Advances in imaging technology have played a key role in the progress made over the last two decades in the treatment of patients with neurological disease, facilitating the identification, classification, and documentation of the different pathologic processes that constitute the subspecialty fields of neurology. As such, neuroimaging is an integral part of both the training and practice of this specialty, with many subspecialists spending a considerable portion of their time in the application of imaging techniques for the diagnosis and treatment of their patients.
Purpose of Imaging: Task-Oriented Choices
The most practical approach to a discussion of the application of neuroimaging techniques in neurological care is to first address the tasks, diagnostic or therapeutic, that require their utilization. From this perspective, the clinical scenarios in which imaging techniques are likely to be needed must be assessed along the following lines:
- What information is being sought, and how quickly is it needed?
- Which of the available imaging techniques is most likely to answer the question being asked?
- How will the information obtained by imaging impact further diagnostic algorithms, the treatment, and the prognosis of the patient?
Based on these considerations, the tasks that require the utilization of imaging during clinical care include initial diagnosis, categorization and therapeutic allocation, imaging of the brain blood vessels, characterization of the pathologic process and its consequences (edema, mass effect, etc.), and assessment of prognosis.
Direct imaging of the brain is necessary in order to assess the status of the tissue. The techniques available to complete this task include computed tomography (CT) and magnetic resonance imaging (MRI). As it will be discussed below, choosing one or the other involves considerations of speed, sensitivity, type of lesion, resource availability, and temporal profile of the event.
Categorization and Therapeutic Allocation
Once the diagnosis is made, a more precise definition of the condition has enormous therapeutic implications, both immediate and long-term. The same technologic advances described above have extended to improvements in image resolution, with better signal-noise ratios, and a superior definition of the pathologic processes. Furthermore, since the cerebral vasculature is so often implicated in the pathogenic process, imaging of the cerebral arteries and veins becomes an additional dimension in the categorization of these patients.
Imaging of the Brain Vessels
Imaging of the brain blood vessels is an integral component of the evaluation of stroke, vascular anomalies and in some cases, tumor evaluation (especially in the surgical planning stage). In general, the tests that are available for the completion of this task are divided into two groups: (1) those that are non-invasive (ultrasonic, radiotomographic, and magnetic-based techniques) and (2) those that are invasive (catheterization and angiography). In addition to its diagnostic capabilities, the latter also allows the application of endovascular therapeutic techniques.
Characterization of the Process
As discussed earlier, documentation of the structural abnormalities can be accomplished using CT and MRI. MRI can be utilized to further identify the brain connectivity (MRI diffusion tensor imaging), vascular perfusion (MRI perfusion-weighted imaging), and chemical activity (MRI spectroscopy). Nuclear-based imaging techniques, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), also provide mechanisms for functional imaging of the brain.
Following identification and categorization of the lesion, it is possible to use the imaging information to approximate the potential outlook and final outcome of each patient. Furthermore, it is often possible to make predictions about the potential benefit of a specific treatment strategy. Clearly, this characteristic of imaging relates to the fact that the existing techniques represent the equivalent of "bedside neuropathology."