NMTCB(CT) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Metformin hold after contrast

Withhold metformin for 48 hours after iodinated contrast when eGFR is below 30 mL/min/1.73 m2 or acute kidney injury is present, and restart only after renal function is confirmed stable.

Withhold contrast below eGFR 30

IV iodinated contrast is generally avoided when eGFR is under 30 mL/min/1.73 m2; values from 30 to 44 require caution with reduced volume and adequate hydration.

Contrast premedication regimen

Prednisone 50 mg by mouth at 13, 7, and 1 hour before contrast plus diphenhydramine 50 mg one hour before; methylprednisolone 32 mg at 12 and 2 hours is an alternative.

Epinephrine for anaphylactoid reaction

Give 0.3 to 0.5 mg of 1:1000 epinephrine intramuscularly into the lateral thigh for bronchospasm or hypotension, repeating every 5 to 15 minutes as needed.

Extravasation management steps

Stop the injection, aspirate residual contrast via the cannula, remove it, elevate the limb, apply cold compresses, and document; large volumes or blistering warrant surgical consultation.

Power injector IV access

Use a 20-gauge or larger catheter in a large antecubital or forearm vein and confirm patency with saline before injecting contrast at rates above 3 mL/s.

CIN prevention measures

Use the lowest feasible contrast volume with normal saline hydration, avoid dehydration and nephrotoxins, and select low- or iso-osmolar rather than high-osmolar contrast media.

Pregnancy screening before CT

Ask about the last menstrual period and the possibility of pregnancy, document the response, and consult radiology before scanning when pregnancy is known or suspected.

CTDIvol defined

Volume CT dose index, in mGy, is the average dose within the scanned volume for one rotation, measured in a standard 16 cm or 32 cm acrylic phantom.

DLP calculation

Dose-length product equals CTDIvol multiplied by the scanned length in centimeters, is reported in mGy·cm, and reflects total radiation output for the acquisition.

Effective dose conversion

Multiply DLP by a region-specific k factor in mSv per mGy·cm; approximate adult values are 0.0021 for head, 0.014 for chest, and 0.015 for abdomen.

Pitch effect on dose

In helical scanning dose is inversely proportional to pitch, so raising pitch from 1.0 to 1.5 lowers dose by about one third while slightly increasing image noise.

Tube current modulation

Automatic exposure control varies tube current along the z-axis and around the patient to match attenuation, holding noise constant while reducing overall mAs and dose.

mAs and image noise

Noise is inversely proportional to the square root of mAs; quadrupling mAs halves noise, and reducing mAs to one quarter doubles image noise.

kVp effect on dose

Radiation dose varies approximately with the square of kVp, so lowering kVp from 120 to 100 reduces dose roughly 30 percent and increases iodine conspicuity.

Lead apron attenuation

A 0.5 mm lead-equivalent apron attenuates more than 90 percent of scattered diagnostic x-rays at typical CT energies and should be worn during any in-room procedure.

Portal venous phase timing

Abdominal portal venous phase images are acquired roughly 70 to 90 seconds after IV contrast, when hepatic and portal veins are opacified and liver lesions are most conspicuous.

Bolus tracking threshold

A region of interest is placed in a target vessel and scanning begins when attenuation reaches a preset trigger, commonly 100 to 150 HU in the aorta or pulmonary artery.

CT pulmonary angiography technique

Use 1 to 1.25 mm collimation timed to pulmonary arterial opacification, typically 50 to 75 mL of contrast at 4 to 5 mL/s followed by a saline chaser.

Iliac crest landmark

The iliac crests approximate the L4 vertebral body, a surface landmark used to center abdominal scans and to set the lower scan boundary.

Gantry isocenter positioning

Center the patient at gantry isocenter because off-center position increases dose and noise and skews the patient size estimates used by tube current modulation.

Arms raised for torso CT

Elevate both arms above the head for chest and abdominal scans to prevent beam hardening and streak artifact through the torso from the humeri.

HRCT chest parameters

High-resolution chest CT uses 1 to 1.25 mm slices and a high-spatial-frequency kernel, with inspiratory, expiratory, and prone images showing air trapping and dependent atelectasis.

Circle of Willis

An arterial ring at the base of the brain formed by the anterior, middle, and posterior cerebral arteries linked by the anterior and posterior communicating arteries.

Hounsfield unit formula

HU equals 1000 times (mu tissue minus mu water) divided by (mu water minus mu air); water is 0, air is -1000, and compact bone is about +1000 or higher.

Window width

Window width is the range of Hounsfield units mapped to the display gray scale; a narrow width raises contrast and apparent noise, while a wide width lowers contrast.

Window level

Window level, or center, is the midpoint Hounsfield unit of the displayed gray scale and should be set near the attenuation of the tissue of interest.

Helical pitch definition

Pitch equals table travel per gantry rotation divided by the total collimated beam width; values below 1 overlap data and increase radiation dose.

Beam hardening artifact

Preferential absorption of low-energy photons produces cupping in uniform objects and dark streaks between dense structures such as bone, metal, or concentrated contrast.

Photon starvation artifact

Very few photons reaching the detectors through thick or dense regions, such as the shoulders, create noisy streaks that higher mA or iterative reconstruction can reduce.

Reconstruction kernel selection

Sharp or bone kernels improve spatial resolution but increase noise, while smooth kernels reduce noise and improve low-contrast detectability in abdomen and brain studies.

Iterative reconstruction benefit

Iterative algorithms model system geometry and photon statistics, allowing 20 to 40 percent lower dose or mAs while keeping noise comparable to filtered back projection.