M6.5 Medical Physics

Question 1

decribe the structure of an x ray tube

Answer 1

a high voltage power supply connected to a heater which accelerates electrons
electrons are shot at a tungsten target
some of their KE is converted to EM waves

Question 2

explain simple scattering

Answer 2

when the x-ray has less than less energy than the work function
photon is absorbed and the electron jumps up an energy level then drops releasing an x ray

Question 3

describe compton scattering

Answer 3

incoming x-ray has enough energy to remove electron
some of the photons energy is converted to the electrons Kinetic enegry
electron is removed
and photon is scattered with less energy

Question 4

explain pair production

Answer 4

high energy interacts with the nucleus
electron and positron is produced annihilate each other produce a pair of smaller energy photons travelling in opposite directions

Question 5

photoelectric effect

Answer 5

x ray is absorbed by electron
electron escapes atom with some KE

Question 6

when using an intensity against wavelength how to find maximum potential difference

Answer 6

use eV = hc/lambda
use minimum wavelength on the graph

Question 7

What happens to an intensity against wavelength graph when accelerting p.d is increased

Answer 7

graph shifts left but k lines stay in the same place
(theyre dependent on the meterial)

Question 7

why are contrast mediums used

Answer 7

because lots of soft tissues have similar attenuation coefficients

Question 8

what are the two commonly used contrast mediums

Answer 8

barium and iodine

Question 9

why do contrast mediums absorb x rays more than soft tissues

Answer 9

because attenuation coefficient is directly proportional to Z^3 (number of protons)^3

Question 10

how do CAT scans work

Answer 10

x ray tube around patient
detector opposite
thin beam of xrays used
gets cross sectional image

Question 11

pros and cons of CAT scan

Answer 11

produce 3d image

slower than xray
longer time exposed to raidation

Question 12

define a medical tracer

Answer 12

a radioactive substance injected into the patient

Question 13

what are the properties of a medical tracer

Answer 13

gamma source
must have a long enough half life to be detectable
and short enough so they dont stay in the system

Question 14

what are two main medical tracers used
And their half lives

Answer 14

fluorine - 18
half life of 110 mins

technitium 99m
half life of 6 hours

Question 15

what are the components of a gamma camera

Answer 15

collimator
scintilator
photomultiplier
computer

Question 16

what does a collimator do

Answer 16

thin lead tubes
only allows the gamma rays parallel to the camera to pass through

Question 17

what doe the scintilator do

Answer 17

converts the gamma rays to multiple visible light photons

Question 18

what does the photo multipler do

Answer 18

convert the visible light photons to electic signals

Question 19

what does the computer do

Answer 19

generates an image based on the electric signals

Question 20

pros and cons of a PET scan

Answer 20

pros
non evasive
can produce real time images

cons
radioactive source is used
patient is exposed to activity
expensive

Question 21

how are sounds produced and what affects their frequency

Answer 21

vibrations
greater the frequency of the oscilation the greater the frequency of the sound produced

Question 22

describe an ultra sound A-scan

Answer 22

ultrasound ray is sent out and some is reflected back
and it uses 2distanc = speed/time
(there and back)

Question 23

decribe an ultrasound B-scan

Answer 23

MANY A-SCANS
and the intensity of the reflected sound is used to generate a light of a matching intensity to produce an image

Question 24

what causes a high proportion of the ultrasound wave to be reflected

Answer 24

A high differenence in acoustic impedence

Question 25

what is impedance matching

Answer 25

things like coupling gel to avoid the sound traveling through air as air and skin and very different acoustic impdednace
so a gel of simiar Z can be used to ensure no air gets between so all/most of the ultrasound are absorbed by skin

Question 26

in the equation
Δf= 2vcos(θ)
f c

what is theta and c

Answer 26

theta is the angle between the blood vessel and the transducer

c is the speed of sound in blood

Question 27

explain how ultrasound is used ot measure the speed of blood flow in an artery

Answer 27

pulses are aimed at the moving blood cells in an artery
transducer is places at an angle usually 60 degrees
the detected frequency is different to the emitted frequency
ratio of frequencies enables the speed of blood flow to be calculated

Question 28

photon energy where simple scattering is an appropriate attenuation method

Answer 28

1-20 eV

Question 29

Photon energy where the photo electric effect is an appropritae attenuation method

Answer 29

30 eV

Question 30

Photon energy where compton scattering is an appropritae attenuation method

Answer 30

0.5 - 5.5 MeV

Question 31

Photon energy where Pair Production is an appropritae attenuation method

Answer 31

1.1 MeV+

Question 32

name some diagnostic applications of a gamma camera

Answer 32

Detection of cancer
scans of organs (in exam maybe name one)
scans of tissue
bone scans
observing the functionality of an organ

Question 33

in an x ray tube what is the cathode and what is the anode?

Answer 33

cathode is the heated filament
anode is the tungsten target