Radiation dose

• Annual background - 3 mSv
• CT - 10 mSv

What test should I order?

• ACR Appropriateness Criteria!!

Radiograph

• Radiolucent = black (air)
• fat
• soft tissue/fluid
• Radioopaque/radiodense = white (calcium/bone, metal)
• CXR, AXR basics

CT

Contrast in CT

• ? risk of contrast nephropathy
• Contrast cut-off Cr 2.0/eGFR 30
• If Cr 1.3-2.0/eGFR 30-45, hydrate before/after
• A lot of attending preference/variation for Cr cut-off...
• Avoid if in AKI
• Avoid in pulmonary edema due to volume load
• Will need at least 18-20G antecubital for high flow IV contrast (PE protocol, aortic dissection, angio)
• Contrast allergy

• Mild (nausea, urticaria)	Pre-medicate (takes a while!): Prednisone 50 mg PO 13, 7, and 1 hr before CT AND Benadryl 50 mg PO/IV 1 hr before CT Rapid pre-medication (4 hrs) possible; call the radiologist (no strong evidence for efficacy)
  Moderate (facial edema, throat tightness w/o dyspnea, bronchospasm)
  Severe (pulmonary edema, angioedema, dyspnea, anaphylaxis)	Contraindicated

• Indications for IV contrast
• Typically want IV contrast for abdominal imaging
• Don't need for: renal stones, lung parenchyma/nodule, ? RP hematoma
• Indications for with and without IV contrast
• Bowel ischemia, active GI bleeding, ingested material (e.g. pill) vs. hemorrhage
• AAA (can show unstable thrombus)
• Solid organ tumors (renal cell, pancreas, liver) for better characterization/management of the tumor
• Indications for PO contrast
• Suspected anastamotic leaks/perforations
• Avoid in n/v, risk of aspiration (can cause chemical pneumonitis)
• Barium - used for barium swallows, enemas
• Gastrografin - water soluble (iodine based); use if concern for bowel perforation as barium can cause peritonitis

Ultrasound

• Ultrasound basics
• FAST exam for trauma

MR

• Hypointense = black = gas
• Hyperintense = fluid

• Typical spin echo (classic T1/T2) images are slow to acquire (not ideal for imaging entire body; more suited to MSK)
• T2 HASTE is faster sequence for body imaging to get "lay of the land"
• Gradient echo (GR) sequence is faster and T1 weighted, used for pre/post-contrast

 MR contrast

• Gadolinium
• Risk of NSF (nephrogenic systemic fibrosis) (scary!)
• Calculate GFR; cut off is < 15-30 mL/min for contraindication
• eGFR 30-60 mL/min - don't give if in AKI (eGFR may be inaccurate)
• eGFR > 60 mL/min = contrast OK

MR safety - is implant safe? Check www.mrisafety.com

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MR physics

• Hydrogen nucleus is a proton, which has an electric dipole
• Every electric field has a magnetic field, so protons have magnetic fields
• Proton magnetic fields can align with an external magnetic field B₀
• # protons aligned ~ B₀ (5/1,000,000 aligned for B₀ = 1.5 T)
• Protons wobble around the B₀ axis at the Larmor frequency f ~ B₀ (e.g. 42.6 MHz @ 1 T)
• MRI alternates B₀ with B₁ (using an RF electric field at 64 MHz), which flips/perturbs the proton at an excitation angle
• When B₁ is shut off, protons realign with B₀, generating an electric field
• T1 constant ~ time it takes for protons to realign with B₀ = repeat time = TR
• Governed by proton interaction with macromolecules/stationary protons (cell wall, lipids, myelin, etc.) - spin-lattice interaction. More macromolecules = ↓T1, ↓T2
• Short T1 = bright
• T2 constant ~ time it takes for protons to lose phase coherence (dephase) with B₁ = echo time = TE
• Governed by proton interaction with other water/mobile protons (diffusion, perfusion, water content) - spin-spin relaxation. More water = ↑T1, ↑T2
• Long T2 = bright
• T1/T2 is different for protons in different environments (white/grey matter, CSF, fat, etc.)
• 

PET

• Avid/hot = bright (increased tracer uptake)
• SUV = standardized uptake value (determined by tracer dose, pt weight, time since injection)
• Use mean SUV in liver as "blood pool" reference - should be 2-3 SUV
• Cut-off between benign/malignant is SUV of 2.0-2.5