Diagnostic Imaging of the Brain and Spinal Cord

Question 1

Radiographs

Answer 1

• primary uses: skull fractures, foreign bodies, tumors in skull, calcified brain lesions
• often used before MRI to make sure there are no metal objects
• fluoroscopy: real time moving radiographic images
• advantages: inexpensive, easy access
• disadvantages: produces tissue ionization (neutral atom/molecule acquires a positive or negative charge which can cause cell mutation or death)

Question 2

Cerebral Angiography

Answer 2

• basics: contrast material injected into carotids or vertebral arteries, radiographs
• primary uses: detection of vessels abnormality (position, occlusion, malformation, aneurysm, fistula), interventional radiography
• advantages: inexpensive, easy access
• disadvantages: tissue ionization

Question 3

Ultrasonography

Answer 3

• basics: ultrasound waves through fontanelles or craniotomy, computer analyzes the time and intensity of echoes from the waves
• primary uses: positioning and size of internal structures, bleeding, edema
• advantages: inexpensive, fast, easy, portable equipment
• disadvantages: limited by echoes from the surrounding skull, cannot penetrate bone

Question 4

Computed Tomography CT

Answer 4

• AKA CAT Scan
• basics: scanning apparatus rotates a narrow radiograph beam around the head, absorption proportions are converted to gray scale by computer
• best at detecting acute bleeds
• black=low density structure
• white=high density structure
• density changes pathologically: hyperemia or freshly clotted hemorrhage appears more dense, edematous tissue appears less dense
• cross section=1.5mm to 1cm thickness
• contrast material can also be utilized
• primary uses: calcifications, edema, hydrocephalus, tumors, cysts, hemorrhages, aneurysms, vascular malformations
• advantages: cross-section, decent resolution, non-invasive, fast, and safe
• disadvantages: tissue ionization

Question 5

MRI

Answer 5

• basics: strong external magnetic field causes hydrogen atoms in water to spin and emit energy; radiofrequency fields are used to alter this magnetic alignment
• T1 weighted: highlights fat deposits, dark=water (CSF, edema), calcium; bright=lipids, gadolinium
• T2 weighted: highlights water, dark=calcium, bone; bright=water (CSF, edema)
• signal is picked up and analyzed
• primary uses: suspected tumors, infrastructure, demyelination, infarcts
• advantages: high resolution (better than CT), image can be obtained in any plane, no radiation
• disadvantages: bone poorly imaged, costly, slow, ferromagnetic implants

Question 6

Specialized MRI Scans

Answer 6

• FLAIR: suppresses liquid signals (CSF); type of T2, free water is now dark but edematous tissue is bright, useful for MS
• DTI: diffusion of water; maps fibers, useful for MS, concussion, traumatic brain injury, ischemic stroke (can detect within 5 minutes)

Question 7

MRA

Answer 7

• A=angiogram
• visualization of arteries
• useful for stenosis and aneurysms

Question 8

fMRI

Answer 8

• measures increase in blood flow that accompanies neural activity; active brain areas
• blood oxygen level dependent imaging technique (BOLD); measures regional deoxygenation of venous blood
• uses T2 MRI

Question 9

PET

Answer 9

• positron emission tomography
• basics: radioisotopes are inhaled or injected; radioactively tagged molecules bind to specific neurons; pairs of gamma rays from decaying positrons are detected by scanner, reconstructed with CT
• primary uses: major research and clinical tool for blood flow, brain metabolism, and other chemical processes
• advantages: visualization of different types of neutrons
• disadvantages: lack of detailed resolution, positron-emitting nuclides decay rapidly