Remote sensing
(Health Physics)
Cambridge IA2 Physics
Syllabus Section VII.29
X-Rays
(a)
explain in simple terms the need for remote sensing
(non-invasive techniques of diagnosis) in medicine.
(b)
explain the principles of the production of X-rays by
electron bombardment of a metal target.
(c) describe the main features of a modern X-ray tube, including
control of the intensity and hardness of the X-ray beam.
(d)
show an understanding of the use of X-rays in imaging
internal body structures, including a simple analysis of the
causes of sharpness and contrast in X-ray imaging.
(e)
recall and solve problems by using the equation I =
I
0e
-μxfor the attenuation of X-rays in matter.
X-Rays
• Production
• Spectrum
• Attenuation
• Imaging
The Electromagnetic Spectrum
electromagneticspectrum.swf
X-Ray Production
• Heater cathode
Thermionic emission
• Accelerating p.d.
• Anode target (often Tungsten, W)
High m.pt,
water cooled to remove heat
• X-rays are produced as electrons are
X-Ray
Spectrum
Background
spectrum
Characteristic
lines
Threshold
X-Ray Spectrum
X-Ray Spectrum
Background
• Bremsstrahlung (‘braking’) radiation
emitted by electrons as they slow down and
rapidly lose energy inside the target
Characteristic lines
Characteristic X-Rays
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html
Characteristic lines
produced by electron
transitions of fixed
energy within the
target atoms.
X-Ray Spectrum
Electron KE
X-ray photon energy:
E
k
= eV = h
max
= hc/
min
Threshold wavelength,
Threshold Wavelength
= hc/eV
=
(6.6 x 10
-34
x 3 x 10
8
)
/(1.6 x 10
-19
x 3.5 x 10
4
)
= 3.5 x 10
-11
m
X-Ray Spectrum
Intensity
• Intensity
tube current (heater current)
Hardness
• Hardness = penetration of the X-rays
electron energy
accelerating voltage
X-Rays – Attenuation
I = I
0
e
-
x
I
0
= incident intensity
(Wm
-2
)
= linear attenuation coefficient
(m
-1
)
x = thickness of medium
(m)
Incident Intensity, I
0Transmitted intensity, I
X-Rays – Attenuation
Worked Example (Page 37 – Health Physics)
For 1MeV radiation incident on tissue, the linear
attenuation coefficient is 7m
-1
. Calculate the
thickness of tissue required to reduce the intensity
of the beam by half.
0.5 = e
-
x
–
x = ln0.5
x = ln2 / 7
XRayInteract.swf
X-Ray Imaging
Sharpness
• Size of anode
• Size of aperture
• Removal of scattered X-rays by Pb collimator grid
Contrast
• Densities of tissue/bone
X-Ray Imaging - Sharpness
Depends on area of target anode
X-Ray Imaging - Sharpness
Pb collimator grid removes scattered X-rays
Film or
detector
Unexposed
area
1. Object Contrast
Dependent on the object under study:
•
Density
• Atomic number
• Thickness
Barium meal
Barium meal (contrast agent)
Investigating swallowing
2. Subject Contrast
Dependent on:
•
Object contrast
and on
• The
energy characteristics of the beam
3. Display Contrast
Dependent upon
• The detection devices
• The display devices
Image Resolution
Image resolution is affected
by
• The design of the X-ray
tube
• The design of the detector
• The nature of the display
X-Rays – Questions
Let’s look at
how X-rays
are used in
Computerise
d Axial
Tomography
Syllabus Section VII.29
CT (Computed Tomography) Scanning
(f) Show an understanding of the purpose of
computed tomography or CT scanning.
(g) Show an understanding of the principles of CT
scanning.
(h) Show an understanding of how the image of an
8-voxel cube can be developed using CT
scanning.
