Medical physics Flashcards
USS signs: DCDA/ MCDA twins
DCDA twins: lamda sign (part of placenta)
MCDA twins: T sign (thinner amniotic demarcation)
When can FH be detected on USS?
5+2 gestation or CRL>6mm
When can gestational sac be seen? How does it grow?
TV: 4+3 weeks (2-3mm)
TA: 5+3 weeks
Grows 1mm/ day in diameter
Yolk sac- when should it be seen and how big?
Visible in chorionic cavity (TV) at 5 weeks (3-4mm).
Should be seen in all pregnancies with gestation sac >12mm.
Reaches max of 6mm (at 10 weeks)
Embryonic pole- when is it visible?
When gestational sac diameter >18mm
Can be seen TV at 37 days (2-3mm)
How do US waves interact with tissues?
Reflection: change of direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originates.
Strength of reflection from an object depends on its acoustic impedance.
Refraction: change in direction of a wave due to a change in its speed as it passes from one medium to another
Diffraction: bending of waves around small obstacles and spreading out of waves past small openings (occurs when a wave encounters an obstacle that has a diameter comparable to its wavelength)
Scatter
Absorption: direct conversion of sound energy into heat as it travels through a medium
Nd: YAG laser & uses
Crystal, solid state laser
Used in hysteroscopic ablation/ endometriosis ablation/ treatment of TTTS
CO2 laser
Highest power continuous wave laser. Microscopic precision with high degree of clinical efficacy.
Preferred for endometriosis because minimal tissue damage without lateral thermal spread
Used for CIN treatment
What is in an USS transducer?
Piezoelectric crystals
Type of atom released during MRI to generate image?
Hydrogen
Type of treatment where device is inserted into vagina to deliver radioactive substance?
Brachytherapy
Type of electrosurgery used in laparoscopy where a pad is placed on the patient?
Monopolar diathermy
Monopolar vs bipolar
With the monopolar device, the current passes from the active electrode to the target lesions through the patient’s body and finally exits the patient via a return electrode. With the bipolar device, the current only passes through the tissue between the two electrodes of the instrument
USS process causing lysis of cell and damage?
Heating
3 biological effects of USS
- Cavitation (i.e. growth, oscillation and decay of small gas bubbles under the influence of an ultrasound wave)
- Microstreaming (formation of small local fluid circulations and can be both intra- and extracellular)
- Heating
USS phase with best resolution
Axial
USS features of complete/ partial molar pregnancy/ choriocarcinoma
Complete:
- Solid collection of echoes with numerous small anechoic spaces (snowstorm/ granular appearance/ honeycomb texture)
- Bunch of grapes sign (swelling of trophoblastic villi)
- Normal interface between abnormal trophoblastic tissue & myometrium
- No identifiable fetal tissue or gestational sac
Partial mole:
- enlarged placenta with multiple diffuse anechoic lesions
- Fetus with severe structural abnormalities or growth restriction
- Oligohydramnios or deformed gestational sac
Choriocarcinoma
- Large irregular haemorrhage mass, invading myometrium (20% occur after TOP)
A CXR/ CT abdomen is equivalent to how many days of natural background ratiation?
Radiation dose of CT pelvis?
CXR: 2.4 days
CTA: 2.7 years (0.8-3rad; 9-30mGy)
CTP: 2.5-8rad; 25-80mGy)
When are urodynamics (multi-channel filling and voiding cystometry) indicated?
Before surgery in women who have:
- Symptoms of OAB leading to clinical suspicion of detrusor overactivity
- Symptoms suggestive of voiding dysfunction
- Anterior compartment prolapse
- Previous surgery for stress incontinence
On TVS USS, at what gestation are the following structures visible from:
- Gestational sac
- Yolk sac
- Embryonic pole
- Gestational sac: 4+3
- Yolk sac: 5- 5+3 (gestational sac should be 10mm when yolk sac visible)
- Embryonic pole: 5+3 to 6 wks (gestational sac should be 16mm when embryonic pole visible)
What is the maximum normal diameter of the yolk sac on TVUS?
