X Ray Physic
Transcript of X Ray Physic
-
8/14/2019 X Ray Physic
1/33
X-RAYSX-RAYS
X-ray (or radiographic images) use the x-ray part of the em spectrum to
expose the subject
short wavelength (l): < 1 nm (more than x10000 shorter than visible) high energy (E=hn=hc/l - n=c/l): > 1keV
high frequency (n=c/l)
E: energy (in eV)l: wavelength
h: planks constant
c: speed of light
n: frequency
Rem: eV is the energy that an e- acquires when accelerated one meter
distance in an electric field of a potential difference of 1 V
(e is charge and V is potential difference).
-
8/14/2019 X Ray Physic
2/33
X-RAY FILMX-RAY FILM
Contains a blue-green sensitive emulsion
Large crystals
Gives optical density related to input exposure via the H&D curve
Large tonal (dynamic range) 1000:1 instead of 100:1 for norm. films
A:screen exposure type
B:direct exposure typeDT: transmission density
H: exposure
R: useful density range (2)
L: useful exposure range (
-
8/14/2019 X Ray Physic
3/33
X-RAY TUBEX-RAY TUBE
Basic x-ray tube is a vacuum tube containing a tungsten filament (anode),
bombarded by e- accelerated by kV. Photons are created from this process
The emission x-ray spectrum has a continuum and line (spike) characteristics
-
8/14/2019 X Ray Physic
4/33
X-RAY TUBEX-RAY TUBE
Spectral properties are changed by: altering kV (electric field ie. speed of the e-)
or mA (current to filament)
Increased mA increases intensity (graph a)
Increased kV gives higher penetration - because of shorterl (graph b)
-
8/14/2019 X Ray Physic
5/33
Use of filter of various materials (Pb, Cu, Al) between source and subject
attenuates wavebands and removes low energy ls (soft x-rays)
which produce scattering. Because of shorterls the beam is more penetrating!!
X-RAY TUBEX-RAY TUBE
-
8/14/2019 X Ray Physic
6/33
Io
x
I
xoeIIm-
=
m: linear attenuation coefficient
x
I
X-RAYSX-RAYSAttenuation by absorption
-
8/14/2019 X Ray Physic
7/33
X-RAYSX-RAYSHalf Value Thickness (HVT)
HVT is the beam power measured as thickness (x) of material
to reduce Io to Io/2 (I.e. x where the intensity is decreased by 2)
2/12/ == xorIo
put
xoo eII
m-=2/
x
e
m-=
2/1
divide by xoo eII
m-=2/
apply loge
xe m-=)2/1(log solve for x
m=
2logex
-
8/14/2019 X Ray Physic
8/33
X-RAYSX-RAYSHalf Value Thickness (HVT)
2/12/ == xforo
put
xoo eII
m-=2/
xe m-=2/1
divide by xoo eII
m-=2/
apply loge
xe m-=)2/1(log solve for x
m=
2logex
x
I
1/2
HVT is the beam power measured as thickness (x) of material
to reduce Io to Io/2 (I.e. x where the intensity is decreased by 2)
-
8/14/2019 X Ray Physic
9/33
X-RAYSX-RAYS
The attenuation mechanism is mostly related to Z (atomic number)
of the material. It includes:
Releigh scattering (proportional to Z2)
Compton scattering (no dependence on Z)
Photoelectric effect (proportional to Z3)
Pair production (proportional to Z2)
For a fixed mA the intensity I is related to the focus to film distance d by
the inverse square low: I=Io/d2
-
8/14/2019 X Ray Physic
10/33
X-RAYSX-RAYS
Z (atomic number) for:
fat ~ 5.9
muscle ~ 7.4
bone ~ 13.9
Attenuation ratio for bone to muscle is 13.9/7.4= 6.6
I.e. Bone attenuates 6.6. (~7) times more than muscleand thus gives better contrast.
-
8/14/2019 X Ray Physic
11/33
SHADOWGRAPHSSHADOWGRAPHS
Most usual mode of radiographs - skiagraphs
The image relies on different absorption of tissuethat is transparent, but there is a large change in Z
(from air to bone gives a good contrast)
Tissue would not be easily differentiated without the aid ofcompounds with different Z introduced to give shadows.
Examples: Barium Sulphate (BaSO4) to image stomach, intestines
Iodine (I2) for blood.