(i) – (k) Ultrasound
www.impactscan.org/rsna99.htm
www.chadwickmedical.com/svc_ctangio.htm
3D Image divided into
voxels
(
Pixel
= 2D)
CAT Imaging
• 3D X-ray scanning of the whole body or
region of a body
P. Lovatt & Hari.R 42/122
This image was
produced with a
window of
1000HU at level
–700HU
(HU<+300)
CAT Imaging
http://cal.man.ac.uk/student_projects/2000/mmmr7gjw/technique9.htm
CAT Imaging
This image was
produced with a
window of
500HU at level
+50HU.
(–450<HU<+550)
Computerised Tomography
CAT Imaging
Rotating the
scanner provides
more information
about the
Physics 2000 website (Colorado)
Physics 2000\tomography\auto_rib_cage.html
Image Planes
Image planes
A Axial
Creating a CAT Image
4 1
2 7
? ?
? ?
Imagine just 4 voxels of tissue (small 3D cubes) to be
imaged using a rotating CAT scanner
X-ray
production
Tissue to
First Image 0
0
4 1
2 7
Image
Angle 0
0+5
+9
4 1
2 7
Angle 45
0Second Image 45
0
Third Image
90
0
4 1
2 7
Angle 90
0+6 +8
22 14
16 6
11 20
4 1
2 7
Angle 135
0Fourth Image 135
0
Image Processing
1. Subtract ‘background’ (4 + 1 + 2 + 7 = 14)
2. Divide by 3 to remove duplication
Scan
Result
26 17
20 35
12 3
6 21
Final
Image
4 1
2 7
– 14
3
Consider
one voxel
1. Subtract the
unwanted
contribution to
each voxel
(background)
4
Scan 1
4 4
0 0
8 4
0 4
Sca
n 2
Sc
an
4
Sc
an
3
12 4
4 4
16 4
4 4
12 0
0 0
Image
4 0
0 0
– 4
3
2. Divide by 3 to
remove the
3D Imaging
• Based on the 8-voxel cube in 3D
• Many scans at different angles
• Linear and angular motion
• Millions of computations per voxel
• Intensity and contrast can be varied
P. Lovatt & Hari.R 58/122
Clockwise from top-left: Volume rendering overview, axial slices, coronal slices, sagittal slices.
Although visually very appealing, the volume rendering is often of limited diagnostic value, and requires substantial computer resources. Qualitative and quantitative information tends to be more accessible on the cross-sectional images, and many operators prefer to forgo the volume rendering for
www.answers.com/topic/ct-workstation-neck-jpg
CAT Images
Stroke diagnosis (brain)
Abdomen
CAT Images
CAT
Images
Sliced CAT
sections
(axial) can be
combined to
produce 3D
or moving
images
P. Lovatt & Hari.R 64/122
CAT Mummies
Another mummy:touregypt.net/featurestories/mummification.htm
CAT Advantages
Can image calcium
More useful than MRI for investigating cortical bone
fractures and calcification of organs.
Rapid imaging
Modern machines can produce images in a matter of a few
seconds, depending on the type of scan.
Good for obese patients
Fat separates the abdominal organs and good CT images can be generated. Ultrasound is difficult in obese patients.Good contrast between
different tissues
Good contrast is seen between tissues which are mainly bone, fat, water, and air. Use of a narrow X-ray beam and windowing can produce detailed images. However CT cannot differentiate well between different parts of the same organ.
Cancer Treatment
CT is particularly useful for staging and treating tumours.
Three-dimensional Imaging
CAT Disadvantages
High ionizing dose
A thoracic CT scan may expose the patient to as much
radiation as 40 chest X-rays.
Bony artefacts
Brain scans may be distorted by bony artefacts
Imaging in the transverse (axial)
plane only
Sagittal
and
coronal
images are generally difficult,
although possible if the patient can be positioned
appropriately within the scanner.
High cost of equipment and
procedure
CT scanners are fairly expensive to purchase and the
number of personnel involved means that each scan is
considerably more expensive than an X-ray.
CAT - Questions
SAQ 5.2 (Page 49,
Health Physics
)
Syllabus Section VII.29
Ultrasound
(i) explain the principles of the generation and detection of
ultrasonic waves using piezo-electric transducers.