6mm (around 10th week)
Yolk sac >6mm is suspicious of failed pregnancy
Monopolar vs Bipolar diathermy
What frequency is typically used?
Monopolar: current passed from small electrode held by surgeon and returned to a large area plate via patient’s tissues
Bipolar: current passes between two electrode held by surgeon as forceps. Used to coagulate rather than cut.
Frequency must be >100kHz (500kHz is typically used), below this, depolarisation (electric shock) can occur
200kHz-3.3MHz
Key abnormalities on ECG for hyper/ hypokalaemia, hyper/hypocalcaemia
Hyperkalaemia: Tall T waves, wide QRS, flat p waves, prolonged PR, bradycardia
Hypokalaemia: flat T waves, U waves, increased amplitude of p waves, long PR, ST depression
Hypercalcaemia: Short QT, long PR, Osborn waves (also caused by hypothermia)
Hypocalcaemia: Long QT, reduced PR, narrow QRS, T wave flattening/ inversion, ST depression
When is lap and dye more appropriate than HSG for assessing tubal patency?
Lap & dye if history of pelvic surgery or PID
MRI
- Principle
- T1 vs T2
- SI unit for magnetic field/ flux
Principle: Uses strong magnetic fields to cause protons to align with the magnetic field. Radio frequency pulses then cause the protons to excite or ‘spin’. When they relax back into alignment of the field, they release radio waves which are detected by MRI sensors.
T1 weighted: fluid is dark
T2 weighted: fluid is bright
MRI magnets typically generate fields of 0.5 to 3.0 Tesla
Tesla = SI unit for magnetic field
Weber is the SI unit for magnetic flux
Radiotherapy dosing
Dosing is in Gray (Gy)
Total dose varies between tumours and stage but typical regimes involve 1.8-2.0 Gy fractions delivered over a number of weeks with total doses accumulating to reach around 50Gy
Principle of PET scan
Uses radioactive tracer, usually fluorodeoxyglucose (FDG), an analogue of glucose. Is given to patients and taken up in areas of high metabolism. The tracer emits gamma rays, detected by the scanner.
Uses ionising radiation
Types of LASER
Gas LASER:
- CO2
- Argon
- Helium- neon
Solid state LASER
- Nd YAG
- Neodynium
- Titanium sapphire
Liquid
- Rodamine
- Stibene
- Coumarin
Semiconductor
- Diode
SI units:
Gray
Sievert
Becquerel
Gray- SI unit of absorbed dose of ionising radiation
Sievert- SI unit of equivalent dose of ionising radiation
Becquerel- SI unit of radioactive decay
USS:
- Doppler effect
- Refraction
- Scatter
- Diffraction
- Attenuation
- Doppler effect: frequency change when an observer moves towards or away from an object
- Refraction: the change in wave direction as it passes from one medium to another
- Scatter: the effect when the sound waves are greater than the structure they come into contact with causing uniform amplitude waves in all directions with little or no reflection returning to the transducer
- Diffraction: bending of waves around small obstacles
- Attenuation: decreasing intensity of a sound wave as it passes through a medium. Due to a combination of absorption of sound waves, conversion into heat energy and loss of scattered and reflected sound waves.
SI unit of diathermy power?
Watt
Oligo/ Polyhydramnios diagnosis
Oligo: AFI <5cm or deepest fluid pocket <2cm
Poly: AFI >25cm or deepest fluid pocket >8cm
AFI = measuring depth of fluid pockets in all 4 quadrants
Laser of choice for ablation in TTTS
Diode or ND: YAG
LASER of choice for CIN/ genital warts treatment
CO2
DEXA scan principles
Measures bone density by: Measuring absorption from two different XRAY beams with different energy peaks at the same time
Soft tissue absorption subtracted to give BMD measurement
What are radio-sensitisers and the 4 main groups?
