Notes f or 1/19/99

Susie El-Saden

In plain film radiography, we see only bone, and we have to infer the location and health of other structures. To a certain extent, calcifications of some structures can be used to form inferences about the health of other tissues.

Pneumoencephalography (a rare form of medieval torture) was used to force air from the spinal sac to the brain, and to outline the soft structures of the brain. This was practiced to just a few years ago.

MRI by comparison, shows native tissue signal.

Pituitary tumors often indicated by visual problems: a secondary diagnosis.

The MRI is capable of exquisite resolution. MRI with contrast is a popular study. Gd (gadallinium, an MR contrast agent) goes to portions of the brain not having an effective blood brain barrier. The Pit has no BBB and enhances richly, because it also has a high blood volume (unless it is the location of cystic disease. The clivus also shows intense. Some vessels, esp. small veins, will enhance. Signal in arteries is a sign of trouble, possibly indicating slow flow.

CT is not dead: it is the first line of defense, because it is quick (3 minutes), relatively cheap, and has exquisite resolution with bones.

Ex: MVA. Using bone windows (contrast adjusted to signal intensity of bone), even very fine structures and abnormalities can be seen. Her example shows a hairline skull fracture. CT can be made quantitative using Hounsfeld units of x-ray density.

Black dots on CT scan are air. Any air in the head is abnormal, a pneumocephalus.

Most skull fractures are in the temporal lobe, and is very likely to lacerate the temporal (medial meningeal artery). Blood in CT shows up as increased density (intensity). The artery is very small (smaller than a piece of spaghetti). When addressed quickly, an epidural hematoma may be effectively treated with surgery. These are frequently associated with loss of consciousness, due to the blood deposition on the brain.

More flexible scanning planes. CT is very limited, rotating only about 20 degrees (plus head tilt) into the coronal plane.

CT often suffers from beam hardening artifact in the skull base and obscures structures there. The leptomeninges are often missed in CT. Even with contrast enhancement, as the LM appear isointense. On the other hand, the MRI shows no skull signal (and cannot be used for bone imaging) so the leptomeninges are more visible.

MRI with Gd shows enhancement of at risk central structures, such as the eighth nerve, before frank structural changes can be seen.

White matter disease with signal changes often cannot be seen at all on CT.

Visibility in the petrous area and skull base is much better.

Ex. Subdural hematoma. In CT, the blood may become more or less isointense with brain as its density changes with clotting. The physician must rely on distorted anatomy to make the diagnosis. Bilateral isodense hematoma is much more difficult, because it may not create a midline asymmetry.

In MRI, the hematoma is much easier to see, based on its large contrast w.r.t. the normal brain. (Perhaps we will mention in a future lecture the breakdown of hemoglobin and its signal changes in MRI).

We usually see the patients subacutely.

In the brain:

As these lesions evolve, they can change radially, with different stages appearing circumferentially. If multiple bleeds have occurred at different times, they may be isolated by their differing relative intensities.

Ex.: Hematoma. Blood mounts an almost immediate inflammatory response, as it is very irritating. The inf. response may be the most important cause of clinical problems.

Ex.: Female with birth control pills and dehydration results in increased risk of clotting. Several hours out from the event, the T1-wtd images can show very high intensity.

An alternative diagnosis might be arteriography. In this case, the diagnosis must be based on the absence of anticipated intravascular signal contrast.

Ex.: Head trauma. With frontal impact, the brain hits the inside of the skull and the top of the orbital roof. These lesions, being up against the bone, are often difficult to see in CT. By comparison, MRI sees these areas very well. Impact lesions often will appear frontally and occipitally as the brain bounces in the head.

Tearing between grey and white matter is fairly common, and frequently seen in the corpus callosum, due to the slightly different mechanical properties of white and gray matter.

In some cases, the trauma results in diffuse axonal dystrophy that can be seen in T2-wtd scans even when otherwise invisible. Because of the increased contrast for small bleeds, the physician will sometimes use gradient echo (no 180 degree pulse) scans to study this.

Some diseases, such as a cavernous hemangioma, are cryptic, and invisible. These are often not recognized in standard radiography, but are easy to see on MR.

