Medical physics and clinical applications

Question 1

Ultrasound

Answer 1

Frequencies >20kHz, usually used in medical imaging between 2-15MHz
- 5-7.5 MHz for TV, 2.5-5 MHz for TA
Longitudinal compression waves - axial plane best resolution
Sound beam originates from mechanical oscillations of crystals in a transducer, which is excited by electrical pulses (piezoelectric effect)
Doppler effect describes the frequency change when an observer moves towards or away from an object

Question 2

Pulse doppler

Answer 2

Allows sampling gate to be positioned over a vessel visualized on grey-scale image
Amplitude of signal approximately proportional to number of moving RBCs
-> info on direction, velocity of blood flow, flow characteristics
- angle dependent (flow perpendicular to transducer hard to detect)

Question 3

Power doppler

Answer 3

= energy / amplitude doppler
Allows detection of larger range of Doppler shifts, so better visualization of small vessels, but less directional or velocity info
Benefits - free from aliasing, no angle dependences, higher sensitivty to detect smaller/lower flow vessels, better penetration
Disadvantages - no directional or velocity info

Question 4

Colour doppler

Answer 4

Provides estimate of mean velocity of flow within vessel by colour coding it
Flow direction arbitrarily assigned colour red or blue, indication flow towards or away from transducer
Also info on overall view of flow in organ, direction and velocity
- but is angle dependent and has poor temporal resolution

Question 5

Endometrial thickness and ovarian follicles

Answer 5

ET
If reproductive age: 5-14mm
Postmenopausal should be <4mm

Follicles
- simple anechoic area within ovaries, with clear well-defined walls
- growth 2mm/day, maximum diameter 20-25mm before ovulation

Corpus luteum - can be solid, cystic or haemorrhagic

Question 6

Stages of IVF

Answer 6

Stage 1 - pituitary desensitization
- patient given GnRH agonist
- lasts 2 weeks

Stage 2 - ovarian superovulation
- daily IM gonadotrophin
- hCG given prior to oocyte collection when largest follicle is 18mm and ET 6mm

Stage 3 - oocyte collection
- oocytes retrieved 36hr after hCG injection

Stage 4 - embryo transfer
- 2-3 days after oocyte collection

Stage 5 - post embryo transfer
- progesterone supplements given to support corpus luteal function

Question 7

Gestational sac

Answer 7

Detectable TV from 31 days (4w+3) gestation, where measures 2-3mm
Detectable TA from 5w+3
Grows 1mm/day

2 fluid-filled compartments:
- inner amniotic cavity - from 8w expands rapidly in chorionic cavity so that it soon occupies most of gestational sac
- outer chorionic cavity - dominates in early pregnancy

Then fusion of chorionic and amniotic membrane by end of first trimester, obliterating chorionic cavity

Question 8

Yolk sac

Answer 8

Becomes visible in chorionic cavity TV at 5w, when measures 3-4mm
Should be seen in all pregnancies with gestation sac diameter >12mm
Grows until reaches maximum diameter of 6mm at 10w gestation

Question 9

Embryonic pole

Answer 9

Should be visible when gestational sac diameter is >18mm, so TV from 37days

Crown rump length is measure of gestation before 12w but unreliable after this (fetus more likely to be flexed)
- if CRL >6mm then should see fetal heart (5w+2)
- should see fetal spine by 9w
Biparietal diameter to measure gestational age after 12w

Question 10

Nuchal translucency

Answer 10

Raised in chromosomal abnormality - T21, T18, Turner’s - between 11-14weeks

If NT > 6mm - link with chromosomal abnormalities, cardiac anomalies, fetal viral infection, rhesus incompatibility

Question 11

Risk of miscarriage in ultrasound findings

Answer 11

Embryonic death
- absence of cardiac activity in embryo with CRL >6mm
- absence of yolk sac/embryo in gestation sac diameter >20mm

Complete miscarriage
- thin regular endometrium

Incomplete miscarriage
- ET >5mm
- hyperechoic tissue within uterine cavity

Question 12

Ectopic pregnancy

Answer 12

= implantation of fertilized ovum outside the uterine cavity
9.6/1,000 pregnancies

USS:
- pseudosac in uterus common (avascular on doppler)
- 78% are ipsilateral to corpus luteum
- free fluid in POD 20%
- ‘sliding organs’ sign

