MEDICAL IMAGING

• Polarisable

• Diffracted by atoms in crystals

• Short wavelengths (10^-8 to 10^-13 m)

• Electromagnetic waves

• Travel through a vacuum at the speed of light, 3 x 10⁸ms^-1

Decrease in the intensity of electromagnetic radiation as it passes through matter/ space

• For photons with energy of 1 - 20 keV

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1. Photon interacts with an electron in the atom

2. Energy of the photon < energy required to remove the electron

3. Photon bounces off

4. No change to its energy

• For photons with energy < 100 keV

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1. Photon is absorbed by an electron in the atom

2. Electron uses this energy to escape from the atom

• For photons with energy ≥ 1.02 MeV

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1. Photon interacts with the nucleus of the atom

2. Disappears

3. Energy used to create an electron and positron

• For photons of energy 0.5 - 5 MeV

• Energy and momentum conserved

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1. Photon interacts with an electron in the atom

2. Electron is ejected from the atom

3. Photon bounces off

4. Reduced energy

I = I_oe^-μx

• I = transmitted intensity

• I_o = initial intensity

• μ = attenuation/ absorption coefficient

• x = thickness

Soft tissues have low absorption coefficients

Contrast media improve the visibilty of their internal structures

Relatively harmless

• ↑ atomic number = ↑ attenuation coefficient (Z³ ∝ µ)

• Contrast medium in the blood

• Injected into blood vessels

• ↑ atomic number = ↑ attenuation coefficient (Z³ ∝ µ)

• Contrast medium in digestive systems

• Barium meal = white liquid mixture swallowed by the patient

Linear accelerators create high-energy X-ray photons

Photons kill off cancerous cells

Through Compton scattering and pair production

Table moves through a gantry containing an X-ray tube and detectors opposite

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Tube produces a fan-shaped beam of X-rays

Irradiates a slice of the patient

X-rays attenuated by different amounts by different tissues

Detectors record intensity of transmitted X-rays

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Tube and detectors make a 360^o rotation

Two-dimensional image acquired

Table moved slightly through the ring

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Images manipulated by a computer to produce a three-dimensional image

Disadvantages

• Patient must remain still- any movement blurs the image

• Expensive

• X-rays are ionising radiation- harmful

• Prolonged- exposes the patient to a high radiation dose

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Advantages

• Three-dimensional image- allows doctors to assess the dimensions/ position of disorders

• Distinguishes between soft tissues of similar attenuation coefficients

Lead-lined to protect radiographer from ionising radiation

Evacuated tube- electrons pass through without interacting with gas atoms

Large potential difference between two electrodes

Cathode produces electrons by thermionic emission

Accelerated towards anode (target metal)

Collisions produce photons

Emitted through a window

Energy ouptut of photons < 1% kinetic energy of electrons

Remainder of energy transferred to thermal energy of anode

Oil circulated to cool anode

λ = hc / eV

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↑ potential difference = ↓ wavelength

↑ current = ↑ intensity

• Gamma photons

• Least ionising- safe to place inside patient

• Most penetrating- can be detected externally

• ↓ t_1/2 for ↑ A

• Little is required

• Patient not subjected to high dosage of radiation continuing long after procedure

• Produced artificially

⁹⁹₄₂Mo → (67 h) ^99m₄₃Tc + ⁰_-1e + v_e

→ (6 h) ⁹⁹₄₃Tc + ɣ

→ (2.1 x 10⁵ y) ⁹⁹₄₄Ru + ⁰_-1e + v_e

Radioisotope chemically combined with elements

Injected into the patient

Targets particular tissues

Ensures the radioisotope reaches the correct organ/ tumour

Progress through the body traced using gamma camera

Concentration used to identify irregularities in function of body

Collimator

• Honeycomb of long, thin tubes made of lead

• Absorbs photons arriving at an angle to the axis of the tube

• Ensures image is clear

Scintillator

• Sodium iodide

• Produces photons of visible light when struck by a single gamma photon

Light guide

Photomultiplier tubes

• Convert a single photon of visible light into an electrical pulse

X-ray: anatomy of the body

Gamma camera: function/ processes of the body

Decays

• ¹⁸₉F → (110 mins) ¹⁸₈O + ⁰₁e + ɣ

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Produced at the hospital using a particle accelerator

