Does an X-ray have more or less energy than light? Long or short wavelength? Why does this matter?
More and shorter wave length.
Because they have more energy, they are able to pass through some substances that light cannot.
What is the basis of X-rays?
- X rays pass through/are blocked to varying degrees by different substances.
- Depends on the density and thickness of substance
What are the methods of obtaining an x-ray image?
- Films with fluorescent phosphorous
- Computed Radiology (CR) where x-rays strike a sensitized plate which is read and digitalized into a computer image.
- Digital Radiography (DR) where x-rays strike a plate of sensors producing the computer image directly
Are x-rays a form of ionizing radiation? Why is ionizing radiation bad?
Yes. This is a hot topic, especially with regard to pediatric patients. The goal is to reduce radiation exposure.
Ionizing radiation can displace electrons creating “free radicals” that harm biologic tissue via cell death.
What imaging study is responsible for most ionizing radiation in the US? How is radiation measured?
The CT scan (especially a pelvis and A/P = 15 mSv)! Radiation is measured in millisieverts (mSv)
What are the deterministic effect of radiation? Can we predict and document these?
Deterministic effects result from cell death.
Examples include skin erythema, epilation, radiation burns and cataracts. These are easily predicted/documented because there is an EXPOSURE THRESHOLD.
Effective dose is estimate of radiation effect to POPULATION, not a specific patient
What are the stochastic effects of radiation? Can we predict and document these? What is ALARA?
The stochastic effects of radiation are cancer or leukemia. These are hard to predict because there is NO THRESHOLD. May/may not occur, after a long delay (think pediatric patients!), at a very low rate. Just the possibility of these effects lead to exposure levels being kept as low as reasonably achievable (ALARA).
Effective dose is estimate of radiation effect to POPULATION, not a specific patient
Why are pediatric patients more at risk for stochastic radiation?
- Stochastic effects generally occur many years after radiation exposure- a child has many more years of life than an elderly patient.
- They have a higher proportion of rapidly dividing cells aka radiosensitive cells
- Harder to shield due to small size
What shape of curve is the lifetime risk of radiation-induced cancer?
A linear, no threshold dose response curve.
Average lifetime attributable risk of 1/1,000 patients receiving a 10mSv effective dose. Approximately half expected to be fatal.
Do medical x-rays pose a threat to a developing fetus?
No, but all females are screened and take a pregnancy test if necessary.
- x-rays are highest risk during the 1st trimester because the fetus is in very early development.
- Risk decreases with fetal age, so must balance risk to mother with risk to fetus. Would still do an x-ray if you suspect a PE or appendicitis.
What is the first line of imaging for evaluation of the lung, heart and skeleton? Why?
X-ray, due to its availability, speed and low cost.
Examples of x-ray images. Are they 2D or 3D?
2D–> ALWAYS get more than one view
- Chest x-ray (CXR)
- Spine films
- Skeletal films
- Abdomen films
How do x-rays pass through bones, metals, air and fat respectively?
- Bones and metals ABSORB x-rays, making them look WHITE.
- Air or fat PASS THROUGH x-rays, making them look black.
What is the basis of ultrasound? Does it involve ionizing radiation?
High frequency sound waves leave probe, travel into body and are differentially reflected back to the machine for analysis. Location of structures is based on time to return, composition of structure is based on intensity of echoes.
- Fluid is black, reflections are white (fat/gallstones)
NO.
How to differentiate a gallstone from tumor in gallbladder? What imaging?
Use ultrasound- the fluid is black while stones are white. if a gallstone, they should be MOTILE and move as your reposition the patient.
What is ultrasound used for?
Evaluating pregnancy and gallbladder!
- Best way to assess abdominal or pelvic pain in women, very good at evaluating the uterus and ovaries
- Good option for children because NO radiation
- Deep vein thrombosis leading to pulmonary emboli
What is the basis for CT?
Electromagnetic energy (ionizing radiation) moves around the patient in a tube (the GANTRY moves) scanning from many angles to generate a 3D image.
What is CT used for? What is it NOT suited for?
- Best test for screening the brain for acute hemorrhage, and looking for mass effect or herniation in the brain.
- Best test for almost all imaging of the chest (PE), tumor and interstitial lung disease.
- Best test for almost all abdominal screening (tumor, inflammation, obstruction, urinary tract disease/stones). Can actually see the KUB (vs KUB x-ray, see none of that)
Not suited for GB disease or abdominal pain in young women (US is best).
When is MRI more advantageous than CT?
- MRI is preferred for seizure foci or MS.
- MRI is preferred for acute, non-hemorrhagic infarction (stroke). CT could miss this initially.
How does CT administered? When is it used?
CT Contrast is an iodine based dye
Oral: opacify the bowel (esophagus, stomach, small bowel)
IV: opacify vessels, demonstrate lesions in organs, identify lymph nodes from their adjacent vessels
What are the risks of CT Contrast? What screening test do we use?
- Allergies:
1. usually mild, rash/hives
2. rarely, severe hypotension, bronchospasm and death - Nephropathy: VERY common source of acute renal failure. MUST TEST FOR RENAL FUNCTION. Risks: preexisting poor renal function, type 2 diabetes, nephrotoxic drugs and reduced IV volume.
How can we prevent CT contrast neuropathy?
RENAL SCREENING
- hydration
- mucomist- antioxidant given to people on some hypoglycemia agents (metformin)
- Try different contrast medias
What is the basis for MRI? How is an image generated?
- Tissues are mostly water with 2 hydrogen atoms/protons.
- In the magnet, dipoles of protons line up with direction of magnetic field
- A radiofrequency pulse is applied to flip the spins
- Protons then relax as electromagnetic field is turned off
- Receiver coils read the radio frequency signal generated by tissues
Image generated by length of TIME for protons to relax, resulting in different signals sent to coils.
Is an MRI safe? When is it contraindicated?
YES! Especially compared to CT- it has no ionizing radiation.
- Risky with patients with pacemakers or implants like aneurysm coils, surgical devices.
- Image may have artifacts due to other metals (like braces or rods in bones etc.)