-
8/14/2019 X Ray Physic
12/33
SHADOWGRAPHSSHADOWGRAPHS
S
X
Film
-
8/14/2019 X Ray Physic
13/33
X-RAY TOMOGRAPHYX-RAY TOMOGRAPHY
Image of slices (tomi) of the body. Now an obsolescent
technique developed to localise an internal site (depth
localisation) in a body. Used instead of stereo radiography
that uses 2 tubes or two exposures.
Detail in plane
K is sharp
-
8/14/2019 X Ray Physic
14/33
CAT SCANNING - 1st GenerationCAT SCANNING - 1st Generation
X: X-ray sourceS: subject
D: detector
Rotation in X intervals Time ~ 4 min!!!!
-
8/14/2019 X Ray Physic
15/33
CAT SCANNING - 2nd GenerationCAT SCANNING - 2nd Generation
Single source with narrow fan of detectorswhich traversed and rotated.
Time ~ 20 sec.
-
8/14/2019 X Ray Physic
16/33
CAT SCANNING - 3rd GenerationCAT SCANNING - 3rd Generation
Moving source with more detectors.
Time ~ 4-5 sec.
-
8/14/2019 X Ray Physic
17/33
CAT SCANNING - 4th GenerationCAT SCANNING - 4th Generation
Stationary 360 degree ring of detectors
and a moving source.
Time ~ 1 sec.
-
8/14/2019 X Ray Physic
18/33
CAT SCANNING - 5th GenerationCAT SCANNING - 5th Generation
Uses no moving parts.
Tube with the patient inside 210 deg.
The detector ring is similar.
An e- beam scans around the body
in multiple adjacent tracks to generate
x-rays.
Time ~ 0.1s to a few ms or real time
-
8/14/2019 X Ray Physic
19/33
NUCLEAR MEDICINENUCLEAR MEDICINE
Uses ingested or injected radioisotopes
Measurement of the distribution and concentration shows abnormalities
Uses include Radiotherapy and Diagnosis by:
tracers (showing the functions of the organs) or
imaging(picture of an organ)
Type of emitters: a particles - not detectable outside the body
b particles - very damaging
g-rays - very penetrating, not damaging (low radiotoxicity)
Detection and imaging is with the aid of thegamma camera.
-
8/14/2019 X Ray Physic
20/33
NUCLEAR MEDICINE - MEASURESNUCLEAR MEDICINE - MEASURES
Half-life (t 1/2 ) of the radiochemicals (ie. measure of radiochemicals) iscalculated, in disintegrations s-1 (1Bq=1 s-1 ), by:
t 1/2 = loge2/l
Biological effects are measured by the absorbed dose D,in J kg-1 or gray Gy, by:
D=energy/mass
Damage effectis measured as a quality factor Q, eg:for x-ray, g-ray and b
particles Q=1, for slow neutrons Q=3, fora particles Q=10.
where l is the decay constant
-
8/14/2019 X Ray Physic
21/33
NUCLEAR MEDICINE - MEASURESNUCLEAR MEDICINE - MEASURES
Dose equivalent, in Sv (sievert), is calculated by:
Dose equivalent= D*Q
Annual significant natural dose ~ 1-3 msV
Additional artificial dose ~ 0.25 mSV
-
8/14/2019 X Ray Physic
22/33
NUCLEAR MEDICINENUCLEAR MEDICINE
RADIOACTIVE TRACERSRADIOACTIVE TRACERS
A variety of radioactive tracers (isotopes) provide diagnostic informationfor specific purposes (e.g. blood, urine, organs, liver functions, tumours, etc.).
The effective half-life of a g-emitter (Te) is related by its half-life (Tr)and its biological half-life (Tb) by the relationship:
1/ Te = 1/Tb + 1/Tr
-
8/14/2019 X Ray Physic
23/33
NUCLEAR MEDICINENUCLEAR MEDICINE
GAMMA CAMERASGAMMA CAMERAS
They use a scintillator as the detector (with excellent quantum
efficiency) for counting the incident g radiation.
