Medical imaging

Question 1

Define X-ray

Answer 1

Em radiation with wavelength ranging from 0.1 - 10 nm

Question 2

Define x-ray tube

Answer 2

a peice of equipment that produces x ray photons by firing electrons from a beta cathode across a large p.d. in an evacuated tube. x-ray photons are produced when the electrons are decelerated by hitting the target metal of the anode

Question 3

Define Thermionic emission

Answer 3

The release of electrons from the surface of a heated material

Question 4

Define attenuation

Answer 4

The decrease of intensity of em radiation as it passes through matter or space

Question 5

Define attenuation coefficient

Answer 5

A measure of the absorbtion of x ray photons by a substance. Also called absorbtion coefficient.

Question 6

Define CAT scan

Answer 6

Computer Axial Tomography; x ray images taken in slices which are built up in softwhere to produce a 3-D image from which any 2-D image can be displayed

Question 7

Define medical tracer

Answer 7

A compound labelled with a radioisotope that can be traced inside the body with a gamma camera

Question 8

Define Collimator

Answer 8

Part of a gamma camera, a honeycomb of long thin tubes made from lead that absorbs any photons arriving at an angle to the axis of the tubes so that a clear picture is obtained

Question 9

Define Scintillator

Answer 9

Part of a gamma camera, that produces thousands of visable photons when struck by a single gamma photon

Question 10

Define Photomultiplier tube

Answer 10

A peice of equipment that converts a photon of visable light into an electric pulse

Question 11

Define PET scan

Answer 11

Positron emission tomography scan; forms an image from the 2 gamma rays produced during the annihilation of a positron and an electron

Question 12

Define Ultrasound

Answer 12

Sound that is higher frequency than the typical limit of human hearing (20kHz)

Question 13

Define Ultrasound transducer

Answer 13

A device used both to generate and to recieve ultrasound which changes electrical energy into sound and sound into electrical energy

Question 14

Define Piezoelectric effect

Answer 14

When electrical charge accumulates in a solid material due to an applied mechanical stress, producing a potential difference

Question 15

Define Acoustic impedance

Answer 15

The product of the density and the speed of ultrasound in a substance

Question 16

Define intensity reflection coefficient

Answer 16

The ratio of reflected intensity to incident intensity at a boundary

Question 17

Define coupling gel

Answer 17

A gel with acoustic impedance similar to that of skin smeared onto the transducer and the patients skin before an ultrasound scan in order to fill air gaps and ensure that almost all the ultrasound enters the patients body

Question 18

Define impedance matching

Answer 18

The use of two substances wih similar acoustic impedeance to minimise reflecion of sound waves at a boundary

Question 19

Define simple scatter

Answer 19

X ray photons in range of 1-20 keV
X ray ionises atom and bounces off
X ray machines have greater than 20keV

Question 20

Define Photoelectric effect

Answer 20

X ray photons less than 100keV
Photon is absorbed and electron is emitted
Dominant mechenism in x ray machines

Question 21

Define Compton scattering

Answer 21

X ray photons in range of 0.5-5 MeV
X ray photon frees electrons but also scatters with reduced energy
Radiographs need to be protected from compton scattering using a sheild

Question 22

Define pair production

Answer 22

X ray photons more than 1.02 MeV
Photon dissapears and positron pair is produced (electron and positron)