MR Imaging (MRI)

Although the MRI procedure is very safe and generally easy to tolerate, because of the large magnetic field, patients should not get an MRI if they have implanted electrical or mechanical devices.
MRI scan of patient with lyme disease who has multiple sclerosis-like lesions

MRI captures the physical structure of the brain, unlike PET and SPECT, which reveal the functioning of the brain.

In neurologic Lyme Disease, approximately 15-45% of patients may have white matter hyperintensities. These are sometimes also called UBOs or "unidentified bright objects". In some patients, antibiotic treatment results in a diminution or disappearance of these hyperintensities. Certain MRI sequences are best able to detect hyperintensities; these include FLAIR sequences and Magnetization Transfer methods. These MRI images in Lyme Disease may appear similar to the demyelinated areas seen in the "white matter" of the brain MRI of patients with multiple sclerosis.

SPECT Imaging Single Photon Emission Computerized Tomography (SPECT) is an imaging device that provides a picture of the functioning of the brain. The advantage of SPECT imaging is that it is widely available and relatively inexpensive (compared to PET imaging). SPECT machines with several "heads" allow for increased imaging sensitivity. In Lyme Disease, the most common finding is of heterogeneous hypoperfusion diffusely throughout the brain. This pattern cannot be distinguished from diseases with a similar pattern, such as Lupus, chronic cocaine abuse, or other vasculitic inflammatory disorders. This pattern of heterogeneously decreased perfusion is different from what one would see in patients with primary depression or Alzheimer's disease.

A. Alzheimer's disease B. Cerebral Vasculitis C1. Lyme Disease prior to treatment C2. Lyme Disease with improvement following treatment D1. Lyme Disease prior to treatment D2. Lyme Disease with deterioration following treatment Note: areas of yellow show normal cortical flow

Although the pattern looks like a vasculitis, such a pattern could be produced even if the blood vessels were normal if the nerve connections to the blood vessels were functioning abnormally. In other words, a SPECT scan cannot tell you whether the problem is due to your blood vessels or due to nerve cell dysfunction. SPECT scans also are limited in that the images obtained are rated relative to a part of the brain that is presumed to be working normally, such as the cerebellum or deep gray matter. This may not be true in Lyme Disease. This lack of "absolute quantification" limits the firmness with which one can draw conclusions regarding a SPECT image. Although SPECT imaging should be considered a very helpful adjunctive tool in the differential diagnosis of Lyme Disease, it does not and cannot "make" the diagnosis of Lyme Disease.

SPECT imaging is also used to determine whether the heterogeneous hypoperfusion has diminished over time, perhaps due to treatment. Repeated SPECT scans should be done at the same medical center to allow for a fair comparison across images.

PET Imaging Positron Emission Tomography (PET), although primarily a research tool, has been used increasingly for clinical purposes. PET imaging can demonstrate biochemical or physiological processes involved in brain metabolism. The assumption behind all functional brain imaging (such as PET) is that there is a close relationship between local brain nerve activity, brain glucose metabolism, and brain blood flow. The advantage of PET (in comparison to SPECT) is that the images have enhanced resolution and, with the use of an arterial line, absolute quantification of the metabolism and flow in different brain areas is possible. In other words, unlike SPECT which requires assumptions about normal and abnormal brain areas for interpretation, PET can provide a definitive absolute number regarding the amount of flow or metabolism in a particular area of the brain. Because a PET center requires a highly trained multi-disciplinary staff of physicists, chemists, computer and mathematical experts, technologists, and physicians as well as a cyclotron in the same building as the patient and the scanner to allow for production of the radiopharmaceuticals, PET scans are more expensive and far less widely available than SPECT scans.

Different radiopharmaceuticals allows us to measure different aspects of brain function. For example, the most widely used "tracer" for measuring regional brain blood flow is ¹⁵O-labeled water.¹⁸F(fluorine) as fluorodeoxyglucose (FDG) is widely used to measure brain glucose metabolism, since the brain uses glucose to function. One way of determining whether the brain blood vessels are functioning normally is to conduct a ¹⁵O-PET before and after a carbon dioxide inhalation challenge. The patient is then asked to breathe through a tube that contains a slightly higher amount of carbon dioxide than the normal atmosphere. In a person with normal blood vessels, this should result in an expansion of blood flow throughout the brain. If certain areas have damaged blood vessels, then the expansion of flow in that area would be less. This is one way of determining whether the problem in a disease (such as Lyme encephalopathy) is due to inflamed or blocked small blood vessels or due to normal blood vessels with abnormal nerve input.