The scintillator emits light
The light emitted is amplified by a photomultiplier tube
-
8/14/2019 X Ray Physic
24/33
GAMMA CAMERASGAMMA CAMERAS
SIMPLE COLLIMATORSIMPLE COLLIMATOR
Designed to measure overall activity
(eg. From the thyroid)
K: shielding
V: output signalB: background
J: subject
g: g ray emission
D: detector (usually a PMT or SPD)
-
8/14/2019 X Ray Physic
25/33
GAMMA CAMERASGAMMA CAMERAS
RECTILINEAR SCANNER TYPERECTILINEAR SCANNER TYPE
Includes a lead septa that gives a
focal spot for scanning action by singleprobe method to synthesize a picture
at low resolution.
Q: septa
F: focal spot
L: lightguide
x,y: scanning directions
-
8/14/2019 X Ray Physic
26/33
GAMMA CAMERASGAMMA CAMERAS
THE USUAL GAMMA CAMERATHE USUAL GAMMA CAMERA
Is a fixed array of multiple detectors
(PMTs* or SPDs*), with a pineholeaperture. The computed output is
viewed on CRTs real time.
P: Pinhole aperture
M: multiple detectors
*Photomultipliers,Silicon Photodiodes
-
8/14/2019 X Ray Physic
27/33
GAMMA CAMERASGAMMA CAMERAS
ROTATING TYPE FOR EMISSIONROTATING TYPE FOR EMISSION
TOMOGRAPHYTOMOGRAPHY
A scanning system records sectorscans in a variety of planes which are
then combined by S/W to give a
composite view. It uses a rotating
gamma camera or a multi-crystalscanner that has better resolution
A: multiple position scans
B: array of multiple detectors
-
8/14/2019 X Ray Physic
28/33
ULTRASOUND IMAGINGULTRASOUND IMAGING
Uses frequencies greater than 20kHz (ie. the audible limit) such
as 1 MHz for biomedical diagnostic use.
The method depends on the detection ofreflections of about 1%
in magnitude at body tissue.
Applications include: brain scan, foetal size and development,
cardiography, tissue abnormalities, stones and others.
Advantages are that: it can differentiate different type of tissue,
no tissue damage, no side-effects.
-
8/14/2019 X Ray Physic
29/33
ULTRASOUND - PROPERTIESULTRASOUND - PROPERTIES
Its velocity c, depends on the medium density, measured in
m s-1 (see table on the back of the handout).
Sound image resolution depends on l. Increased resolutionis achieved by reducing l, but penetration is decreased.
Reflection strength depends on density mismatch in the body,
e.g. between bone and muscle. The presence of air blocks
aids the transmission of the ultrasound. Use of a jelly between the transducer and the skin to assist the
transmission of ultrasound.
The absorption of ultrasound in tissue depends on frequency, temperature,
density etc and is given by:
xoeII
m-=
k
Where k is the attenuation coefficient and x the thickness.
-
8/14/2019 X Ray Physic
30/33
ULTRASOUND GENERATIONULTRASOUND GENERATION
Sonar Contains a transmitter
and a receiver (T).
Voltage is applied to
emit ultrasounds.
Received energy is
converted back to voltage.
Emits beams of variable
widths (pulses).
Note that otherwise we can
generate continuous ultrasound.
D=ctD: range, c:velocity, t:time of travel
-
8/14/2019 X Ray Physic
31/33
ULTRASOUNDULTRASOUND
SCANNING TECHNIQUESSCANNING TECHNIQUES
A type scan Measures with a static transducer.
The echo time is measured by
synchronizing the CRT display to
the transceiver.
Applications: brain scan.
S: subject
C: transducerJ: jelly
T:transmitter, R:receiver
P: pulse rate generator
Q: time base generatorK: CRT display
-
8/14/2019 X Ray Physic
32/33
ULTRASOUNDULTRASOUND
SCANNING TECHNIQUESSCANNING TECHNIQUES
B type scan (moving) Uses a rocked transducer and
pulsed emissions to increase theprobability of obtaining normal
reflections.
Applications: tumours and stones.
Positions 1-3 to give composite
image.
-
8/14/2019 X Ray Physic
33/33
ULTRASOUNDULTRASOUND
SCANNING TECHNIQUESSCANNING TECHNIQUES
Time Position (TP) type scan Doppler type scan
It is a modified B scan type suitably
pulsed with low speed time.
Applications: determination of foetal
heart rates.
Uses continuous waves.
Frequency changes due to the velocity
(V) of the subject or the source.
Movement towards the receiver
gives a higher frequency.
Frequency change Dn=2nV/c
n: frequency
V: velocity of subject or source
c: velocity of ultrasound
Applications: heart functions, blood flow