2.2 Principles of imaging Flashcards
Spatial resolution
Ability to separate fine structures (line pairs/cm)
ie. matrix size
Contrast resolution
Ability to separate different tissue density (grey scale)
Plain radiography
Crookes tube/X rays
-vacuum tube with current between filament and target (anode/cathode)
e-density of tissues, ie. atomic number and [ ]
- transmission of xrays
- increase in AN, increased # of e and therefore increased stopping of e passing through
- material with high [ ] of high atomic number (ie. Gold) is very hard to penetrate
Photographic film (negative) -more recently "xray detector"
Light box
-more recently computer monitor
Plain radiography PROS vs CONS
PROS
- cheap
- readily available
- excellent spatial resolution
CONS
- ionising radiation
- relatively poor contrast resolution
- planar technique, 3D becomes 2D
Plain radiography uses
- often the initial examination
- bones for trauma
- chest for all symptoms
- abdomen for pain (constipation, bowel obstruction)
- breast for detection of cancer (mammography)
Computed tomography, CT
Crookes tube/X rays
-vacuum tube with current between filament and target (anode/cathode)
e-density of tissues, ie. atomic number and [ ]
-transmission of xrays
X-ray detector
- solid state scintillation crystal
- gas ionisation chamber
Computer monitor
CT PROS vs CONS
PROS
- cross sectional, 3D, can look inside
- better contrast resolution radiography
- good spatial resolution (less than PR, so wouldn’t use for mammograms yet)
CONS
- more ionising radiation
- expensive
CT uses
- head for acute symptoms (trauma, stroke)
- chest/abdominal advanced imaging (malignancy, pain investigation)
- bones for subtle lesions (trauma, tumour, infection)
- “non-invasive” angiography
Ultrasound
Piezoelectric crystal
(Piezoelectric = electricity resulting from pressure)
-Produces sound waves
-Converts sound to an electric signal
-When crystal is exposed to a current, high freq sound is produced
-Changes to physical structure of crystal produces electric current
Sound velocity and tissue reflectivity
-transmission of high freq sound
Computer monitor
Ultrasound PROS vs CONS
PROS
- no ionising radiation
- cheaper than CT
- freely available
- solid vs cystic (“fish in the sea”)
CONS
- operator dependent
- sound penetration problems with bone, fat and gas
Ultrasound uses
- children
- pregnancy
- screening for abdominal symptoms, esp GB
- pelvic pathology in females
- small parts - thyroid, testis, salivary glands, breast
- musculo-skeletal esp. muscles, tendons and ligaments
- blood vessels (Doppler)
MRI
Very strong magnet (1.5 Tesla) and radio wave generator
Proton density and microenvironment of those protons
-emission of FM radio waves (64 MHz, requires use of Faraday cage)
Radio receiver
Computer monitor/film
MRI PROS vs CONS
PROS
- no ionising radiation
- excellent spatial resolution
- reasonable contrast resolution
CONS
- expensive
- strong magnet (ferromagnetic materials)
- long scan times of 30-60mins
- claustrophobia
- air/calcification/metal distort the magnetic field
MRI uses
- brain except acute trauma
- spine for pain or neurological deficit
- joints for pain, bones where xray not helpful
- liver and biliary tree where US not helpful
- breast in young females for CA
- pelvic malignancy
- non-invasive angiography
Nuclear medicine
Gamma emitting radioisotope and chemical
- radiopharmaceutical (tag)
- using a particular chemical that will go to a specific part of the body (ie. liver, tumour) and will start emitting radiation when it gets there
Accumulation of radiopharmaceutical - chemical interaction with organ/tissue
- emission of gamma rays
- will penetrate fat people too
gamma camera
-scintillation crystal
computer monitor/film
