years10CT abdomen
1.3 years3.9PET FDG Exam (10 mCi)
158 days1.3NM Bone Scan (20 mCi)
* Time to receive equivalent effective dose from
natural background radiation

Typical doses in mGy during CT in adults
(Shrimpton et al. 1991)
Exam Eyes Thyroid Breast Uterus Ovaries Testes
Head 50 1.9 0.03 * * *
Cervical
spine
0.62 44 0.09 * * *
Thoracic
spine
0.04 0.46 28 0.02 0.02 *
Chest 0.14 2.3 21 0.06 0.08 *
Abdomen * 0.05 0.72 8.0 8.0 0.7
L. spine * 0.01 0.13 2.4 2.7 0.06
Pelvis * * 0.03 26 23 1.7
The symbol * indicates that dose is < 0.005 mGy

Patient Dose Path

REFFERENCES
Edward M. Hsiao, Frank J. Rybicki, and Michael Steigner.
(2010) CT Coronary Angiography: 256-Slice and 320-
Detector Row Scanners. Curr Cardiol Rep. 12(1): 68–75.
Hameed T, Teague S, Vembar M, et al. Low radiation dose
ECG-gated chest CT angiography on a 256-slice
multidetector CT scanner. Int J Cardiovasc Imaging.
2009;25:267–278.
Kalendar, W.A. (2005) Computed Tomography.2nd Ed.
Mahesh, 2002. Physics Tutorial for Residents. Radiographics
22: 249-962
Nagel, H.D. (2004). MSCT Technology. Philips Medical
system Germany. www.multislice-ct.com
Philips. MSCT 256 Slices. PT BERCA NIAGA MEDIKA
Seeram, E. (2009). COMPUTED TOMOGRAPHY. Physical
Principles, Clinical Applications, and Quality Control. 3rd
Ed. St Louis: Saunders.
Walker M, Olszewski M, Desai M, et al. New radiation dose
saving technologies for 256-slice cardiac computed
tomography angiography. Int J Cardiovasc Imaging.
2009;25:189–199.

TERIMA KASIH
• ‫شكرا‬
• THANKS YOU
• ありがとう
• 감사합니다
• Спасибо
• Merci
• Danke
• igracias
• Matur nuwun
84

PHYSICS of COMPUTED TOMOGRAPHY

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