Medical physics and clinical applications Flashcards
Ultrasound
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
Pulse doppler
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)
Power doppler
= 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
Colour doppler
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
Endometrial thickness and ovarian follicles
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
Stages of IVF
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
Gestational sac
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
Yolk sac
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
Embryonic pole
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
Nuchal translucency
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
Risk of miscarriage in ultrasound findings
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
Ectopic pregnancy
= 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)
Placenta praevia
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
Amniotic fluid index
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
Polyhydramnios
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
Oligohydramnios
AFI <5cm or when largest pocket of amniotic fluid is <2cm in vertical depth
CAUSES
- uteroplacental insufficiency
- amniotic membrane rupture
- abnormal fetal renal function
Fetal growth restriction
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
Doppler high resistance patterns
Bilateral notches with mean resistance index >0.55
Unilateral notch with mean resistance index >0.65
Total pulsatility index >2.5
Doppler findings in uteroplacental insufficiency
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
Fetal biophysical profile
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
Neural tube defects
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
Anterior abdominal wall defects
- 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
Renal pelvic dilatation
Associated with postnatal uropathies - pelviureteric junction obstruction, duplex kidney, reflux
- upper limit of normal in 2nd T is 5mm, or in 3rdT is 10mm
Amniocentesis vs CVS
Amniocentesis
- from 15+0w
- 1% miscarriage risk
CVS
- 11-13+6w gestation (NOT before 10, can be 10-14+6)
- 1-2% miscarriage risk
Ionizing radiation
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
Alpha particles
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
Beta particles
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)
Gamma rays
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)
Radiation poisoning
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
X-radiation
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
Pulmonary embolism
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
Radiotherapy
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)
Types of tumour and their radiosensitivity
Sensitive - leukaemia, lymphoma, germ cell tumours
Resistant - renal cell ca, melanoma
Radiosensitizing drugs are cisplatin, nimorazole, cetuximab
Nuclear medicine
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)
Radionuclide
= atom with unstable nucleus
Emits ionizing radiation
Magnetism units
Tesla = SI unit for magnetic field
Weber = SI unit for magnetic flux
MRI magnets produce 0.5-3T
MRI uses
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
Types of MR image
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
Laser
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
Light
= 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)
Laser applications in medicine
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
Microwaves
= 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
Electrosurgery
= 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
Electrosurgical modalities
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
Diathermy
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
Ultrasound appearance of molar pregnancy
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
Umbilical artery doppler waveform
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
CT abdo doseage
= 400 CXRs or 2.7yrs background radiation