(j)
explain the main principles behind the use of
ultrasound to obtain diagnosticinformation about
internal structures.
(k)
show an understanding of the meaning of
acoustic impedance and its importance to the intensity
reflection coefficient at a boundary.
Ultrasound is used for the
diagnosis (finding) and treatment
(destruction) of kidney stones
Ultrasound Imaging
• Ultrasound >20kHz
•
Ultrasound transducer
generates sound
pulses using the
piezo-electric
effect
• Pulses are
reflected
where they meet a
boundary between regions of different
density
Piezo-electric Effect
www.physikinstrumente.com/tutorial/4_15.html
Piezo-electric Effect
piezocompressMS.avi
Change in the dimensions of a
crystal when a p.d. is applied
Piezo-electric ‘Poling’
An electric field of 2000Vmm
-1
is applied to a
heated crystal (above the Curie temperature)
Piezo-electric Effect
Ultrasound
- transducer
Frequency range <600MHz
Earthed case
Co-axial cable
Piezo-electric crystal
Plastic
cover
Silver electrodes
Backing material
Input/output
p.d.
Pulse of high frequency p.d.
Ultrasound pulse
Reflected pulse
Acoustic Impedance
• Intensity of a reflected pulse depends on
differences in density
across a boundary
• Acoustic impedance,
Z =
c
where:
= density
(kgm
-3
)
Acoustic Impedance
Intensity of the reflected pulse:
I
R
= I
0
(Z
2
– Z
1
)
2
(Z
2
+ Z
1
)
2
Medium 1 (
1, Z
1) Medium 2
(
2, Z
2)
I
0I
TTransmission
Acoustic Impedance
Medium
Z =
c / kgm
-2
s
-1
Air
430
Quartz
1.52 x 10
7
Water
1.50 x 10
6
Blood
1.59 x 10
6
Fat
1.38 x 10
6
Muscle
1.70 x 10
6
Soft tissue
1.63 x 10
6
Bone
5.6 – 7.8 x 10
6
See also
Table 7.1
(page 58,
Questions
SAQ 7.1 (Health Physics, Page 59)
Calculate the ratio of sound
reflected
at the boundary
between muscle and soft tissue, using the data from
table 7.1 (page 58).
Extension
Calculate the ratios across
(i) an air-tissue boundary
(ii) a bone-tissue boundary
Ratio = 0.000921
(Virtually ALL the energy is transmitted)
Coupling Gel
Air-tissue boundaries reflect
most of the ultrasound energy
A
coupling gel
is applied
Absorption Of Ultrasound
I = I
0
e
-kx
I
0
= incident intensity
(Wm
-2
)
k = absorption coefficient
(m
-1
)
x = thickness of medium
(m)
Incident Intensity, I
0Transmitted intensity, I
Absorption Co-efficient
Medium
k at 1MHz / m
-1
Air
120
Water
0.02
Muscle
23
Bone
130
Transmission pulse
Ultrasound A-scan
• Similar to early RADAR
systems (radio waves)
Timebase
(calibrated to give distance)
Skull echoes
Mid-line echo
from median
fissure at centre
of brain
Scalp-air echo
Skull-tissue echo
Inner skull echo
Ultrasound
A-scan
Ultrasound B-scan
P. Lovatt & Hari.R 88/122
Doppler Ultrasound
Doppler image of umbilical cord artery & vein
www.centrus.com.br/.../chapter_01.htm
Doppler Ultrasound
Measuring blood flow
Ultrasound in use
• Essentially
Safe
– to
patient and operator
• Portable & low power
• Can obtain
images in 3D
and in
real-time
• Can analyse blood flow using
doppler effect
Ultrasound Therapy
Ultrasound Questions
Health Physics textbook
SAQ 7.2 (Page 61)
(ultrasound v X-rays)
SAQ 7.3
(Doppler effect)
Questions (Page 62)
RADAR
Imaging
Similar
SONAR
Imaging
http://surveying.wb.psu.edu/psu-surv/
SURIs/hydrographic_surveying.htm
Syllabus Section VII.29
Magnetic Resonance Imaging (MRI)
(l) explain the main principles behind the use of magnetic resonance
to obtain diagnostic information about internal structures.
