Physics G485 Revision

A2 PHYSICS: G485 - PIGUY
MEDICAL
IMAGING
>>REVISION>>
X - R AY S
• created when high energy electrons from
thermionic emission strike metal target &
rapidly decelerate
• Also when electrons make transitions
between lower atomic energy levels
I = I0 "
differing attenuation coefficients
• barium increases μ of soft tissue, allowing
distinction of it e.g. intestines
-μx
Intensity is constant in
collimated beam
INTERACTION
photoelectric effect
• intensity is power per cross-sectional area
• x-ray image occurs when tissues have
Key
I - intensity
μ - attenuation
coefficient
x - distance
INTENSITY
e
e
compton effect
e
e
pair production
n
ee+
• image intensifier contains a scintillator
sodium iodide crystal, for every ɣ photon
absorbed it emits thousands of visible
light photons, so less exposure needed
& response time is quicker
IMAGE INTENSIFIER
plastic coating
double sided film
metal backing
intensifying
screens
DIAGNOSIS
METHODS I
COMPUTER IMAGING
CAT
x-ray slice
PET
ɣ detector
3D
• medical tracers!
1. radioactive tracer e.g. technetium-99 is
bonded to biologically active molecule
e+ e-
guy
2. injected and absorbed by organ
detector
3. ɣ camera detects & builds 2d image
1. lead tubes absorb all photons except
those travelling along the axis of tubes
2. sodium iodide crystal as scintillator
absorbs a ɣ photon and emits
thousands of visible light photons
3. photomultiplier tubes detect visible
light, produces an electrical signal
4. image quality improved by narrower
lead tubes or longer scanning time
GAMMA CAMERA
ɣ
photolead multiplier
dynodes
NaI
e-
photocathode
to computer
• gamma camera
ɣ detector
DIAGNOSIS
METHODS II
MRI SCANNERS
• non-invasive techniques (no surgery)
x-rays
CAT
+ve
-ve
low exposure to x-ray;
quick
ionising radiation;
can’t see tissues with
similar attenuations
differentiates between
x-ray exposure v. high;
soft tissues; scans takes
ionising radiation
seconds; high res. 3d
PET
high res. 3d; shows
function of organs;
ionising radiation;
MRI
no ionising radiation;
high res. 3d
slow (~45 minutes)
no metal implants;
can’t see functions
ultrasound
no ionising radiation;
shows movement (real
time images)
lower res 2d; does not
reflect off bone
1. nuclei with uneven no. of nucleons
possess spin, act like tiny magnets
B
S
N
2. they experience a torque
and precess in a magnetic
field
3. this occurs at the
Larmor frequency
4.25x107 x B
4. by resonance, it absorbs energy if radio
waves of Larmor frequency is received
5. so nuclei flip from low to
high energy state & drop
back down after time
B
B
6. relaxation time indicates tissue
7. gradient of magnetic field at all points
varies Larmor frequency indicates position
U LT R A S O U N D
U LT R A S O U N D
• ultrasound (>20kHz) is produced by the
piezoelectric effect
1. alternating pd applied to piezoelectric
crystal (quartz) causes it to oscillate.
2. alternating pd is at resonant frequency
to maximise amplitude
3. epoxy resin backing prevents backwards
propagation of ultrasound & rapidly damps
oscillations after alternating pd stops
Z= c
2
I
r
(Z
Z
)
__ = _______
2
1
I0
(Z2 + Z1)2
!
A-scans are voltage
peaks, where
reflected ultrasound
creates a p.d. by the
piezoelectric effect
!
B-scan is multiple Ascans for 2d image
• ultrasound scanning
1. transducer is used to send pulses of
ultrasound into the patient.
Key
f - frequency
c - speed of ultrasound
v - speed of blood
towards source
I - intensity
z - acoustic impedance
- density of tissue
DOPPLER
2. pulses are reflected at tissue boundaries
3. intensity of the reflected pulse depends
upon the acoustic impedance
4. delay time of the retuning pulse is used
to determine the depth
f
___
f’ = __
2v
1-
c
Change of reflected
ultrasound frequency
indicated speed of the
blood flow.