X Ray Physic

X-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: plank’s 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).

X-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 type DT: transmission density H: exposure R: useful density range (2) L: useful exposure range (<0.5)

X-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

X-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 shorter l (graph b)

X-RAY TUBE 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 shorter ls the beam is more penetrating!!

X-RAYS Attenuation by absorption I

Io

x I = I o e -mx m: linear attenuation coefficient

I x

X-RAYS Half 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)

put

I = Io / 2

I o / 2 = I oe

-mx

for

x = 1/ 2

divide by

1 / 2 = e -mx

apply loge

log e (1 / 2) = -mx

solve for x

x=

log e 2 m

I o / 2 = I o e -mx

X-RAYS Half 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)

put

I = Io / 2

I o / 2 = I oe

-mx

for

x = 1/ 2

divide by

1 / 2 = e -mx

apply loge

log e (1 / 2) = -mx

solve for x

x=

log e 2 m

I o / 2 = I o e -mx I 1/2

x

X-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

X-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 muscle and thus gives better contrast.

SHADOWGRAPHS • Most usual mode of radiographs - skiagraphs • The image relies on different absorption of tissue that 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 of compounds with different Z introduced to give shadows. Examples: Barium Sulphate (BaSO4) to image stomach, intestines Iodine (I2) for blood.

SHADOWGRAPHS X

S

Film

X-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

CAT SCANNING - 1st Generation

X: X-ray source S: subject D: detector • Rotation in X intervals • Time ~ 4 min!!!!

CAT SCANNING - 2nd Generation

• Single source with narrow fan of detectors which traversed and rotated. • Time ~ 20 sec.

CAT SCANNING - 3rd Generation • Moving source with more detectors. • Time ~ 4-5 sec.

CAT SCANNING - 4th Generation • Stationary 360 degree ring of detectors and a moving source. • Time ~ 1 sec.

CAT 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

NUCLEAR 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 the gamma camera.

NUCLEAR MEDICINE - MEASURES • Half-life (t 1/2 ) of the radiochemicals (ie. measure of radiochemicals) is calculated, in disintegrations s-1 (1Bq=1 s-1 ), by: t 1/2 = loge2/l

where l is the decay constant

• Biological effects are measured by the absorbed dose D, in J kg-1 or gray Gy, by: D=energy/mass • Damage effect is measured as a quality factor Q, eg:for x-ray, g-ray and b particles Q=1, for slow neutrons Q=3, for a particles Q=10.

NUCLEAR 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

NUCLEAR MEDICINE RADIOACTIVE TRACERS • A variety of radioactive tracers (isotopes) provide diagnostic information for 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

NUCLEAR MEDICINE GAMMA 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

GAMMA CAMERAS SIMPLE COLLIMATOR Designed to measure overall activity (eg. From the thyroid) K: shielding V: output signal B: background J: subject g: g ray emission D: detector (usually a PMT or SPD)

GAMMA CAMERAS RECTILINEAR SCANNER TYPE Includes a lead septa that gives a focal spot for scanning action by single probe method to synthesize a picture at low resolution. Q: septa F: focal spot L: lightguide x,y: scanning directions

GAMMA CAMERAS THE ‘USUAL’ GAMMA CAMERA Is a fixed array of multiple detectors (PMTs* or SPDs*), with a pinehole aperture. The computed output is viewed on CRTs real time. P: Pinhole aperture M: multiple detectors *Photomultipliers,Silicon Photodiodes

GAMMA CAMERAS ROTATING TYPE FOR EMISSION TOMOGRAPHY A scanning system records sector scans 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-crystal scanner that has better resolution A: multiple position scans B: array of multiple detectors

ULTRASOUND IMAGING • Uses frequencies greater than 20kHz (ie. the audible limit) such as 1 MHz for biomedical diagnostic use. • The method depends on the detection of reflections 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.

ULTRASOUND - 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 resolution is 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:

I = I o e -mk x Where k is the attenuation coefficient and x the thickness.

ULTRASOUND 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=ct D: range, c:velocity, t:time of travel

ULTRASOUND SCANNING 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: transducer J: jelly T:transmitter, R:receiver P: pulse rate generator Q: time base generator K: CRT display

ULTRASOUND SCANNING TECHNIQUES B type scan (moving)

• Uses a ‘rocked’ transducer and pulsed emissions to increase the probability of obtaining normal reflections. • Applications: tumours and stones. Positions 1-3 to give composite image.

ULTRASOUND SCANNING TECHNIQUES Time Position (TP) type scan • It is a modified B scan type suitably pulsed with low speed time. • Applications: determination of foetal heart rates.

Doppler type scan • 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