Management
- expectant (40% failure rate)
- medical - methotrexate, success 80%
- surgical - salpingectomy or salpingostomy

Tubal - 93%
Interstitial
Cervical
Ovarian
Abdominal
Heteropic (combination of intrauterine and ectopic)

Question 13

Placenta praevia

Answer 13

If leading edge of placenta is within 5cm of internal cervical os
To describe placental position in relation to uterine lower segment after 28w gestation

• at 22w 5% women have low-lying placenta
• 25% of women who have low-lying placenta will get placenta praevia

Minor = placenta encroaches to lower segment
Major = placenta encroaches to or covers internal cervical os

Question 14

Amniotic fluid index

Answer 14

Sum of measurement of the depth of the largest cord-free vertical pool in each of the four uterine quadrants

AFI<5cm = oligohydramnios

AFI >25cm = polydramnios

Question 15

Polyhydramnios

Answer 15

If AFI >25cm or largest pocket of amniotic fluid >8cm in vertical depth

CAUSES:
Increased fetal production - maternal diabetes, fetal macrosomia, fetal anaemia (Rh disease or parvovirus), AV fistula, structural fetal abnormalities (open NTDs, teratomas)
Decreased fetal swallowing - upper GIT obstruction, fetal neurogenic disease
Idiopathic

Question 16

Oligohydramnios

Answer 16

AFI <5cm or when largest pocket of amniotic fluid is <2cm in vertical depth

CAUSES
- uteroplacental insufficiency
- amniotic membrane rupture
- abnormal fetal renal function

Question 17

Fetal growth restriction

Answer 17

If birth weight <10th centile for gestation
First change in fetal artery dopplers -> reduced biophysical profile, abnormal venous dopplers -> suboptimal CTG

Symmetrical
- if equal reduction in growth velocity of both fetal head circumference and abdominal circumference
- due to constitutionally small fetus, or pathologically small fetus (uteroplacental insufficiency or triploidy)

Asymmetrical
- classically, AC growth slows quicker than HC growth
- due to uteroplacental insufficiency

Question 18

Doppler high resistance patterns

Answer 18

Bilateral notches with mean resistance index >0.55
Unilateral notch with mean resistance index >0.65
Total pulsatility index >2.5

Question 19

Doppler findings in uteroplacental insufficiency

Answer 19

1st - reduction in end diastolic flow. Increasing RI values, PI values, and S/D ratio.

2nd - absent EDF, when >75% of placental bed obliterated - likely to give hypoxaemia and acidaemia. RI = 1.

3rd - Reversed end-diastolic blood flow
- 10x increase risk perinatal mortality

Fetal artery redistribution - when ratio of MCA pulsatility index to umbilical artery PI is above 95th centile

Abnormal fetal ductus venosus
- severe fetal acidaemia

Question 20

Fetal biophysical profile

Answer 20

To assess fetal wellbeing
5 criteria each score max 2 points for total out of 10:
- fetal tone (over 30mins)
- fetal movements
- breathing movements
- amniotic fluid volume
- fetal HR

<6 is abnormal (lower = worse)
7-8 is suspicious

Question 21

Neural tube defects

Answer 21

2-5/1000 births
On USS see:
- lemon sign - abnormal scalloping of frontal bones
- banana sign - crescent shaped cerebellum that produces abnormally small transcerebellar diameter
- cisterna magna reduced

Spina bifida occulta - unfused vertebral arch
Spina bifda cystica - neural tubes and coverings protrude through vertebral arch, eg meningocoele (NT in normal position but cyst from protruding subarachnoid membrane) or myelomeningocoele (NT ectopic in cystic space), both assoc with hydrocephalus
Rachischisis - no neural tube, neural tissue is fused with skin
Anencephaly

Question 22

Anterior abdominal wall defects

Answer 22

• assoc with raised maternal AFP
• 1/4000 births

Omphalocoele has sac covering, due to umbilical ring defect
- commonly with associated anomalies

Gastroschisis has no sac

Question 23

Renal pelvic dilatation

Answer 23

Associated with postnatal uropathies - pelviureteric junction obstruction, duplex kidney, reflux

