Use of Contrast Media in Neuroimaging

The goals and indications of central nervous (CNS) or neuroimaging are multiple and involve detection of pathologies, including making a precise diagnosis and a differential diagnosis, and detection of disease complications that require an immediate intervention and proved to be an essential part of the decision-making process for therapy. In the case of neoplastic disorders, for example, neuroimaging can precisely define the location and accurately delineate the lesion before intervention. In the case of radiation oncology, it should precisely define and demarcate the margins for targeted intervention. Neuroimaging is also mandatory after any therapeutic intervention for monitoring of disease and for detection and monitoring of possible side effects.

In emergency medicine, computed tomography (CT) assumes a critical role for the evaluation of traumatic and nontraumatic evaluations.¹ CT scanners are fast and widely available, which makes CT a great choice for assessment of neurologic or neurosurgical emergencies. Usually, a conventional noncontrast brain CT is performed in an emergency to detect, for example, fractures and intracranial bleeding, and also to exclude tumors and subdural/extradural hematomas. Nevertheless, the intrinsic tissue contrast is limited in the CT images. Neuroimaging with CT relies, therefore, deeply on contrast media to improve lesion detection (sensitivity) and characterization (specificity).² Furthermore, contrast agents can be used for functional assessment of physiologic process: the blood perfusion and for the depiction of the vessel compartment (angiography/venography). In CT angiography, the images are obtained a few seconds after high-flow contrast injection and most of the observed enhancement is intravascular. When CT acquisition is delayed for 10 to 15 minutes after contrast agent injection, most of the observed enhancement is interstitial. When CT imaging is done at intermediate times, the enhancement reflects a combination of both intravascular and interstitial components.

Because of its high tissue contrast and noninvasiveness, magnetic resonance (MR) imaging is accepted as the most sensitive method for diagnosing diseases of the CNS.3, 4 MR imaging enables accurate recognition and determination of the dimensions of CNS pathologies and their surrounding tissue. This requires a high CNS-to-lesion contrast, which both depends on the signal intensity of the lesion relative to that of the surrounding normal tissue and the best description of the physiologic tissue and vasculature not affected. Furthermore, detailed information on the internal morphology of the lesion is essential for differential diagnosis, grading, and for the selection and planning of therapy.

Although MR imaging using standard nonenhanced T1-weighted and T2-weighted sequences has proven to be very sensitive in the detection of pathologic lesions, for most CNS diseases and for many of the currently available functional MR imaging methods, the use of MR contrast media is mandatory. The standard dose used for MR imaging of the CNS is 0.1 mmol/kg body weight, although numerous studies have shown that lesion detection may be improved with the use of higher doses and dedicated sequences.5, 6 A higher dosage also allows for the combination of a number of advanced, mostly contrast-enhanced MR imaging techniques that have been developed that provide new insights into the pathophysiology of CNS diseases, as well as to better describe the angioarchitecture using contrast-enhanced MR angiography (CE-MRA). One of these techniques, perfusion MR imaging, is now recognized as an important new means for assessing tumor grading and follow-up of various treatment strategies or to better assess cerebrovascular diseases. Another of these techniques, dynamic contrast-enhanced MR imaging, is also gaining acceptance for the same purposes. In this article, the reader is provided with the fundamental features of the contrast mechanisms in MR imaging neuroimaging, the basic methodologies and the first clinical experience with the contrast-enhanced functional imaging tools, and some of the classical indications for contrast media. The different properties of the currently available contrast media and the dosage and field dependencies have been discussed in the article by Dr. Kanal, elsewhere in this issue.

In neuroimaging, pathologic enhancement can occur in several regions:

• 1.

  Abnormal enhancement within vessels without breakdown of the blood-brain-barrier (BBB), which reflects neovascularity, macrovasodilatation or microvasodilatation (aneurysm), and shortened transit time or shunting, including arteriovenous malformations (Fig. 1).

• 2.

  Extra-axial lesions with no BBB, such as meningioma, acoustic schwannoma, or granulomatous disease.

• 3.

  Breakdown of BBB with leakage of contrast, including neoplastic disease, infection, infarction, inflammation with demyelinating disease, and trauma.