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. What would you like to do?
really basic idea behind Bayesian Analysis
pretest probability affects post-test probability
what should the clinical history look like in a radiology report/request
- one sentence, like a tweet, including
- chief complaint
- relevant medical history
- relevant exam findings (localize, lateralize)
- what are you trying to rule out
ex: HIV c 2 days acute back pain at L5 and fever, R/O DJD, DDD
Bayesian analysis formula
post-test odds = pretest odds x likelihood ratio
ex: was the employee who tested positive on the drug test using drugs? = 0.5% prevalence of drug use in the company x 99% specificity and sensitivity of test
.67 = .0005 x .99 -- so there's a 67% chance the person isn't using drugs
the .0005 would increase if the person has bloodshot eyes or needle marks in the arm
volar vs dorsal means what?
palmar vs dorsal
how are radiographic views named?
- for the lower body part, the one closer to the cassette -- so, a R lateral side has the pt laying on her right side,
- RAO, LAO, RPO, LPO (anterior/posterior oblique) -- has pt laying on front or back but on an angle
what's a voxel?
it's a 3-d pixel (vo stands for volume)
attenuation of x-rays is based on what 3 things?
- thickness of material/body
- marterial density (fxn of atomic number)
- energy of the x-ray beam
two variables that determine the amount of radiation
- mAs (milliamperes per second) - volume of x-rays
- kVp (peak kilovoltage) - energy of x-rays (like the height of a waterfall)
allowing passage of x-rays or other radiation, not opaque
- opposite of radiolucency
- not allowing the passage of x-rays or other radiation
spectrum of materials in order of increasing radiodensity (decreasing radiolucency)
air, fat, water, bone, contrast, heavy metals
- air lets the x-rays pass thru, so it'll look black
- bone contrast catches the x-rays, so it looks white
basics on how distortion and magnification happens in x-rays
the closer to the source of light (further from the x-ray cassette/film), the more the object gets magnified, (the bigger it'll look) so you need to have objects flat on the screen, or be aware of why distortion is happening
3 points on how to hang x-ray films
- hang anatomically - like you're looking at someone (their L is on your R)
- laterals are hung in same dir as when image is taken, otherwise facing to the R
- hands and feet are hung so you're looking at dorsal surface and digits point up
ABCs of x-rays (just what each letter stands for)
- soft tissue
things to look for under the A of the ABCs
things to look for under the B of the ABCs
- cortical erosions (cortical is the outside of a Crunchie, cancelous is inside)
- degenerative changes
- radiodense or radiolucent lesions
things to look for under the C of the ABCs
- joint spaces
- joint effusions
things to look for under the S of the ABCs
- SOFT TISSUE
- foreign bodies
- subcutaneous air
where's the pedicle?
btwn the vertebral body and the superior articular process
where's the lamina?
btwn TP and SP
superior vertebral notch and inferior vertebral notch locations?
posterior to the vertebral body, the indentations before the sup/inf articular processes
what's Satisfaction of Search?
You look at an x-ray, find one pathology, say, "ahah!" and stop looking.
The phenomenon in which detection of one radiographic finding interferes with that of others .... results in false negative interpretation
CAT scan aka CT scan is made how?
- x-ray beam and dectector are rotated around the patient who's laying in a tube
- intense math happens
- a measure of attenuation (Hounsfield Units) is given for each pixel, and the end result is a cross-sectional image of the pt based on the attenuation of the x-ray beam
- these are for measuring the attenuation of the x-ray beam in CT scans
- water = 0 Hounsfield units
- air = -1000
super cool thing about helical CTs
volume data at <1mm thickness can be reconstructed in any plane with very high resolution - you can have diff materials (bone, fat, soft tissue) in focus
how many grayscale values in CT? how many can a human eye detect? how does this impact CTs?
- we adjust the window and level to select the part of the scale we want to see (lung, mediastinal, fat, bone....)
motion artifact in CT scans
motion blurring the pic - the machine moves fast but we, or our heart beats, can move faster
artifacts in CTs
complications that make the image a less good representation of the body - motion, partial volume averaging, metal, beam hardening
partial volume averaging artifact in CT
- if the blip of color is directly over one voxel, it's fine, but if it lies on a cross section, each voxel will average the darkenss in with the lightness on those squares, and you'll lose the fact that there was one concentrated spot
- this is more of an issue with thicker slices, less with smaller voxels
metal artifact in CTs
- some metal in the body - it's super dense (high radiodensity), a bright white spot, and it limits the eval of soft tissue directly adjacent
- the metal radiates white off it in a star shape
- this effect can be minimized with tweaks to imaging techniques
beam hardening artifact
a less dense area between much more dense areas may appear esp less dense (a darker line)
"as x-rays pass thru substances that are dense, it filters out the low energy portion of the spectrum and "hardens" the beam"
4 types of CT contrast
how does intravenous contrast work?
- iodinated compounds are injected into a vein as scan is performed, usually approx 100ml
- scan is timed to get images in desired phase - arterial, venous, excretory, delayed
- evaluates blood pool structures (arteries and veins) and vascularity (infection, tumor, inflammation)
why IV contrast is good at id-ing a tumor
contrast leaks into tumor, and also the angiogenesis of a tumor (new blood vessels) draw in blood and contrast from the nearby vein or artery
allergy & CT contrast
- mild: rash - not a contraindication, just an inconvenience
- severe: hypotension, laryngeal swelling, CV collapse (contraindications, of course)
renal failure & CT contrast
- renal failure (creatinine > 1.5-1.8) can be triggered in pts w diabetes, multiple myeloma, -- decreased risk w non-ionic iso-osmolar contras agents
- once you're on dialysis it's fine to do contrast - it won't hurt you now!
a leakage of contrast material into the fatty tissue around a vein.
Extravasation happens in one out of every 250 to 500 procedures.
3 advantages of CT
- fast (seconds)
- high resolution
- multiplanar reformatting (MPR) - 3D dataset can be reconstructed in any plane
3 disadvantages of CT
- risks of trouble from the IV contrast
spectrum of things that give increasing amounts of electromagnetic radiation
- visible light
- UV light
- x-rays (x-rays, CT)
- gamma rays (nuc med -used for destroying tissue)
can ionizing radiation result in damage to DNA?
what is ionizing radiation?
- electromagnetic waves w the energy to knock an outer shell electron out of its valence shell --> an ion
- can --> damage to DNA
the Inverse Square Law of Radiation and Distance
- radiation dose decreases as a function of distance from the source, squared
- ie. double the distance, get a quarter of the radiation
as low as reasonably achievable - how much radiation you want to use
but it's a compromise btwn dose and quality of image
depends on rate of exposure, tissue exposed, strength of radiation, etc
health effects caused by radiation (erythema, sterility, chateracts...)
neg effects that happen years after radiation exposure - cancer
"linear no threshold model"
it's an assumption - risk is directly proportional to the dose at all dose levels - so you need to look at the sum over time, not just at some one-time high level tx
What would you like to do?
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