Physics of Radiology
Lior Copel, M.D.
Assaf Harofeh Medical Center, Zerifin
Sackler School of Medicine,
Tel-Aviv University
Clinical Case
Jaundice and Fever
• Subjective:
– 42 year old female
– Malaise and jaundice for 2 months
– Fever for 1 week
• Objective:
– Fever – 38.5
– Leukocytes – 16700 ; PMN – 78%
– Skin and corneal jaundice:
Total bilirubin – 11.3 mg/dL
Direct bilirubin – 6.7 mg/dL
Diagnosis
• Pancreatic adenocarcinoma (head)
• Obstructive jaundice
• Ascending cholangitis due to
obstructive jaundice
Conventional Radiology
The Electromagnetic Spectrum
Photons
• Electromagnetic radiation is quantized in
discrete quantities called photons
• Photons behave as waves or particles but
have no mass
• Photons energy (E) – Frequency
Wavelength
• C = velocity of light = 300,000 km/sec
E = h x f = h x (C / λ)
Photons
• Photons  X-Rays
• Photons – high energy - 20 – 200 keV
short wavelength - 10-10 m
X-Ray Tube
X-Ray Interaction in Material
• Pass through (penetrate)
• Absorbed (transfer energy to the
absorbed medium)
• Scattered (change direction and
possibly lose energy)
The Cassette
Radiology Examination Room
X-Ray Advantages
•
•
•
•
•
•
Excellent imaging of the chest and skeleton
Good evaluation of GIT and GUT
Good spatial resolution
Can be performed dynamically
Availability
Relative low radiation dose to patient
X-Ray Disadvantages
• Poor 3D geometry
• Poor soft tissue resolution
• Insensitive to small lesions (esp. lung fields)
• No evaluation of CNS and PNS
Ultrasound (US)
Natural Ultrasound
Sound Wave
Sound Waves
• Infrasound - < 20 Hz
• Audible sound – 15 – 20,000 Hz
• Diagnostic ultrasound – 1 – 20 MHz
(1 MHz = 106 Hz)
• Diagnostic US uses transducers for the
production of the waves
Velocity of the US Wave in Different Tissues
Doppler Physics
• Doppler effect – the change in frequency
that results from a moving sample
• Object Movement:
– Toward the detector  higher frequency,
lower wavelength
– Away from the detector  lower frequency,
higher wavelength
Doppler Shift
US - Advantages
• Excellent soft tissue
contrast resolution
• Dynamic
• No radiation
• Safe in pregnancy
• Available, cheap
US - Disadvantages
• Operator dependent  not imagined-not seen !
• Relatively no anatomic landmarks
• Cannot penetrate air and bone
• No imaging of lungs, CNS, PNS and GIT
Computed Tomography (CT)
Computed Tomography
• Tomography – Greek:
– tomos  slice or section
– graphein  to write or record
• CT scan – a diagnostic test that combines
X-rays with computer technology
• A series of X-rays from many different
angles are used to create a cross-sectional
image of the patient’s body
Computed Tomography
• Reconstruction “builds” the CT image
from the data collected and represents a
cross section of the patient
Computed Tomography
• Four hardware components:
– Radiation source
– Radiation detector system
– Mechanical manipulator
– Computer with display
Contrast-Enhanced CT
• Abnormal tissues (e.g. tumor, inflammation)
enhance differently than normal tissues with IV
contrast. This enables the abnormal tissue to be
identified and characterized
• Contrast media opacify specific structures
(blood vessels, liver, spleen and the urinary tract)
enabling abnormal findings to be detected in
those structures
Hounsfield Units
The Gantry
PROPOSALS FOR THE IONISING RADIATION
(MEDICAL EXPOSURE) REGULATIONS 1999
REPIACING
The Ionising Radiation (Protection of Persons Undergoing Medical
Examination or Treatment) Regulations 1988
CONSULTATIVE DOCUMENT
Comments to be sent to:
Ms P M Brown
Room 525 Wellington House
133-155 Waterloo Road
London SEI 8UG
No later than 31 May 1999
DEPARTMENT OF HEALTH
MARCH 1999
T y p ic a l e ffe c tiv e d o se s fro m d ia g n o stic
M e d ic a l e x p o su re s in th e 1 9 9 0 s
D ia g n o s t ic
p ro c e d u re
X - r a y e x a m in a tio n s :
L im b s a n d j o in t s
( e x c e p t h ip )
C h e s t ( s in g le P A f ilm )
S k u ll
T h o r a c ic s p in e
L u m b a r s p in e
H ip
P e lv is
A bdom en
IV U
B a r iu m s w a llo w
B a r iu m m e a l
B a r iu m f o llo w t h r o u g h
B a r iu m e n c m a
C T head
C T chest
C T a b d o m e n o r p e lv is
T y p ic a l
E f f e c t iv e
D ose
(m S v )
E q u iv .
