NEUROIMAGING

MR generally best tool for imaging the brain Non-contrast CT still used as first test for 2 common emergency indications: head trauma and stroke In both cases looking for acute blood which is white relative to brain, because of lipid content of brain

Head Trauma

3 trauma cases with blood in meningeal compartments going from superficial to deep: epidural, subdural, and subarachnoid bleeds Looking for blood collections that create mass effect on brain and need to be evacuated surgically Usually no mass effect with subarachnoid bleed, but need to consider if subarachnoid bleed was from aneurysm that then caused patient to crash car (which came first)

Stroke

Stroke: sudden neurologic deterioration on vascular basis (CVA) Key reason for doing CT is to determine if stroke is hemorrhagic or not. Causes and treatments are different. May give thrombolytic emergently for acute infarct, but not for bleed How soon does hemorrhagic stroke show up on CT? : Immediately! Following case is typical hypertensive basal ganglia bleed

Next case is non-hemorrhagic left occipital stroke (infarct) How soon does infarct show up on CT?: Depends on the size. Large one may have subtle swelling in 6 hours, but smaller one may take a couple days. Therefore, emergent head CT is done to see if patient has bleed or not, not to necessarily to see an infarct Infarct becomes low density as edema develops

This is the same patient with infarct shown on MR (first 2 series: T1- and T2-weighted) As opposed to CT, MR can measure more than one variable, e.g., T1 and T2, so if lesion not well seen on one sequence, may be better seen on another. CT can measure only one variable, X-ray attenuation MR more sensitive to soft tissues differences than CT. Can see edema of infarct sooner. Water is dark on T1 and bright on T2

Typical MR unit is superconducting magnet which is always on. Need very strong magnetic field to magnetize hydrogen nuclei. Net vector of magnetization is then tipped by adding RF energy. Precessing vector then creates sine wave signal captured by coils placed around body part of interest. The decay of the signal depends on a couple effects, and the images created can depend more on one factor than the other (T1 or T2)

Same patient with additional common MR sequence (FLAIR) FLAIR is like T2 sequence, but pure water signal is set to zero, getting rid of all of the bright signal from CSF. The edema (water) in infarct stays bright since it is in the brain with a different surrounding molecular environment, and therefore a different signal than protons in pure water (CSF). It makes the visual search for pathology easier.

MR signal depends on another variable, motion. The basilar artery on the T2 image is a black dot anterior to the brain stem. Although there is a lot of water in the artery, and it might be expected to be bright on T2, it has a flow void because the protons have moved out of the imaging volume and haven’t gone through entire process of signal generation. Therefore, can do MRA without contrast Diffusion-weighted scan signal depends on microscopic motion/diffusion. Infarct is bright on DW because acute ischemia causes shift of water from extracellular space to intracellular where water can’t move as much. There is less of a “microscopic flow void” and infarct is bright. This occurs much earlier than edema, and is sensitive and fairly specific for acute infarct. Following is the very commonly performed diffusion-weighted sequence on the same patient

62-year-old patient with progressive right arm weakness over the last week

3 images are T1, T2, and T1 gadolinium-contrast-enhanced MR T1 and T2 are very sensitive to pathology, but often not specific, since most pathologies all over body are dark on T1 and bright on T2, like infarct, because they have increased water content T2 sequences good for screening because most pathologies are bright Gadolinium adds specificity. There are 2 enhancing (therefore solid) masses. They light up because Gd doesn’t cross blood- brain barrier into normal brain, but does enter tumor extracellular space Brain mets from lung CA, most common malignancy presenting with brain mets

42-year-old obese patient with chronic low back pain

No imaging recommended. Won’t usually change management.

70-year-old female with sudden severe low back pain when lifting and turning to empty a heavy garbage can

Sounds like osteoporotic compression fx Well shown on plain X-ray

36 year-old-patient with severe left leg sciatica not responding to physical therapy, and now with calf weakness

Imaging not usually indicated initially for sciatica, because will usually resolve with conservative treatment (PT, meds) Do imaging if conservative Rx fails or if neurologic damage develops (weakness, loss of reflex) MR exam of choice because it has excellent soft tissue contrast. CT has very high spatial resolution for high-contrast tissues (like nodules in lung or fracture in bone), but not as good for soft tissues MR shows L4-5 disc protrusion compressing left L5 nerve root.

67-year-old patient with bilateral leg pain increasing with distance walked (non-smoker, no diabetes, normal pedal pulses)

History suggests claudication, but also suggests not vascular This is spinal claudication caused by spinal stenosis

Normal axial lumbar CT for comparison

Severe spinal stenosis on CT due to facet joint degenerative disease and hypertrophy CT will show bony stenosis well, but won’t show soft tissue component as well as MR

L4-5 spinal stenosis on MR

Lumbar normal vs spinal stenosis on MR

73-year-old male S/P prostatectomy for aggressive prostate cancer, now with bilateral leg weakness

While CT will show bony anatomy well, do MR to see soft tissue component Cauda equina compression by prostate CA mets

Spine Trauma

Plain X-rays still often used for lesser degrees of trauma CT for clarification of X-ray findings or as initial imaging for high-suspicion cases with greater trauma Same as using CT to further evaluate CXR findings. Lung and bone are both high- contrast tissues, so use the high spatial resolution of CT. Don’t need the contrast resolution of MR for bone

Normal open mouth view of odontoid for comparison. Note lateral masses of C1 aligned with those of C2

C1 ring fracture. Note spreading of lateral masses of C1 Use CT to better show anatomy

Compared to plain X-rays CT will better show compromise of spinal canal

CT will better show injury to posterior elements that may indicate instability and need for orthopedic mechanical stabilization of spine

When neurologic injury is question, such as spinal cord injury, use MR because that’s soft tissue issue

Facial Trauma

Compared to plain X-rays, CT provides more definitive evaluation for possible facial bone fractures, detecting fractures not visible on X-ray The following axial CT image is a single slice on a patient with a trimalar complex (tripod) fracture The tripod fracture breaks the 3 connections of the zygoma to the rest of the face –Through maxilla including floor of orbit and lateral wall of maxillary sinus –Through lateral wall of orbit, typically at fronto- zygomatic suture –Through zygomatic arch

The 2 following 3-D CT surface reconstructions (same patient as for preceding image) make the 3 components of the tripod fracture easier to see The face is constructed of multiple rigid bony rings, and it is difficult have a displaced fracture through one side of a ring without the fracture completing itself in space (hence the multiple components, analogous to fractures of other bony rings such as the pelvis)

Following coronal CT slice is from patient who had direct blow to left eye Note orbital floor blow-out fracture with prolapse of inferior rectus muscle into fracture gap. Orbital blow-out fractures occur due to direct blow to eyeball resulting in increased intra- orbital pressure causing bony confines of orbit to give way at weakest point (typically orbital floor, but also commonly medial wall of orbit)

Following case is 3-D CT surface reconstruction for a patient who had a blow to mid-face This is a Le Fort I fracture extending through superior alveolar ridge of maxilla on both sides. LeForte fractures break different levels of the central bony face off from the more superior face. Le Fort II fracture is higher, extending from lateral wall of maxillary sinus, across inferior orbital rim and floor of orbit, to bridge of nose (on both sides) Le Fort III fracture is higher still, extending from lateral wall of orbit (usually at fronto-zygomatic suture to bridge of nose (on both sides)