At present, there is no useful MR method for looking at subarachnoid bleeds. With an arterial aneurysm (outpouching of the vessel wall, that is often secondary to flow problems) there may be subsequent rupture. If they do not die, and complain of the "worst headache in my life", they will get an emergent head CT based on suspicion of a subarachnoid aneurysm. There is a huge risk of re-bleeding within the first few hours. This is a common UCLA referral. It is not yet understood why MRI does not pick up subarachnoid blood - once this has taken place, the MRI exam will gradually supplant CT for these exams. Now, however, there can be no tolerance for reduced sensitivity.

Ex.: an MR study of subarachnoid hemmorhage. A rare example.

Ex.: Pulsatile changes in certain vascular lesions can be used to identify motion of structures such as pulsating aneurysm. Flowing blood is usually black, but still produces high intensity artifacts. Angiography will show these of course, but requires injection of contrast material through a intracarotid catheter.

MRA is a method for collecting angiograms with MR. The relative advantage of MRA and spiral CT are still under investigation. MRI does not show anatomy in a simple manner: it shows flow - not necessarily structure. CTA (ct angiograms) do not have a large volume coverage, and the 150 cc injections of iodine cannot be easily repeated without a long clearance time (days). MRA, however, is prone to artifacts. In Time of flight MRA, anything with short T1 will appear bright in Dr. El-saden's example, small hematomas were misinterpreted as vascular structure. Often radiologists must resort to the source images (used for maximum intensity projections) in order to better interpret the images.

Metastases are secondary tumor growths. For example, in lung cancer, cells may make their way up to the brain and begin their rapid division there. Pre-contrast, the CT may or may not show the lesions. With injected contrast, the CT is better, but does not compare with MRI. Given this, there may not even be an indication for contrast-enhanced CT in metastatic disease.

Toxoplasmosis results in edema and enhancement with Gd due to BBB defects, for example at the border of the abscesses.

Typically high intensity lesions on T2 I white matter. Preferred involvement of corpus callosum, which is very difficult to see in CT, even with contrast added.

Shows normal on CT, but is easily seen as increased intensity on T2 wtd MRI. Presentation is often cognitive change.

Many small diffuse lesions.

This disease causes difficult only after the organism starts to die, at which point the pt. suffers from the brain inflammatory response to the dead organism. The MRI may show this disease well before there are florid clinical signs, and it is difficult to anticipate treatment, as the medicines may cause more death of the organisms.

Contrast enhanced MRA is an emerging method that might displace x-ray based angiography. The main limitation for FOV in the magnet is the "sweet spot" of the coil. MRA does not require injections, and may be used to prescreen for angiograms so that the physician may limit overall dose of contrast.

We only have about 6 hours after stroke in which to treat stroke, because of the rapid death of brain tissue with infarct. Urokinase can be effective in as little as 15 minutes. But: urokinase may also potentiate hemmorhage and must be used conservatively. If the drug is given after the arterial walls have started to deteriorate, it may case more morbidity.

Infarct staging

The cause of cell death may be cytotoxic edema.

MRI can demonstrate even very small lesions, and can show changes in only a few (<20) minutes after stroke, when using Gd contrast.

Brownian (random) motion of water particles is present everywhere. It is thought that extracellular motion is greater than inside the cells. In ischemia, with Na/K pump failure, more water molecules become intracellular and result in a decrease in apparent diffusion coefficient (ADC). The ischemic penumbra, that is, the tissue on the edge of the lesion, is perhaps salvageable, even if the core is dead. The penumbra is though to be "stunned" due to reduced, but not lethally, blood flow. The penumbra while bright on T2 is often isointense on diffusion scans. The areas of T2 and diffusion mismatch may be saveable tissue, and this is a hot and active topic of radiologic research.

Perfusion scanning is also now being used to look for brain blood abnormalities. The mismatch between the perfusion and diffusion is also an indicator of salvageable tissue. A large area of mismatch can suggest an opportunity for intervention.

Simple cytotoxic edema may not be visible on T2. This is because there may not yet be a net shift of water to that region, instead it is only redistributed.

Even with the limited spatial resolution of MR diffusion images, it is possible to see diffusion abnormalities, perhaps because of the relatively large contrast available.

…remains limited in large part because of the problems of motion. In the chest, however, the MRI is popular, where the large structures (e.g., the heart) can be seen well.

MR has supplanted myelography, as MRI shows not only the CSF, but the soft tissues of the cord and nerves.

Orthopedic MR is big and growing, as it can detect marrow edema and secondary effects of damage to the bone trabeculae, even if cannot see the small fractures. MR is also particularly valuable in the seeing soft tissue changes (e.g., the muscle) following orthopedic injuries.