No. of
chest xra y s
A p p ro x .
e q u iv .
P e r io d o f
n a tu ra l
b a c k g ro u
nd
r a d ia t io n
< 0 .0 1
< 0 .5
< 1 .5 d a y s
0 .0 2
0 .0 7
0 .7
1 .3
0 .3
0 .7
1 .0
2 .5
1 .5
3
3
7
2 .3
8
10
1
3 .5
35
65
15
35
50
125
75
150
150
350
115
400
500
3 days
11 days
4 m o nths
7 m o nths
7 weeks
4 m o nths
6 m o nths
1 4 m o nths
8 m o nths
1 6 m o nths
1 6 m o nths
3 .2 y e a r s
1 year
3 .6 y e a r s
4 .5 y e a r s
CT - Advantages
• Excellent anatomical data
• Excellent spatial resolution
• Good contrast resolution (fair in CNS and
MSK imaging)
• Reconstructions (MPR, MIP, VR, navigation)
• Fast exam (20sec.- 1min.)
• Available
CT - Disadvantages
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Radiation dose
Reactions to contrast material
Static exam
Availability
Cost
Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging
• Concept – 1970
• Present – 60 million
examinations per year
Hydrogen Nuclei
The Hydrogen Nuclei
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Have the largest magnetic moment
Abundant in the body
1023 H protons in each 1 cm3 of tissue
These protons are normally randomly oriented
and have no net magnetic moment
(magnetization vector)
The Hydrogen Nuclei
Hydrogen Nuclei in Nature
The Magnet
1 Tesla = 10,000 gauss (G)
Earth’s magnetic field 50 µT (0.5 gauss)
MRI magnet is 20,000 more powerful than earth’s magnetic field
Protons in a Magnetic Field
Protons in a Magnetic Field
90 Degree RF Pulse
90 Degree RF Pulse
Time Constants Measurements
• T1
– Time to recover longitudinal magnetization
– Realigning to B0
– Range  200 – 2000 msec
• T2
– Time to dephase  loose horizontal magnetization
– Range  30 – 500 msec
• Differences in T1 and T2 provide basis for
variations in signal intensities and tissue contrast
T1 Relaxation Time
The time it takes to each tissue to return to its
steady state position is the base for MR imaging
T2 Relaxation Time
Relaxation Time
• Different tissues return to equilibrium
in different time
• Those difference in relaxation time
enables different contrast resolution
to different tissues and make it
possible to differentiate normal
tissue from abnormal one
Contrast Agents – Gadolinium-DTPA
• Abnormal tissues (e.g. tumor, inflammation)
enhance differently than normal tissues with iv
contrast. This enables the abnormal tissue to be
identified and characterized
• Contrast media opacify specific structures
(blood vessels, liver, spleen and the urinary
tract) enabling abnormal findings to be
detected in those structures
Planes of Imaging
• Coronal
• Sagittal
• Axial (Horizontal)
MRI Advantages
• Excellent contrast resolution (esp. CNS,
MSK, mediastinum, pelvis and heart)
• Each tissue / pathology can be
evaluated with different sequences
• No radiation
MRI Advantages
• Multiplanar imaging
• Can be performed in pregnancy
(not in first trimester)
• No risk with contrast materials
• Can be performed dynamically
MRI Disadvantages
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Problem with cortical bone and calcifications
Hazard to patients with metallic objects
Problems in trauma and ventilated patients
Claustrophobia
Long examination
Availability
Expensive
Be Careful !!!