27, Medical Imaging

Question 1

How are X-rays produced?

Answer 1

A high voltage is placed across a vacuum with a hot filament (cathode) at one end and a sheet of metal at the other (anode), this creates a sort of electron gun which hit the metal sheet, slowing them down abruptly which releases X-ray photons (X-rays are also produced at a specific wavelength when electrons are knocked out of the shell and become replaced), these then leave the X-ray tube through a small window.

Question 2

Why are very high P.D.’s required to produce X-rays?

Answer 2

Because the electrons need to have very high K.E., especially since most is transferred to thermal energy at the anode.

Question 3

How could you find the wavelength of X-ray emitted from an X-ray tube?

Answer 3

Equate E = hf to E = QV (= eV) and use f = c/wavelength.

Question 4

What is attenuation?

Answer 4

The decrease in intensity of EM radiation as it passes through matter.

Question 5

What are the four mechanisms for attenuation of X-rays?

Answer 5

Simple scattering, photoelectric effect, Compton scattering and pair production.

Question 6

What is simple scattering?

Answer 6

When a low energy X-ray bounces off an electron in an atom.

Question 7

What is the photoelectric effect?

Answer 7

When an electron absorbs the X-ray and escapes from the atom.

Question 8

What is Compton scattering?

Answer 8

When an electron absorbs the X-ray, escapes the atom and releases a lower energy photon.

Question 9

What is Pair production?

Answer 9

When a very high energy X-ray interacts with the nucleus of an atom and an electron-positron pair is produced.

Question 10

What does the equation I = I_0 e^μx show?

Answer 10

Intensity = initial intensity * e^ attenuation/absorption coefficient * thickness of substance.

Question 11

What are the units of the attenuation coefficient?

Answer 11

m^-1.

Question 12

How can soft tissues like blood vessels be seen in X-ray images?

Answer 12

By using a contrast medium, a harmless substance with a high Z number such that the X-rays become absorbed by the tissue where the substance is present.

Question 13

How can X-rays be used in therapy?

Answer 13

Can be used to target specific groups of cells to kill cancers etc.

Question 14

What substances can be used as a contrast medium?

Answer 14

Iodine in blood and barium can be used to view the gut.

Question 15

What is a CAT scan?

Answer 15

Where an X-ray tube spins around the object with a detector array at the other end to produce 3D images of the object.

Question 16

What are the advantages and disadvantages of X-rays and CAT scans?

Answer 16

X-rays are much cheaper but CAT scans can produce 3D images which can be more useful. CAT scans also expose the patient to much more radiation and require the patient to remain very still for an extended period of time, both have the advantage of being non-intrusive.

Question 17

What makes an ideal radioisotope?

Answer 17

One which has a very short half-life, decays into a stable isotope, emits mostly gamma rays and is safe inside humans.

Question 18

What is a tracer and how does it work?

Answer 18

You chemically bind it to the radioisotope and the compound accumulates in the area you want to investigate.

Question 19

What is a gamma camera?

Answer 19

The device used to find decays of the radioisotope.

Question 20

What components make up a gamma camera and what do they do?

Answer 20

First there is the collimator which ensures that only photons travelling perpendicular to the camera are detected, then there is a scintillator which attempts to have as many gamma rays interact with it as possible producing a burst of visible light. Then the visible light travels down a light guide (think ethernet cable) into a photomultiplier which uses the photons to induce a current (by letting it interact with charged plates) causing a pulse of current which can be detected by the computer to create a display.

Question 21

What is a PET scanner?

Answer 21

A device similar to a CAT scanner which uses a ring of gamma detectors to produce a 3D image from the gamma rays produced by the annihilation of positrons produced by radioactive materials in the human.

Question 22

Why is the fact that two gamma photons are emitted in opposite directions important for PET scans?

Answer 22

Because the decay itself will also produce gamma photons and these need to be ignored so only photons which appear at opposite sides of the scanner at the same time are used, also the slight time difference between the photons can be used to find the origin of the decay.

Question 23

What is the issue with setting up PET scans in hospitals?

Answer 23

It is very expensive to buy PET scanners and also the tracers used need to be produced on site since they have very short half-lives, this requires a particle accelerator which is very expensive.

Question 24

What is ultrasound?

Answer 24

Any sound with a frequency beyond the point that humans can hear ~20kHz.

Question 25

What is the name given to the effect of crystals producing and E.M.F. when compressed or stretched?

Answer 25

The Piezoelectric effect.

Question 26

What is an ultrasound transducer and how does one work?

Answer 26

An AC current with the same natural frequency is placed across a crystal in a short pulse, this causes the crystal to produce a high frequency pulse, the reflection of this pulse can also be detected with the same crystal.

Question 27

What is an A-scan?

Answer 27

An ultrasound scan where a single transducer is used, the distance to different structures can be detected with this method.

Question 28

What is a B-scan?

Answer 28

Multiple A-scans compiled into one image to allow you to get a 2D image of the object.

Question 29

Explain the equation Z = ρc.

Answer 29

It is the defining equation for acoustic impedance (= density * speed of wave in substance)

Question 30

What does the equation I_r / I_0 = ((Z2-Z1)/(Z2+Z1))^2 mean?

Answer 30

The ratio of reflected intensity to incident intensity is equal to the square of the ratio of the difference and sum of the acoustic impedance of the two materials.

Question 31

Why is a coupling gel used with ultrasound imaging?

Answer 31

To overcome the fact that most sound would be reflected by the skin air barrier.

Question 32

What is Doppler imaging?

Answer 32

When ultrasound is targeted on something that is moving like blood causing a Doppler shift in the ultrasound which can be detected and used to find the velocity of the blood.

Question 33

What is the equation Δf = 2fv cos(θ)/c used for?

Answer 33

Calculating the velocity of the blood in Doppler imaging, f is the original frequency, c is the speed of ultrasound in blood and theta is the angle between the direction of blood flow and the ultrasound beam.