Leigh Vaughan, MD June 5, 2012 MUSC

Outline CT Advantages, disadvantages Modality basics Appropriate uses With or without contrast MRI Advantages, disadvantages MRI Physics With or without contrast T1, T2, Flare, diffusion Anatomy Tutorial Resources and References

CT Basics Benefits: More accessible Less expensive compared to MRI Disadvantage: More hazardous radiation exposure Risk of nephrotoxicity or adverse reaction with IV contrast Motion or metal artifact Display: Transverse images view with right of patient on the left (“foot of bed” view)

CT Basics Types: helical or spiral, multiplanar reformating, ultrafast (electron beam) Resolution: varying thickness from 1mm to 10mm, with varying intervals (high res 8-10mm) Measurements: Tissue density measured in Hounsefield units (-1000 to +1000); the more negative the HU, the blacker the image, the less dense the object Reading: High attenuation or density= white & low attenuation or density= black

CT Windows Lung Window- parenchyma, bronchial anatomy. Negative HU Increased opacity: consolidation, collapse, mass, interstitial disease, pleural disease Decreased opacity: destroyed parenchyma (emphysema, bullae, cyst) decreased blood or air flow (infarct, emboli, pneumothorax) Mediastinal Window- hilar, pleural, & mediastinal anatomy Increased opacity: LN, hematoma, goiter, mass Decreased opacity: pneumomediastinum Bone Window- Most dense, highest HU

Mediastinal window, with contrast

Lung window

Indications for Contrast With Contrast IV: vasculature (evaluation of PE, aortic dissection) pancreatitis brain abscess avascular tissue lymph tissue pleural disease tumor delineation PO: non-intestinal abdominal structures (abscess, mass) Without Contrast Uses: pulmonary nodules renal stone* sinus disease hydrocephalus acute stroke (unknown type)* trauma calcium scoring in CAD interstitial disease (HIGH RES)

Gallstone-induced pancreatitis in 27 year-old woman Balthazar, Emil J. Radiology. 2002; 223: Copyright © 2002 by RSNA

MRI Basics Advantages: No ionizing radiation Safer in pregnancy Better soft tissue contrast Disadvantages: More expensive Less available Unsuitable in unstable or claustrophobic Unsuitable with foreign objects (aneurysm clips, pacers, cochlear implant, cardiac stents, shrapnel) Not optimal for bone Precautions: Remove transdermal patches (aluminum)

MRI Physics MRI machine uses strong magnetic field to detect the location and local chemical environment of protons in water molecules T1/T2 relaxation times- the time it takes for nuclei to return to its original alignment in longitudinal (T1) or transverse (T2) axis of the magnetic field Use “signal” when speaking about tissue (rather than “density” used with CT’s) Planes: sagittal, axial, coronal

MRI With or Without Contrast Contrast with Gadolinium Gadolinium slows down relaxation phase (shorten T1) & increases signal on T1 weighted images- relatively more contrast goes to vascular structures, producing increase in T1 weighted signal intensity Water/pathological areas appears brighter on T1 contrast. Contrast contraindicated in ESRD requiring renal replacement (not recommended with GFR < 30)

MRI: T1 & T2 T1 ANATOMY– longitudinal tissue relaxation -water (CSF, urine) is dark/ fat is bright T2 PATHOLOGY– transverse tissue relaxation -water (CSF, urine) is bright/ fat is dark Good to establish edema (white or increase signal) Distinguish pathologic tissue from normal T2 with FLAIR (which speeds up imaging time)- - most helpful in multiple sclerosis/demyelinating -free water is now dark, but edematous tissues remain bright.

Diffusion weighted images Images reflect random motion of water Most helpful in evaluation of early, acute stroke (<6 hrs) In acute stroke, there is decrease water diffusion/motion, injured tissue appears white Images can be shown in ADC maps (apparent diffusion coefficient ) which reverses the signal (ie. acute stroke is then black)

CT Radiology Anatomy Tutorial Chest: Abdomen and Pelvis Head oRad.htm

Other references Textbooks, online e-tools, website links, journals, student and resident specific material, & CME Anatomy tutorial: