13 Imaging and Angiography

  1. CATscan (AKA CTscan)?
    • CAT scans employ ionizing radiation (x-rays)
    • Attenuation of the x-ray beam on a CT scan is defined in Hounsfield units

    • no attenuation (air) –1000 
    • water 0 
    • dense bone +1000 

    • Cranial CT
    • brain (grey matter) 30 to 40
    • brain (white matter) 20 to 35
    • cerebral edema 10 to 14
    • CSF + 5
    • bone +600
    • blood clot 75 to 80 (acute SDH or EDH, fresh SAH)
    • fat –35 to –40
    • calcium 100 to 300
    • enhanced vessels 90–100

    • Spine CT
    • disc material 55–70
    • thecal sac 20–30
    • (Hct < 23% will cause an acute SDH to be isodense with brain)
  2. Noncontrast CT scans
    • employed in emergency situations (to quickly rule-out most acute abnormalities), to evaluate bone in great detail, or as a screening test
    • Eg: acute blood (EDH, SDH, IPH, SAH), fractures, foreign bodies, pneumocephalus and hydrocephalus
    • Weak in demonstrating acute stroke (DWI MRI is preferred)
    • Has poor signal quality in posterior fossa (due to bone artifact)
  3. IVcontrast enhanced CTscan (CECT)?
    • primarily for imaging neoplasms or vascular malformations, especially in patients with contraindications to MRI
    • All CT contrast agents contain iodine.
    • Typical IV dose of contrast: 60–65 ml of e.g. Isovue 300® which delivers 18–19.5 grams of iodine.
  4. CT angiography (CTA)
    Employs rapid injection of iodinated contrast at 3–4 cc/sec....typically 65–75 ml of e.g. Isovue 300
  5. Magnetic resonance imaging (MRI)
    • Abbreviations:
    • ● TR: time to repetition
    • ● TE: time to echo
    • ● TI: time to inversion
    • ● T1: spin-lattice relaxation time (“time to magnetize”) (regrowth)
    • ● T2: spin-spin relaxation time (“time to demagnetize”) (decay)

    • T1 weighted image (T1WI)
    • Short T1 →high signal (bright). “Anatomic image”, somewhat resembles CT
    • Short T1 →high signal (bright). “Anatomic image”, somewhat resembles CT
    • -T1WI: CSF is black, subcutaneous fat is white, TR and TE are short (hundreds and double digits, respectively)
    • -white on T1WI are: fat, melanin, subacute blood (>48 hrs old)
    • White matter is higher signal than grey matter (myelin has a high fat content)
    • Most pathology is low signal on T1WI
    • Image Upload 1

    • T2 weighted image (T2WI)
    • Long T2 →high signal (bright). “Pathological image.”
    • -T2WI: CSF is white, TR & TE are long (thousands and hundreds, respectively)

    • Spin density image
    • -AKA proton density image or balanced image
    • -Partway between T1WI and T2WI
    • -CSF= grey, approximately isodense with brain
    • -Becoming less commonly use

    • FLAIR
    • -FLuid-Attenuated Inversion Recovery. Long TR and TE.
    • -Resembles a T2WI except the CSF is nulled out (appears dark)
    • -The grey/white intensity pattern is reversed from T1WI 
    • -Most abnormalities including MS plaques, other white matter lesions, tumors, edema, encephalomalacia, gliosis and acute infarcts appear bright
    • -Good for demonstrating abnormalities in CSF
    • Differential diagnosis of increased signal in subarachnoid spaces on FLAIR:
    • 1. subarachnoid hemorrhage (SAH): 􀀀 the best sequence for detecting acute SAH on MRI
    • 2. meningitis: occurs in some cases
    • 3. meningeal carcinomatosis
    • 4. superior sagittal sinus thrombosis
    • 5. stroke
    • 6. adjacent tumor: ? if related to higher protein
    • 7. previous administration of gadolinium

    • “STIR” image
    • “Short Tau Inversion Recovery.”
    • -Summates T1 & T2 signals
    • -Causes fat to drop out –sometimes also called fat suppression or “fat sat”
    • -Useful primarily in spine and orbit. Very good for showing bone edema (can help in dating spine fractures)

    • Contraindications to MRI
    • 1. Pregnancy--During the first trimester, MRI can cause reabsorption of products of conception (miscarriage).Gadolinium contrast is contraindicated during all of pregnancy, and is not approved for use in age <2 years. Breast-feeding must be interrupted for 2 days after administration of gadolinium to the mother.
    • 2. cardiac pacemakers/defibrillator, implanted neurostimulators, cochlear implants, infusion
    • pumps: may cause temporary or permanent malfunction and  ferromagnetic aneurysm clips (see below): some centers exclude all patients with any type of aneurysm clip
    • 3. Metallic implants or foreign bodies with large component of iron or cobalt (may move in field, or may heat up)
    • 4. Swann-Ganz catheter (pulmonary artery catheter)
    • 5. Metallic fragments within the eye
    • 6. Placement of a vascular stent, coil or filter within the past 6 weeks
    • 7. Shrapnel: BB’s (some bullets are OK)
    • 8. claustrophobic patients, critically ill patients

    • MRI concerns in patients with a cerebral aneurysm clip:
    • 1. the danger of the MRI magnetic field causing the aneurysm clip to be pulled or torqued off of the aneurysm or to tear the neck
    • 2. the artifact produced by the metal of the clip in the magnetic field
    • 3. heat generated in the region of the clip: not clinically significant
    • The more ferromagnetic the clip, the larger the force exerted on it by the magnetic field and the greater the image distortion near the clip.

    • Hemorrhage on MRI
    • -FLAIR is the best sequence for detect ing SAH on MRI
    • Phase Approximate time after onset T1 MRI T2 MRI
    • Hyperacute  0-6 hrs                           I     B 
    • Acute         6-72 hrs                          I     D 
    • Early subacute 3-7 d                         B     D 
    • Late subacute 7-14 d                        B     B 
    • Chronic     >2 weeks                         D     D
  6. MRI contrast
    • Current agents are mostly based on gadolinium (a rare earth metal which is paramagnetic in solutions)
    • Adverse reactions:
    • 1. anaphylactic reactions: rare (prevalence: 0.03–0.1%)
    • 2. nephrotoxicity
    • 3. nephrogenic systemic fibrosis (NSF): a rare, but serious illness characterized by fibrosis of skin, joints and other organs, which is associated with certain gadolinium containing agents given to patients with severe renal failure (most were on dialysis).Gadolinium is now relatively contraindicated with a GFRof 30–60 ml/min, and is contraindicated with GFR<30
    • 4. gadolinium allergy: use the same allergy prep as for iodine allergy
  7. Magnetic resonance angiography (MRA)
    • Two ways to obtain an MRA:
    • 1. Gadolinium enhanced: usually for extracranial vessels (e.g. carotids)
    • 2. Noncontrast images using flow related enhancement techniques (most common: 2D time of flight)--Usually for intracranial vessels
  8. Diffusion-weighted imaging (DWI) and perfusion-imaging (PWI)
Card Set
13 Imaging and Angiography