8.1 - Medical Imaging Flashcards
how is an image produced using x ray
within x ray tube…
- electrons are accelerated to metal target
- interaction of electrons with target produces photons (x rays)
- some of the x-rays pass through patient, then hit a detector behind patient
- some are attenuated by the patient (absorbed, scattered, or lose energy)
amount of attenuation depends on…
- density and atomic number of tissue/material
- energy of the x-ray beam
detected x rays are digitized and processed, creating an image which is uploaded
densities on x rays
most dense → attenuates lots of x-rays. lighter
* metal
* bone
* muscle
* blood
* liver
* lung
* fat
* air
least dense → darker/black
systematic interpretation of chest x-ray
A - airways
B - breathing
C - circulation
D - disability (bones)
E - everything else
systematic interpretation of abdominal x-ray
A - air
B - bowel
D - densities (bones, stones, tubes, foreign bodies)
O - organs
clinical indications for use of x rays
chest
dyspnoea, cough, haemoptysis, chest pain, tube/line poisioning, post procedure, cancer mets
abdomen
neonatal, passing of urinary tract calculi, presence of foreign bodies
MSK
trauma, pain, deformity, swelling, post relocation of joint
haemoptysis - coughing up blood
diagnoses from x-ray
chest
infection, pulmonary oedema, pleural effusion, pericardial effusion, pneumothorax, cancer
abdomen/pelvis
obstruction, volvulus, perforation, colitis (can see inflammation), calculi (eg calculi), abdominal aortic aneurysm (can see widening of aorta)
MSK
fracture, dislocation, effusion, soft tissue injury, tumor, infection
pros + cons of x ray
advantages
- quick
- portable
- cheap
- simple
disadvantages
- radiation (very low)
- one plane, and two dimensions only
- cannot see all pathology
- poor soft tissue imaging (particularly if they’re close together as they are difficult to distinguish)
pros + cons of x ray
advantages
- quick
- portable
- cheap
- simple
disadvantages
- radiation (very low)
- one plane, and two dimensions only
- cannot see all pathology
- poor soft tissue imaging (particularly if they’re close together as they are difficult to distinguish)
what is flouroscopy
- similar to x ray process
- continuous/pulsed x-rays are used
- creates a moving image
- can examine anatomy, pathology, motion and function
- images often enhanced using contrast (eg barium or iodine)
- high atomic number = good absorber of x-rays = dense on image
clinical use of flouroscopy
- diagnostic and interventional
- vascular (angiography) eg cerebal, coronory, embolisation, angioplasty and stenting
- GI eg barium swallow/meal, barium enema
- GU eg urogram, nephrostomy, hysterosalpinogram
- MSK eg arthogram, therapeutic joint injections, orthopaedic surgery
pros and cons of flouroscopy
advantages
- dynamic studies, real time - can assess function, carry out intervention
- quick
disadvantages
- higher radiation dose than single x-ray due to it being continuous
- radiation exposure to interventional radiographer
- one plane, two dimensions, cannot see all pathology
- poor soft tissue imaging
how does CT scan work
- x rays produced as normal
- x ray tube on one side of rotating gantry (ring), with detectors on opposite side
- patient table moves through gantry
- same principle of x ray attenuation
- cross sectional slices of the patient imaged
- detected signal processed by computer to produce cross sectional images
what view is CT
axial / transverse view
looking from feet up
clinical use of CT
diagnosis / guiding further investigation / management
- trauma
- bleeding + clots
- ischaemia / infarcts
- cancer + staging cancer
- perforation
- obstruction
- calculi
- weight loss
- fever
monitor conditions
eg cancer and interstitial lung disease (ILD)
interventional
radiotherapy, CT guided biopsies/drains
ischaemia = when blood flow restricted
pros and cons of CT scan
advantages
- quick
- good spatial resolution
- can scan most parts of the body well
disadvantages
- significant radiation
- does not delineate soft tissues well (eg gynae)
- affected by artefact (eg movement/metal)
- requires breath holding, which not all patients can do
- overuse
- incidental findings (have to follow this up, and this causes patient anxiety)
- contrast reactions
delinate = describe or portray something precisely
PET scans how do they work
positron emission tomography
- work by gamma rays
- administration of radiopharmaceutical (more info on different card)
- nuclear decay of the radionuclide occurs within tissues of interest, emitting positrons
- positrons collide with electrons, producing gamma radiation
- this is detected by gamma camera close to patient
- gamma camera contains scintillator (converts signal to light)
- light signal amplified and processed by computers
- gives functional and anatomical infomation
how do radiopharmaceuticals work
administration
- injection
- ingestion
- inhalation
two parts to radiopharmaceutical
- pharmaceutical part: takes the compound to tissues of interest
- radionuclide part: sends signal from tissue of interest
how it works
- nuclear decay of radionucleotide occurs within tissues of interest
- emit positrons during decay
- positrons collide with nearby electrons
- these produce gamma photons
- these are detected by the gamma camera
what is a common radiopharmaceutical
18F-FDG
- flourodeoxyglucose is pharmaceutical part (glucose analogue) which is taken up by areas of high glucose metabolism
- this then releases flourine-18 (radionuclide)
- some tissues are naturally metabolically active
- therefore some tissues naturally glow on PET
- other metabolically active tissues will also glow eg tumours
clinical use of PET
oncology
detection, staging and response to treatment
neurological
early diagnosis of Alzheimer’s disease and localisation of seizure focus
cardiac
identification of poorly perfused myocardium
infection/inflammation
pyrexia of unknown origin and vasculitis
pyrexia = fever
clinical use of PET
oncology
detection, staging and response to treatment
neurological
early diagnosis of Alzheimer’s disease and localisation of seizure focus
cardiac
identification of poorly perfused myocardium
infection/inflammation
pyrexia of unknown origin and vasculitis
pyrexia = fever
pros + cons of PET
advantages
- good contrast and spatial resolution
- can analyse anatomy and function
- can identify cancers
disadvantages
- physiological uptake of radiopharmaceutical
- radiation dose to patient
- risk of radiation to others
- radioactive waste produced
- expensive and time consuming
- radionuclide shortages
- confusion: eg if patient has done heavy exercise → tissues more metabolically active + will show up on scan
how does MRI work
- MRI scanner creates strong magnetic field
- this aligns all the hydrogen atoms in the matient
- radiofrequency pulse is applied
- this ‘tips’ the aligned hydrogen atoms, which then create detectable magnetic field
- this field induces an electric current in nearby coils in MRI machine
- varying signal intensities are produced by different tissues
- signals then processed to create images
- after radiofrequency pulse ends, hydrogen atoms relax back into alignment
imaging colours / weighting
high signal = bright = ‘hyperintense’
low signal = dark = ‘hypointense’
→ T1 weighting = fat is bright, water is dark
→ Τ2 weighting = fat is dark, water is bright
clinical use of MRI
- central nervous system
- head and neck imaging
- MSK imaging (bones/joints/soft tissues)
- GI eg liver
- cardiac MRI, angiography
- gynaecological imaging and prostate imaging
- in paediatrics and pregnancy to avoid radiation (and therefore theoretical damage to DNA)
pros and cons of MRI
advantages
- no radiation
- good contrast resolution, especially soft tissues
disadvantages
- expensive
- time consuming
- fewer machines and radiographers available
- contraindications (pacemakers, cochlear implants, metal, claustrophobia, inability to stay still)
- contrast reactions
- some other risks eg overheating
- magnetic field strength - need to remove all metal objects from room
pros and cons of MRI
advantages
- no radiation
- good contrast resolution, especially soft tissues
disadvantages
- expensive
- time consuming
- fewer machines and radiographers available
- contraindications (pacemakers, cochlear implants, metal, claustrophobia, inability to stay still)
- contrast reactions
- some other risks eg overheating
- magnetic field strength - need to remove all metal objects from room
how does ultrasound work
- utilises soundwaves (not part of electromagnetic spectrum)
- crystal in transducer probe oscillates
- this creates high frequency sound waves
- sound waves travel through tissues and are refelected back from boundaries between tissues of different density (acoustic impedence mismatch)
- probe detects reflected sound waves and converts them into electrical signal
- time taken for echo to return is used to calculate where it was reflected from
- proportion of reflected waves are used to calculate acoustic impedence mismatch in that place
hyperechoic = more reflection = white on image
hypoechoic = less reflection = dark
acoustic shadowing = large acoustic impedence mismatch = sound waves completely reflected back, non pass through, dark area behind bone, air, stones
doppler ultrasound
- if something is moving towards or away from the soundwave, eg blood flow in a vessel, the frequency of the reflected wave is affected
- moving towards the soundwave = increased frequency of echo wave
- moving away from the soundwave = decreased frequency of echo wave
what is duplex ultrasound
normal 2D imaging + doppler
clinical use of ultrasound
solid organs (liver, kidneys, spleen, pancreas, thyroid, fetal brain, lymph nodes etc) can see tissue, massess and bleeds etc
hollow structures (eg heart, vessels, gallbladder, common bile duct, appendix, uterus etc) can see function, flow, obstruction, masses, stones etc
breast assess for lumps or abnormality seen on mammogram
obstetrics pregnancy dating, fetal anomaly, placental location, fetal growth
MSK assessing ligaments, tendons, joints, muscles, nerves, soft tissue masses
interventional eg guided injections, biopsies, drains and aspirations
can also have transvaginal, transrectal, transoesopageal ultrasounds
clinical use of ultrasound
solid organs (liver, kidneys, spleen, pancreas, thyroid, fetal brain, lymph nodes etc) can see tissue, massess and bleeds etc
hollow structures (eg heart, vessels, gallbladder, common bile duct, appendix, uterus etc) can see function, flow, obstruction, masses, stones etc
breast assess for lumps or abnormality seen on mammogram
obstetrics pregnancy dating, fetal anomaly, placental location, fetal growth
MSK assessing ligaments, tendons, joints, muscles, nerves, soft tissue masses
interventional eg guided injections, biopsies, drains and aspirations
can also have transvaginal, transrectal, transoesopageal ultrasounds
pros + cons of ultrasound
advantages
- lack of radiation
- low cost
- portable
- dynamic, eg can see blood flow
- quick
disadvantages
- operator dependent
- no bone or gas penetration
- diffiult with obese, frail and unwell patients
- risk of overheating