Radioprotectors
Radio-sensitisers increase the effect of a given dose of radiation
4 main groups:
- Oxygen
- Hypoxic cell sensitizers
- Halogenated pyrimidines
- Bioreductive agents
e.g. metronidazole, cisplatin, cetumixab
Radioprotectors are agents that reduce the effects of radiation e.g. amifostine. Limited use in clinical practise due to possible protection of tumours
At what gestation is the FH first detectable on US
6 weeks
Main advantages/ disadvantages of power doppler mode?
Adv: Good penetrance and able to detect flow in small vessels
Dis: No information on direction of flow or velocity of flow
What are the 3 doppler modes?
Power
Pulse
Colour
Pulse & colour give flow and direction information, but are angle dependent and have poorer penetration than power mode
USS features consistent with miscarriage
Mean gestation sac diameter >=25mm (with no obvious yolk sac or fetal pole).
OR
Fetal pole & CRL >=7mm & no evidence of FH activity
USS features in keeping with molar pregnancy
Complete mole:
- Enlarged uterus
- Solid collection of echoes with numerous small (3-10mm) anechoic spaces (snowstorm or granular appearance)
- Bunch of grapes sign, which represents hydronic swelling of trophoblastic villi
Partial mole:
- Placenta enlarged containing areas of multiple, diffuse anechoic lesions
- Foetus with severe structural abnormalities, growth restriction, oligohydramnios or a deformed gestational sac may be noted
- Colour doppler may show high velocity, low impedance flow
B & M rules in IOTA USS guidance on ovarian masses
B rules (Benign)
- Unilocular cysts
- Presence of solid components where largest solid component <7mm
- Presence of acoustic shadowing
- SMooth multilocular tumour with largest diameter <100mm
- No blood flow
M- rules (malignant)
- Irregular solid tumour
- Ascites
- At least 4 papillary structures
- Irregular multilocular solid tumour with largest diameter 100mm
- Very strong blood flow
Non-reassuring and abnormal features on CTG (NICE classification)
Variability:
NR: <5bpm for 30-50min, >25bpm for 15-15min
Ab: <5bpm for >50min, >25bpm for >25min
FHR range:
NR: 100-109 or 161-180
Ab: <100, >180
Accelerations:
NR/Ab: Absence is of uncertain clinical significance
Decelerations:
NR:
Variable decels with no concerning characteristics for >90min
OR
Variable decels with any concerning characteristics in up to 50% of contractions for >30min
OR
Variable decels with any concerning characteristics in >50% contractions for <30min
OR
Late decels in >50% of contractions for <30min with no maternal or fetal clinical risk factors like vaginal bleeding or significant meconium
Ab:
Variable decels with any concerning characteristics in >50% contractions for 30 mins (or less if any maternal/ fetal clinical risk factors)
OR
Late decels for 30 min (or less if any maternal/ fetal clinical risk factors)
OR
Acute bradycardia, or single prolonged deceleration lasting >=3 min
*Concerning characteristics: lasting >60s, reduced baseline variability within the decel, failure to return to baseline, biphasic (W) shape, no shouldering
USS frequencies & those used in medical imaging
> 20kHz
Medical imaging: 2MHz to 15 MHz
(1MHz = 1000kHz)
What frequency is used for trans-abdominal US vs TVUS?
Trans-abdominal: 3-3.5 MHz
TVUS: 5-7.5 MHz
Approximate dose of radiation to breast tissue when performing CTPA in pregnancy vs to foetus
How does this effect risk of breast cancer
10-20mGy to breast
0.1mGy to fetus
Increases lifetime risk from 12% to 13.6%
SI unit for power generated by electrosurgery units
Watts
Often 50-400 watts
Absolute CI to MRI/ relative CI
Absolute contraindications
- Implanted electric and electronic devices especially heart pacemakers (especially older types)
- insulin pumps
- implanted hearing aids
- neurostimulators
- intracranial metal clips
- metallic bodies in the eye
Relative contraindications
- Metal hip replacements, sutures or foreign bodies in other sites are relative contraindications to the MRI
- The first trimester of pregnancy
