1. ) What is a CT image composed of ? 2. )CT will vary according to? 3. ) What CT is assigned to air and what CT is assigned to water?

1. )tiny squares called pixels- will be assigned a number from -1000 to +1000 measured in Hounsfield units (HU's) 2. )-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 3. )air: -1000 Hu water: 0 Hu

CT Imaging Flashcards by Sydney DeArmond

what size should you take the slices on CT?

always less than 3-5mm or you’ll miss things

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) 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

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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

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what are 2 characteristics of denser substances in CT?

absorb more xrays
↑ attenuation (brighter)
whiter on CT

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what is attenuation?

stopping of xrays

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After CT scans are obtained they can be?

windowed to optimize visibility of different types of pathology (enhance different structures)

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CT can be ordered to evaluate which 4 conditions?

tumors
trauma
infection
bleeding

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CT is structural imaging not? Because it represents?

Not functional imaging because it represents a moment in time

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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

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Axial plane

cut body in half like magician would

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Coronal plane

think of a tiara like cut through the body→ anterior and posterior sections

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Sagittal plane

think about the lab where we split the gonads in half or when we wanted to view the sinuses

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which 3 ways can contrast be administered?

IV
enteric (oral or rectal)
intrarticular

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what are the risks of omitting contrast when it is indicated?

diagnostic/treatment errors

unnecessary morbidity & costs

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what are 2 ADRs of contrast?

acute adverse reactions

contrast induced nephropathy (CIN)

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all modern contrast is made of what substance?

iodine base

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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)

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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

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What is the most important thing when it comes to giving contrast?

timing

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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

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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