• upper limit of normal in 2nd T is 5mm, or in 3rdT is 10mm

Question 24

Amniocentesis vs CVS

Answer 24

Amniocentesis
- from 15+0w
- 1% miscarriage risk

CVS
- 11-13+6w gestation (NOT before 10, can be 10-14+6)
- 1-2% miscarriage risk

Question 25

Ionizing radiation

Answer 25

Process of converting an atom/molecule into an ion by adding or removing charge particles

Occurs in radioactive decay (where unstable atomic nucleus spontaneously loses energy by emitting ionizing particle and radiation, impossible to predict)
- alpha, beta and gamma radiation

Question 26

Alpha particles

Answer 26

Emitted from nucleus of an atom
Similar to Helium-4 nucleus
2 neutrons + 2 protons
Heavily ionizing
High mass, low penetration depth (can be stopped with sheet of paper), high energy
Alpha decay is responsible for 99% helium production on earth
Used in smoke detectors, and power source for cardiac pacemakers

Question 27

Beta particles

Answer 27

Can be shielded by few cm of metal

Beta minus is an electron
Arises from beta minus decay of a neutron (neutron -> proton + electron + electron antineutrino)

Beta plus is a positron
Has high energy
Arises from beta plus decay where energy is used to convert a proton (energy + proton -> neutron + positron + neutrino)

Question 28

Gamma rays

Answer 28

Electromagnetic radiation of high frequency (above 10^19 Hz)
Produced by radioactive decay, fusion and fission
Consist of photons emitted by nucleus of an atom
Short wavelength
Used to sterilize medical equipment, removing decay-causing bacteria from foods, gamma knife surgery, nuclear medicine

Ionization via photoelectric effect, compton scattering, pair production

Need large amount of mass to shield (lead, aluminium, concrete)

Question 29

Radiation poisoning

Answer 29

Due to exposure with excessive ionizing radiation, in acute exposure
No treatment to reverse effects
Can try to mitigate effects with 5-androstenediol or potassium iodide (to protect thyroid)

Mild - vomiting within 48h exposure, depressed immune system, stillbirth, spontaneous abortion, 10% fatality in 30 days
Moderate - vomiting within 24h, fever, hair loss, infection, haematemesis, maelena, 30% fatality in 30 days
Severe - vomiting within 1h, diarrhoea + as above, 50% fatality in 30days
Very severe - vomiting within 30mins, as above + disorientation, dizziness, hypotension

Question 30

X-radiation

Answer 30

Form of electromagnetic radiation
Emitted by electrons
Wavelength 10-0.01nm

Exposure is measure of X-ray’s ionizing ability, in Coulombs or Roentgens

Absorbed dose is measure of energy absorbed
Equivalent dose is measure of biological effect of radiation on human tissue

Fetal CNS most sensitive at 25w

Question 31

Pulmonary embolism

Answer 31

DVT in 70% of those with PE
CTPA -ve predictive value over 99%
- increases lifetime risk of maternal breast ca by 13.6%
- additional risk of childhood ca of 1:1,000,000 above baseline

VQ scan fetal radiation dose is 3x greater, additional risk ca 1:280,000 above baseline

Question 32

Radiotherapy

Answer 32

Radiation causes DNA damage via direct or indirect ionization of atoms that make up DNA chain
- in hypoxic environments tumours are more resistant to damage (O2 is radiosensitizer)
- for solid epithelial tumour, 60-80Gy, lymphoma 20-40Gy

Fractionation allows tumour cells in radio-resistant phase of cell cycle during treatment to cycle into a more sensitive phase for the next treatment dose, allows time for reoxygenation

SEs - oedema, infertility, fibrosis, epilation, dryness, cancer (1/1000)

Question 33

Types of tumour and their radiosensitivity

Answer 33

Sensitive - leukaemia, lymphoma, germ cell tumours
Resistant - renal cell ca, melanoma

Radiosensitizing drugs are cisplatin, nimorazole, cetuximab

Question 34

Nuclear medicine

Answer 34

Uses radioactive isotopes internally, based on radioactive decay, emissions captured on gamma camera
Used for imaging (shows physiologocal function not anatomy, eg scintigraphy, PET, gallium scans) and treatment (iodine131 for hyperthyroid, strontium89 for palliation of bone pain, brachytherapy)

