Who discovered X-rays?
Wilhelm ROentgen 1895
Describe the general theory behind x-rays?
x-rays are waves of ionizing energy that will pass through the patient and strike a cassett which contains a screen of fluorescent phosphors that will expose the film
the x-rays will pass thorugh material differently depending on density
they will not pass through bone, so bone swill appear white.
They will pass through air, so lungs appear black
they will pass through water and gas to a lesser extent = gray
Why are x-rays capable to harming biological tissue?
it is short wave length, high energy rays that can displace electrons from their energy level or shell around the nucleus
the displaced electron can harm the tissue, as can the resultant free radical
What form of medical imaging is responsible for the majority of excess radiation exposure?
CT
What percentage of our radiation exposure is due to medical imaging?
48%
What are some deterministic effects of radiation?
these are actual cell death:
skin erythema, epilation, radiation burns, or cataracts
these are easier to document and predict because there is an exposure threshold (.5-2 Gy dose to les leads to risk of cataracts)
What are some stochastic effects of radiation?
cancer like leukemia
these are more difficult to predict - we don’t know the threshold level
WHat is ALARA?
the practice of keeping exposure levels as low as reasonably achievable
What are three reasons why children are at greater risk for stochastic effects?
They live longer, so there is more time for the cancer to develop after exposure
they have a higher proportion of rapidly dividing cells
they are smaller, so it’s more difficult to shield them effectively
What is the scientific unit of measurment for radiation?
millisieverts (mSv)
What is the yearly background radiation we’re exposed to on average?
WHere is the radiation coming from?
about 3 mSv
from radon gas from the earth, natural radioactivity from rocks, cosmic rays, smoke detectors, TVs, etc.
Where does our evidence of cancer from radiation come from?
- people treated for thymic disorders can develop tumors in those areas
- people with TB or scoliosis have multiple chest studies and have higher risk for breast cancer
- atomic bomb survivors
What amount of radiation dose is needed for a significant increase in cancer risk?
studies vary, some say 5-125 mSv, others say 50-120 mSv
What is the most widely accepted risk model estimating the lifetime risk of radiation-induced cancer>
the linear no-threshold dose response curve
What does the lienar no-threhold dose response curve model say is the cancer risk to radiation?
1/1000 patients exposed to 10 mSv effective doses of radiation will develop a radiation-induced cancer
What are some technical things we can do to reduce radiation dose?
- decrease tube current or voltage (but this will increase image noise)
- Choose wider collimation or change the speed at which the table moves
At what point during pregnancy is x-ray imaging advised against?
Try to avoid exposure during first trimester because that’s the time of organ formation (especially in first two weeks)
In an abdomen film, what shows up well and what will not be seen?
bones and gas will show up well
abdominal orgens like the liver, spleen and kidneys are not well seen
Why is it necessary to always get more than one view when taking x-rays
x-rays are two dimensional pictures of three dimensional objects, so you need at least two to be able to figure out what you’re dealing with and where it is in the body
What are the different views you can take in a CXR?
In an extremity film?
PA, AP, Lateral, Lateral decubitus view (laying down
AP, Lateral and Oblique
How does ultrasound work?
it uses high frequency sound waves that leave a probe, travel through the body, and are reflected back to the machine to be analyzed
the location of the structure is based on the amount of time required for the sound waves to return to the prove
the intensity of the echnoes on the image is dependent on the composition of the structures
What are some of the things we almost always use ultrasound for?
evaluating pregnancy
evaluating the gallbladder (this is the absolute best way for this)
asses abdominal or pelvic pain in a woman (really good at evaluating the uterus and ovaries)
evaluate abdominal pain in children and thin females (they don’t have fat to act as a contrast for CT anyway)
evaluating veins for DVT
How does computed tomography (CT) work?
it uses electromagnetic energy
the patient moves through a gantry and it makes scans form many angles and generates a 3D image
In terms of the brain, what is CT best for?
it’s the best test for screening the brain for acute hemorrhage
also the best to look for mass effect or herniation
NOT as god as MRI for seizure foci, MS, acute or non-hemorrhagic infarction
In terms of the chest, what is CT best for?
it’s the best for almost all imaging of the chest, including PE, tumor, and interstittila lung disease
In terms of the abdomen, what is CT best for?
This is the best test for almost all abdominal screening, including tumor, inflammation, obstruction, or urinary tract disease, including stones
Why and when is oral contrast given?
oral contrast is used to opacify the bowel in abdominal studies
IV contrast can opacify the vessels
What are the risks of IV contrast?
allergic reaction - most mild, but some severe
contrast-induced nephropathy
How can we avoid contrast induced nephropathy?
- screen patient’s renal function
- only use contrast when indicated
- keep the patient well hydrated
- give mucosmist to diabetics on metformin (an antioxidant)
How does MRI work?
It uses a high field strength magnet to align hydrogen ions
a radio frequency pulse specific to hydrogen ions causes them to spin
over time, the hydrogen atoms will return to alignment with the magnetic field
different tissues will return to alignment in different amounts of time
FOr what two body systems is MRI most useful?
brain and musculoskeletal system - especially the spine
it’s typically only a problem solving technique in the chest abdomen and pelvis
What are some pros and cons of MRI?
It does not use ionizing radiation, so no risk of cancer
but….it takes a long time and it’s expensive
plus, people have to hold still for a long time in a really some gantry
metal causes lot so f artifacts and there are contraindications: pacemakers, aneurysm clips, other surfical devices
How does nuclear medicine work?
radiotracers are injected into the body and then the patient is imaged with a special scanner to visualize the tracers within the body
this allows the test to be more functional, but you get less detailed anatomic information
How does a barium examination work?
it utilizes x-rays
the barium contrast material is placed into the GI tract - either a barium swallow or a barium enema, depending
less common now - we use endoscopy and CT more
How does PET work?
it uses fluorine 18 fluorodeoxyglucose or FDG
, which is an analog of glucose
it’s good for loking at tumors because tumors take up glucose at higher rates
What are some downfalls of PET?
images often lack anatomic landmarks for precise morphologic orientation - that’s why they are oten overlayed with CT scans
the sites of pathologic uptake could be confused with normal physiologica uptake or vice versa (i.s. the brain, bladder, and left ventricle take up more glucose naturally, but that doesn’t mean a tumor can’t grow there as well)