Medical Physics

Question 0

How are X-rays produced?

Answer 0

+X-ray tube. Electrons are accelerated from the heated cathode–>the anode.
+High p.d. between electors, so they hit the anode w/ high speeds/energy.
+When they collide w/ anode, electrons decelerate rapidly.
+K.E. converted to X-rays.
+X-rays characteristics or target metal.
+1% of K.E. used to make X-ray photons, 99% heats target.
+Oil circulation cools anode by convection.

Question 1

What is an x-ray? Describe their nature.

Answer 1

Powerful, short wavelength electromagnetic radiation. Wavelength ~10^-10m, frequency ~10^18Hz.

Question 2

What are the three ways that X-rays can interact with matter?

Answer 2

The photoelectric effect, the Compton effect (scattering, and pair production.

Question 3

What is the photoelectric effect?

Answer 3

Happens when the energy is binding energy of electron in atom.
+Then electrons emitted from atom as photoelectrons.

Question 4

What is the Compton effect (scattering)?

Answer 4

+X-ray hits free electron at rest.
+Election recoils, X-ray photon deflected through angle 0.
+The greater the angle of deflection 0, the greater the loss of energy.
+So the change in wavelength is greater.

Question 5

What is pair production?

Answer 5

When energy is >1.02MeV.
+High energy X-ray photon interacts with electric field of nucleus.
+Electron-positron pair emerge.
+Energy lost by ionisation.
+Positron annihilates electron producing two identical photons.
+Not significant in diagnostic X-ray imaging as high energies required.

Question 6

How do you find intensity?

Answer 6

I=Power/Cross-sectional area.

Question 7

How do you find the change in intensity I of a collimated X-ray beam in different mediums.

Answer 7

I=I0e^-mx.
Where the attenuation coefficient depends on the energy of an X-ray photon and proton number of the material it passes through.
Exponential.

Question 8

How do X-rays image internal body structures?

Answer 8

+X-ray photons penetrate patient.
+Bone is denser than soft tissue–>attenuation of bone is greater.
+Less X-rays reach film under bone giving a shadow effect.
+Intensity of X-rays proportional to darkening of film.

Question 9

How do X-ray image intensifiers work?

Answer 9

+X-ray photons hit and are absorbed by fluorescent crystals in intensifying layer.
+Atoms become excited, re-emitting energy as visible light photons as they return to ground state.
+Visible light photons hit photographic film.
+Film is more sensitive to light photons–>gives greater blackening and contrast.
Pros: less x-rays needed–>less exposure to patient.

Question 10

How can soft tissues like the intestines be imaged using a barium meal?

Answer 10

+X-rays don’t differentiate soft tissues well.
+Soft tissues have low and similar Z values, but contrast medium has high Z value (attenuation is proportional to Z^3).
+So it absorbs X rays strongly.
+Barium is ingested/injected into the body, and the outline of organs can clearly be seen.
+Used to image digestive tract/throat/stomach.

Question 11

Describe the operation of a computerised axial tomography scanner (CAT scan).

Answer 11

+X-ray source and detectors placed around the patient.
+X-ray source shielded so that the rays emerge from a point & spread through patient, thin & fan shaped to produce thin slice.
+Source is rotated around patient & passed through same section of body from many angles.
+Thin slice of cross section produced & image stored.
+Patient moved small distance, repeat.
+Computer analyses & produces a 3-D image.

Question 12

What is the advantage of a CAT scan?

Answer 12

+Can image bone, soft tissue and blood vessels.
+Provides very detailed image of many types of tissue.
+Can be taken quickly –> suitable for emergencies.
+Low dose of radiation required due to sensitivity of sensors.

Question 13

Describe the use of a medical tracer like technetium-99 to diagnose the function of organs.

Answer 13

+Radioactive tracer is injected into the body.

+Tracer emits gamma rays, so its path through the body can be monitored.

Question 14

Describe the properties of a medical tracer like technetium-99.

Answer 14

+MUST decay predominantly by gamma radiation, as alpha and beta are too damaging and would be absorbed by the body.
+Half-life must be long enough for diagnosis to take place but short enough not to remain in the body for too long.
+Must be possible to monitor.
+NOT chemically poisonous.
+Must be able to reach part of body being diagnosed.

Question 15

Describe the main components of a gamma camera.

Answer 15

Lead collimator:
+Gamma rays leave the patient and travel towards the collimator. Only gamma rays that are parallel to the collimator pass through the crystal of sodium iodide.
+Others are absorbed by lead. This allows the system to know the position from which the gamma rays were emitted–>more accurate image.
Scintillator (sodium iodide crystal):
+Crystal absorbs gamma photons, which cause it to emit a photon of visible light (scintillation.)
Photomultiplier tube:
+Visible light photons pass through the photomultiplier tube & fall onto a photocathode, emitting an electron by the photoelectric effect.
+Electron accelerated towards a dynode at a positive potential w/ respect to the photocathode.
+The electrons emitted accelerate towards another dynode of high positive potential and the process continues over about 10 dynodes until it reaches the amplifier.
+Computer processes the data to give a display.

Question 16

What is a dynode?

Answer 16

A material that emits several electrons when hit by a single electron.

Question 17

Describe the principles of positron emission tomography (PET scans).