• ¹₁p + ¹⁸₈O + ¹⁸₉F + ¹₀n

FDG

• Glucose tagged with a fluorine-18 atom in place of an oxygen atom

• Body treates it like normal glucose

• Accumulates in tissues with a high rate of respiration

• Activity monitored

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Carbon monoxide

• Carbon-11 emits a positron

• Clings onto haemoglobin molecules in red blood cells

• Concentration monitored

Advantages

• Non-invasive

• Identify the onset of certain brain disorders

• Asses the effect of new drugs/ medicines on organs

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Disadvantages

• Facilities required to roduces the radiopharmaceuticals are expensive

• Found only at large hospitals

• Only for patients with complex health problems

• Audible range = 20 Hz - 20 kHz

.....Ultrasound = > 20 kHz

• Short wavelength- can identify features a few millimetres small

• Longitudinal waves

• Can be reflected, refracted and diffracted

• Non-ionising

• Non-invasive

Production of an electromotive force

...by some crystals such as quartz

......when they're compressed/ stretched/ twisted/ distorted

Reversible

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Applying forces

• Compressed: electromotive force produced

• Stretched: electromotive force of opposite polarity induced

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Applying voltages

• External voltage applied: compressed

• External voltage of opposite polarity applied: stretched

Emits ultrasound

• Alternating voltage applied across ends of crystal

• Crystal compressed + stretched

• Chosen frequency = natural frequency of crystal

• Crystal resonates and produces ultrasound signal

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Detects utrasound

• Ultrasound incident on crystal makes it vibrate

• Crystal compressed + stretched

• Generates alternating electromotive force across ends of crystal

• Detected by electronic circuits

A = amplitude

Determines thickness of bone/ distance between lens and retina

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Single transducer emits ultrasound pulses

Each pulse partly reflected and transmitted at boundary between two different tissues

Reflected pulse recieved by transducer with less energy

Pulsed voltage displayed on oscilloscope as V-t graph

Amplitudes attenuated

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t = time taken for pulse to travel from transducer to point of reflection and then back to transducer

Total distance travelled = 2L

L = distance between transducer and point of reflection = vt / 2

B = brightness

Series of A-scans

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Transducer moved over the patient’s skin

Output connected to a computer

At each position, the computer produces a row of dots on the screen

Dot = boundary between two different tissues

Brightness ∝ intensity of the reflected ultrasoud pulse

Produces a two-dimensional image

Z = ρc

• Z = acoustic impedance of the substance

• ρ = density of the substance

• c = speed of ultrasound in the substance

I_r / I_o = (Z₂ - Z₁)² / (Z₂ + Z₁)²

• I_r = reflected intensity

• I_o = incident intensity

• Z₁ = acoustic impedance of substance 1

• Z₂ = acoustic impedance of substance 2

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↑ difference in acoustic impedance = ↑ reflection

Same acoustic impedance = no reflection

Air pockets trapped between transducer and skin

99.9% incident ultrasound reflected at air-skin boundary

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Acoustic impedance of coupling gel similar to that of skin

Smeared on transducer and skin

Fills air pockets

Negligible incident ultrasound reflected

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= Acoustic impedance matching

Transducer on skin above blood vessel

Emits pulses of ultrasound and recieves reflected pulses

Reflected off tissues: constant frequency and wavelength

Reflected off moving blood cells: changed frequency

• Moving towards transducer = ↑ frequency

• Moving away from transducer = ↓ frequency

Transducer connected to computer

Colour-coded image produced to show direction/ speed of blood flow

Δf = 2fvcosθ / c

• Δf = change in the observed ulrasound frequency

• f = original ultrasound frequency

• v = speed of the moving blood cells

• θ = angle to the blood vessel

• c = speed of the ultrasound in blood

FDG injected into patient

Patient surrounded by diametrically opposite gamma detectors

Increased activity where fluorine-18 accumulates

Positrons from fluorine-18 annihilate electrons inside patient

Each annihilation produces two gamma photons travelling in opposite directions

Difference in arrival times used by computer to determine point of annhilation

Different concentrations of FDG show as areas of different colours/ brightness on image