Question 35

Radionuclide

Answer 35

= atom with unstable nucleus
Emits ionizing radiation

Question 36

Magnetism units

Answer 36

Tesla = SI unit for magnetic field
Weber = SI unit for magnetic flux

MRI magnets produce 0.5-3T

Question 37

MRI uses

Answer 37

Magnetic fields to align atomic nuclei (usually hydrogen protons) within body tissues
Radiofrequency fields to systemically alter alignment of magnetisation, hydrogen nuclei make a rotating electromagnetic field
- after field is turned off, protons decay to original spin-down state and different between states is released as photon - this is what the scanner detect

Question 38

Types of MR image

Answer 38

T1 weighted - spin-lattice / longitudinal relaxation time
- decay constant for recovery of z component of nuclear spin magnetisation
- water/fluid is dark, fat is bright
- good for white/grey matter contrast in brain

T2 weighted - spin-spin /transverse relaxation time
- water/fluid is bright, fat is dark
- sensitive to water content

FLAIR sequence - free water is dark but oedematous tissue remains bright

Usually T1 time longer than T2 time

Question 39

Laser

Answer 39

Type of electromagnetic radiation
Beams are spatially coherent, narrow low-divergent, monochromatic, highly collimated
Produced by stimulated emission - photon emitted from excited atom when it is stimulated by other photons
Can be any state (gas, liquid, solid, plasma, semiconductor)

When laser beam interacts with living tissue - thermal effect (coagulation, volatilization), mechanical effect, photoablative, photodynamic effect

Question 40

Light

Answer 40

= electromagnetic radiation of a wavelength that is visible to human eye
- spectrum from radio/microwaves/UV/X ray/gamma ray

Photon is basic unit of light, governed by quantum mechanics, mass-less with no electric charge
- short wavelengths have high energy (blue), long wavelengths have low energy (red)

Question 41

Laser applications in medicine

Answer 41

Gynae
- treatment of CIN with CO2 laser (emits infrared light), or genital warts
- endometriosis
- laparoscopic surgery

Obstetrics
- selective laser photocoagulation in treatment of twin-twin. transfusion syndrome (Diode or ND:YAG)

Urology - lithotripsy, ablation of prostate

+ eye surgery, hair removal, skin treatments

Question 42

Microwaves

Answer 42

= electromagnetic waves with frequency ranging from 0.3-300GHz
Are absorbed by molecules that have dipole moment in liquids
Medical applications - microwave ablation for AF, endometrial ablation, treatment of BPH

Question 43

Electrosurgery

Answer 43

= application of high frequency electric current to tissue, to cut/coagulate/desiccate/fulgarate
Uses alternating current to directly heat the tissue
Monopolar / bipolar / tripolar
Generates smoke plumes which contain chemical by-products, intact cells, viral DNA, viable bacteria

Electric current = flow of electrons or ions
- conduction in tissue is due to interstitial fluid
- higher frequency = lower sensitivtiy

Question 44

Electrosurgical modalities

Answer 44

Cutting - high-power density to vaporize tissue water content

Coagulation - lower average power waveforms

Desiccation

Fulgaration - electrode held away from tissue, so air gap between becomes ionized and superficial tissue burning occurs

Question 45

Diathermy

Answer 45

Dielectric heating
Process where radiowave or microwave electromagnetic radiation heats dielectric material
Produced by rotation of molecular dipoles in high frequency alternating electric field
Diathermy uses higher frequencies (500 KHz) as lower frequencies cause depolarisation and may cause electric shock
A plate dissipates current over a large area. if it becomes partially detached current is concentrated to a smaller area and may cause burns

Question 46

Ultrasound appearance of molar pregnancy

Answer 46

Complete Hydatidiform Mole
- enlarged uterus
- solid collection of echoes with numerous small (3-10 mm) anechoic spaces (snowstorm or granular appearance).
- ‘Bunch of grapes’ sign which represents hydropic swelling of trophoblastic villi

Partial Mole
- placenta enlarged containing areas of multiple, diffuse anechoic lesions
- fetus with severe structural abnormalities, growth restriction, oligohydramnios or a deformed gestational sac may be noted.
- colour Doppler may show high velocity, low impedance flow

Question 47

Umbilical artery doppler waveform

Answer 47

Usually has a “sawtooth” pattern with flow always in the forward direction, that is towards the placenta
Abnormal waveform shows absent or reversed diastolic flow
- before the 15th week, the absence of diastolic flow may be a normal finding

Question 48

CT abdo doseage

Answer 48

= 400 CXRs or 2.7yrs background radiation