Answer 17

+A radioactive isotope e.g. ^18F is introduced to the body.
+The nuclei decays by emission of positron & meets an electron.
+They annihilate & form 2 gamma photons each of energy 0.51MeV.
+Photons move in opposite directions to conserve momentum.
+Gamma detectors around the patient can determine positron position from the time delay between each photon.
+A composite image is formed from all pairs of photons.

Question 18

Outline the principles of magnetic resonance.

Keywords: nuclei, Larmor frequency, resonance, relaxation times.

Answer 18

+Hydrogen nuclei have a property known as spin. In a strong magnetic field, nuclei precess (change the direction of their spin axis) about the direction of the field with a frequency known as the Larmor frequency.
+A radio wave pulse at the Larmor frequency causes resonance and the nuclei precess as a higher energy state; the spin axis reverses.
+After the pulse is gone, the nuclei relax, emitting a radio frequency signal, returning to its equilibrium state.

Question 19

Describe the main components of an MRI scanner.

Answer 19

Large main magnet: produces strong magnetic field (1.4T)
Additional magnets:
+Calibrated to produce non-uniform magnetic field.
+As Larmor frequency depends on B strength, the L frequency will vary from place to place.
+This allows H nuclei at different parts of the body to be detected.
Radio-frequency coil:
+Emits pulses of RF waves to patient through coil.
+Picks up RF waves emitted from patient as a result of de-excitation of H nuclei.
Computer:
+RF signals are processed and displayed so as to construct an image of the no. density of H atoms in patient.
+As non-uniform field changed, atoms in different parts of the patient’s body are detected and displayed.
+Image of cross-section of patient is produced.

Question 20

Outline the use of MRI to obtain diagnostic info about internal organs.

Answer 20

+Patient is subject to strong magnetic field & calibrated non uniform magnetic field.
+RF pulse at larmor frequency transmitted to patient.
+RF emissions from patient are detected and processed.
+H nuclei within patient have a Larmor frequency dependant on magnetic field strength so that location and density of H nuclei can be detected.
+An image is built up by varying the non-uniform field to give specific field strength at different positions in the patient.

Question 21

What are the advantages and disadvantages of MRI?

Answer 21

Advantages:
Does not use ionising radiation; no radiation hazard to patient/staff
Better soft tissue contrast than CAT scan
High quality image, 3D image
Radio waves pass through all materials, no known side effects.
Disadvantages:
Expensive, time consuming.
NO METAL OBJECTS: they heat up.
All external radio waves must be eliminated.

Question 22

Why are non-invasive techniques needed?

Answer 22

Diagnosis not always achievable from external symptoms.
Internal structure info needed.
Surgery costly & time consuming, has high risk complications eg infections
Non invasive techniques quicker & less risky.

Question 23

What is the Doppler effect?

Answer 23

The apparent change in frequency registered by an observer when there is a relative motion between the source and observer.

Question 24

How can the Doppler effect be used to determine the speed of blood?

Answer 24

Ultrasound a wave motion - if a moving object reflects US there will be a change in frequency.
Ultrasound stationary - blood in blood vessel is moving towards it.
Apparent frequency of ultrasound increases bc blood moving towards source.
Monitor change in frequency of ultrasound of known frequency, possible to find average speed of blood in vessel: f’=cf/c-2v or change in f-2fvcos0/c.
Can be used to monitor fetal heartbeat.

Question 25

What are the properties of ultrasound?

Answer 25

Sound waves above audible frequency (<20000Hz). Medical applications use fqc of 1-15MHz
Non ionising.

Question 26

Describe the piezoelectric effect & how ultrasound transducers emit and receive high frequency sound.

Answer 26

+Transducers have piezoelectric crystal & backing material. Piezoelectric crystals deforms when p.d. applied across it, so it oscillates when a.c. current applied across it.
+If frequency matches resonant frequency of crystal, ultrasound produced. Causes crystal to resonate - causing a.c. across it.
+Pulsing needed as reflected signal needs to be compared to initial signal.
+W/ continuous a.c. signal, comparison not possible. Backing material e.g. epoxy resin used to damp crystal vibration after a.c. pulse ends so it’s ready to receive reflected signal.

Question 27

What are the properties of ultrasound scanning?

Answer 27

+Ultrasound transmitted into body where it’s reflected at the boundaries between diff. tissues.
+Reflected wave detected and processed.
+time for echo to reach detector indicates depth of tissue boundary.
+intensity of ech gives info abt tissue boundary.

Question 28

What is the difference between an A scan and a B scan?

Answer 28

A scan shows 1 dimensional measurements.

B scan is A scans from diff angles, 2D image.

Question 29

Define acoustic impedance.

Answer 29

The fraction of the intensity of an ultrasound pulse reflected at a boundary between two materials. Calculated by Z=pc, where Z=a.i., p=density of materia, and c=speed of sound in material.

Question 30

How do you find the fraction of the reflected intensity?

Answer 30

Ir/I0=(Z2-Z1)^2/(Z2+Z1)^2.

Where Ir = reflected intensity and I0=original intensity, and the Zs are diff. acoustic impedence values.

Question 31

Why is impedance matching and gel required?

Answer 31

The amount of reflection of a wave at a boundary depends on the difference in acoustic impedance of the two media.
The intensity reflection for air–>soft tissue is very high so very little ultrasound is transmitted to the body.
Gel excludes the air between boundaries & has an impedance close to skin, so greatly reduces reflection at skin surface.