CT Imaging Flashcards
what size should you take the slices on CT?
always less than 3-5mm or you’ll miss things
- ) What is a CT image composed of ?
- )CT # will vary according to?
- ) What CT # is assigned to air and what CT # is assigned to water?
- )tiny squares called pixels- will be assigned a number from -1000 to +1000 measured in Hounsfield units (HU’s)
- )-CT number will vary according to density of tissue scanned and is a measure of how much of the x-ray beam is absorbed by the tissue at each point in the scan
- )air: -1000 Hu
water: 0 Hu
what is the order of the densities detected in CT in Hounsfield units?
air: -1000
fat: -40 → -100
water: 0
soft tissue: 20-100
bone: 400-600
metal >1000
what are 2 characteristics of denser substances in CT?
absorb more xrays
↑ attenuation (brighter)
whiter on CT
what is attenuation?
stopping of xrays
After CT scans are obtained they can be?
windowed to optimize visibility of different types of pathology (enhance different structures)
CT can be ordered to evaluate which 4 conditions?
tumors
trauma
infection
bleeding
CT is structural imaging not? Because it represents?
Not functional imaging because it represents a moment in time
What are the CT limitations?
- Size of patient (<500lbs)
- Claustrophobia
- Can they hold still?
- Can they hold their breath?→ issues with elderly patients
- Use water-soluble oral contrast if perforation is suspected or aspiration concern
Axial plane
cut body in half like magician would
Coronal plane
think of a tiara like cut through the body→ anterior and posterior sections
Sagittal plane
think about the lab where we split the gonads in half or when we wanted to view the sinuses
which 3 ways can contrast be administered?
IV
enteric (oral or rectal)
intrarticular
what are the risks of omitting contrast when it is indicated?
diagnostic/treatment errors
unnecessary morbidity & costs
what are 2 ADRs of contrast?
acute adverse reactions
contrast induced nephropathy (CIN)
all modern contrast is made of what substance?
iodine base
what are the 3 classifications of IC contrast? which causes the most allergic reactions? what is most common?
- High-osmolarity contrast media (HOCM)**allergic reactions, not used
- Low-osmolarity contrast media (LOCM)→ most common
- Iso-osmolariry contrast media (IOCM)
what is enteric contrast made of and how likely are they to cause allergic reaction?
some is iodine based
all have small potential to cause acute reaction
What is the most important thing when it comes to giving contrast?
timing
what are the uses for positive and negative enteric contrasts? what are they made of?
positive:
- high attenuation in CT
- dilute suspension of barium or iodinated agent
negative:
- water attenuation on CT, distends belly to delineate bowel mucosa, detect active inflammation or active GI bleeding
- plain water for stomach and duodenum but absorbed after that
where can you find recommendations for CT indication?
ACR appropriateness criteria
in what 3 scenarios should you not use contrast in a head CT? why?
trauma, stroke, intracranial hemorrhage
-blood will provide enough contrast
when should you not use contrast for cervical spine CT?
trauma unless arterial injury possibility or penetrating MOI
→ MOI: mechanism of injury
when SHOULD you use contrast for cardiothoracic CT?
evaluating heart and thoracic vessels
trauma
staging of primary thoracic neoplasms
when should you not use contrast for an abdominopelvic CT?
When should you use it?
CT colonography
renal stone eval (stone will show)
extraparenchymal lymphoma
SHOULD USE: for virtually all other indications -→ GI → hepatopancreaticobiliary → GU → GYN
when should you not use contrast in a MSK (orthopedic) CT?
When should you use it?
extremities
spine
SHOULD USE IT: evaluation of soft tissue masses
when should you not use contrast in CT angiography?
- monitoring a known aneurysm for growth
- detection of a hematoma
when should you use enteric contrast?
- gastric/small bowel perf post surgery
- separate bowel loops in pts with minimal visceral fat
- neg contrast for GI bleed
when should you not use enteric contrast?
-diagnosis of appendicitis and diverticulitis
-eval of: Kardashians BALS Bones Very Retro liver, kidney, bladder, spleen, adrenal glands, retroperitoneum, bones, vasculature
what are 2 contraindications for enteric contrast?
- aspiration risk (?mental status)
- high grade bowel obstruction
Avoid barium based oral contrast in patients who have?
-Pts that are at risk for bowel perf because it can lead to ↑ risk for mediastinitis & peritonitis
What patients are at low risk of complications when using barium based contrast? Why?
-patients with chronic leaks/fistulae because these cavities are already experience chronic exposure to bowel content
can you use LOCM/IOCM with previous allergic reaction to HOMC?
yes, much less likely to occur
what should you ask about if someone reports a previous allergic reaction to contrast dye?
iodinated or not (not iodinated ex: gadolinium for MRI)
what are 6 symptoms of anaphylaxis?
I See U Eating Hot Ass Brownies
itching urticaria (hives) erythema stridor→ high pitch wheezing hoarseness bronchospasm anaphylactic shock (hypotension, bradycardia)
Signs and symptoms of physiologic symptoms of IV contrast are?
These are directly related to ?
- transient warmth or chills
- nausea
- vomiting
- metallic taste
the dose
→ dose-dependent rxn’s
what regimen should you give to a patent with a previous allergic reaction to contrast requiring contrast presently?
corticosteroid (solo-medrol) +/- antihistamine
PO over 12-13 hrs or
IV over 5hrs
If patient the patient comes to the ER needing an emergent and necessary CT scan requiring contrast but had a prior rxn to iodine contrast (mild) should you give it to them
yes
will someone with a previous history of iodine based contrast react to gadolinium?
no not molecularly similar and not cross-reactive
when should you check renal function prior to IV contrast admin?
- pt >60yrs old, but healthy
- younger with DM
what should you coadminister with contrast?
bolus IV fluids to protect the kidneys
what medication needs to be held for 1-2days before contrast admin?
metformin
what food allergy predisposes to IV contrast allergy?
true seafood allergy (can be premedicated outpatient with Benadryl and solumedrol
what scan should you do for trauma?
non-contrast CT
what’s the next step if a CXR shows a nodule?
contrast CT
what scan should you do for ?PE?
CT angiogram (contrast)
should you use contrast for renal CT?
no, don’t need it
what are the differences with abdominal CT in thin and BMI >25 in terms of contrast?
BMI>25 IV contrast
thin: need to give oral contrast which takes some time to get into abdomen
when should you do an abdominal CT for abdominal pain?
no evidence of gall bladder, ovarian cyst, and can’t convince them they’re fine
a thin teen patient with a ?appendicitis should have which test first?
ultrasound
which 2 tests should you preform for a patient with previous kidney stones with abdominal pain?
ultrasound and KUB
→ KUB= kidney ureter bladder x-rays