(m)
show an understanding of the function of the non-uniform
magnetic field, superimposed on the large constant magnetic
field, in diagnosis using magnetic resonance.
Note:
The technology is called
Nuclear Magnetic Resonance
imaging (NMR)
. However, because of the negative associations
with the word
‘
nuclear
’
, hospitals prefer to use the term
MRI
MRI
A brain scan showing a tumour
A hospital MRI scanner
MRI Scanner
• Powerful magnet 0.5-3.0T (60T in research)
• Non-linear (gradient) field
• RF generator coil
• RF received
MRI Theory
• The human body is 60% hydrogen
• Hydrogen (single proton) has a high
MRI Theory
All of the hydrogen protons
will align with a strong
magnetic field in one direction
or the other.
The vast majority cancel each
other out, but in any sample
there is not complete
No applied B-field
(random, no net
alignment)
With applied
magnetic field
(small net dipole)
MRI Theory
• An Radio Frequency (
RF
) pulse is
generated using a coil
• It causes the uncancelled aligned
proton spin axes
to
precess
around
the
field axis
at certain frequencies
• When the pulse turns off, the
protons ‘
decay
’
back to their
original state, releasing RF energy
• The returning RF energy is detected
Protons have random spin orientation
Protons align with the field
MRI Theory
Gradient magnets
vary the field (and
the return RF
frequencies detected)
allowing emissions
MRI Safety
• Non-invasive
• Safe, except for patients with
metal objects inside their bodies
e.g. pacemakers, metal implants, shrapnel, bullets,
surgical staples etc
Functional
MRI
(fMRI)
MRI Advantages
Non-ionizing radiation
Because MRI does not ionize tissue it is considered amongst the safest of radiological techniques. There are no known physiological side effects of being exposed to a magnetic field.High soft-tissue contrast
MRI images provide very detailed information about soft tissues. They can differentiate between normal and abnormal tissues and may show damage missed on CTImages can be produced in any plane This is of great value in studies of the Central Nervous System.
Visualisation of areas deep within bony structures
MRI is thus invaluable for the diagnosis and treatment of brainstem tumours.
Shows vasculature (veins and
arteries) without contrast media
Because the patient is not subject to any invasive procedure (i.e. the injection of contrast media) the technique is less unpleasant.
Good for angiography
MRI is excellent for imaging blood flow and studying heart
function. Functional MRI maps changes in blood flow in the brain during specific tasks. This provides valuable information about how the brain works.
MRI Disadvantages
High cost of equipment An MRI scanner can cost over GB£1,500,000 (US$ 3,000,000) to purchase.
Claustrophobia Up to 10% of patients experience claustrophobia during an MRI scan and 1% of scans have to be aborted because of it.
Long imaging time
A complete image may take up to 30mins and movement at the wrong time can cause artefacts in the image. New techniques have reduced imaging time.Strong magnetic field
MRI imaging is unsuitable for many patients with metal implants (e.g. artificial joints) and is especially dangerous for patients with pacemakers, neural
stimulators, or cochleal implants. Loose metal objects must be removed before coming near to the scanner otherwise they may be attracted so strongly by the magnet that they fly through the air towards the patient like tiny missiles!
Unable to image calcium Because MRI detects water rather than molecular density, calcium is not well visualized. This means that tissue calcification, a feature of a number of disease processes, can not be detected. Bone is also less obvious than on a CT scan.
Acoustic noise
Switching on and off of the gradient coils causes repeated loud bangs. Noise levels may reach 95 dB for much of the scan. Ear plugs are advisable to reduce the risk of temporary or even permanent hearing loss.Various minor biological effects
