Objectives

 To understand the role of magnetic resonance imaging (MRI) in intracranial disease

 To recognize the advantages and disadvantages of MRI in the diagnosis of intracranial disease

 To learn basic interpretation principles using selected diseases of the brain

 Physics of MRI image formation will not be covered

Role of Imaging

MRI is the gold standard for imaging the contents of the brain case and spinal canal. MRI is also used to assess response to therapy, plan radiation therapy treatments, and to decide whether or not euthanasia is the proper course of action. MRI is an expensive venture and should not be used as a screening method to overcome lack of a proper medical examination.

Advantages and Disadvantages

Advantages: MRI eliminates superimposition of the bones of the brain case and spine. MRI does not produce ionizing radiation. The MRI unit can produce sectional imaging in different planes without the need to change a patient's position within the magnet. MRI has the ability to depict the highest degree of brain tissue contrast resolution of any imaging modality available to date. Grey matter can be differentiated from white matter. Hemoglobin within a hematoma can be differentiated from perilesional edema, and a cystic mass filled with CSF can be differentiated from a mass filled with inflammatory material.

Disadvantages: These include the cost of the unit and of setting up an MRI compliant room. MRI requires general anesthesia. A typical standard brain MRI exam (with all basic sequences) requires approximately one hour. Animals with ID microchips or soft-tissue foreign material near the area of interest cannot be imaged with MRI, as they create void artifacts that prevent visualization of the area of interest. Therefore, postoperative studies in which metallic implants have been placed cannot be followed up with MRI. An MRI study of the brain takes longer than would a CT. The animal is "buried" within the magnet and, therefore, relatively less accessible during anesthesia.

Interpretation

(Correct terminology in bold type)

Three planes are commonly generated (veterinary terminology in bold type and the human medicine in parentheses): sagittal plane (sagittal), transverse plane (transaxial), and dorsal plane (coronal).

As a general rule, pathophysiological processes associated with inflammation, edema, or tumoral infiltrations show increased water content. This increases the relaxation time of the affected tissue, which translates in a hyperintense lesion on a T2 spin echo sequence. Exceptions to this rule include lesions with short relaxation time, and which are composed of fat, hemorrhage, melanin, or contrast-enhancing characteristics.

Bright (white) structures on the screen are described as hyperintense. Dark (black) structures are called hypointense, and grey are isointense.

Spin Echo Techniques

On a T1-weighted image, tissues such as fat gadolinium, methemoglobin, and melanin are depicted as white. Tissues such as brain parenchyma appear less intense (as a medium shade of gray); tissues such as free water appear black.

On a T2-weighted image, tissues with the strongest signal are depicted as white. These include free water or watery tissues such as tumors, edema, and inflammation. Brain with an intermediate signal appears as a shade of grey, while fat appears darker than on a T1 sequence.

Proton density (PD-W) weighted images can be used to differentiate cystic from solid structures but exhibit less tissue contrast resolution than T1 and T2 sequences.

Tissue with small amounts of hydrogen protons tends to appear black on spin echo sequences; dense cortical bone, calcification, fibrous tissue, hoof of P3, gas and implanted materials are good examples of tissue with void hypointense signals. Flowing blood may also show as a black (void) signal in some sequences.

Inversion Recovery Sequences

FLAIR (fluid-attenuated inversion recovery). Fluid with low cellular content, such as CSF, appears black. However, fluid with a component of edema or inflammation appears white. This sequence helps to differentiate cystic from solid lesions.

STIR - Short TI inversion recovery sequence suppresses signals from fat; therefore, lesions surrounded by fat appear more conspicuous.

Gradient Echo Sequences

These sequences include many different acquisition parameters. They can be T1-, T2-, or PD-weighted, as well as T2* (FLASH). The Fast Long Angle Shot sequence (FLASH) is also known as susceptibility-weighted sequence. It causes any tissue with inhomogeneous magnetic field to show as black or void. It assists identification of mineralization or hemorrhage and is used for magnetic resonance angiography (MRA) and for perfusion brain scanning.

MRI Contrast Material Sequences

Gadolinium is the compound used. Typically, contrast enhancement is assessed under a T1-weighted protocol. The compound shortens the relaxation time of a tissue; therefore, the tissue enhancing behaves similarly to fat. The contrast makes a lesion appear hyperintense on a T1-weighted protocol. The lesion must be vascularized and a break in the blood-brain barrier must be present if contrast enhancement is to show.

Abnormalities can be characterized by assessing size, shape, number, margination, intensity, and location. In MRI, the pattern of contrast enhancement and whether or not the lesion causes a mass effect is also assessed.

Size: Standard imaging post acquisition software allows to measure with millimeter precision any lesion.

Shape: The list of differential diagnoses varies according to whether the lesion is broad based and in close relationship with the bones of the skull or meninges. Other shapes such as wedge-shaped or spherical may also help in formulating the list of differentials.

Margination: Refers to whether the lesion can be differentiated with precision from the normal parenchyma (well defined) or if the reader cannot accurately pinpoint an area as being normal or abnormal tissue.

Intensity: Refers to whether the lesion is hyper-, iso-, or hypointense in relationship to the surrounding normal tissue.

Location: If the lesion arises from the meninges or from an epidural area, it is described as extra-axial. If the lesion arises from within the parenchyma, it is described as intraaxial. Other classifications assess whether the mass is ventricular or limited to the grey or white matter.

Contrast enhancement: Homogenous enhancement suggests a solid lesion with ruptured or incomplete blood-brain barrier and with well-organized vascularization. Heterogenous enhancement suggests regions with no vascular supply, necrosis, or hemorrhage.

Mass effect: The effect of masses on normal tissues is that of displacement. The commonly used term to refer to this effect is to describe it as a "space-occupying lesion." In cross-sectional imaging, where symmetry between the two identically looking cerebral hemispheres is easily assessed, the mass effect of a lesion is also easily noted. Abnormal findings suggesting a mass effect include asymmetric narrowing of one of the lateral ventricles and deviation of the falx cerebri laterally from the midline.

In general terms, the following two selected diseases provide examples of how to use the terms above.

Meningiomas

This infiltrative process is considered an extra-axial (peripheral) lesion. It is usually broad-based in close apposition to the bones surrounding the skull and with well-defined margins. Due to the close relationship to the intracranial surface of the flat bones of the skull, it may cause thickening of the adjacent calvarium. A meningioma tends to be hyperintense to normal brain parenchyma on MRI. Though exceptions to this rule exist, especially when the meningioma is mineralized or necrotic. As a general rule, a meningioma exhibits marked homogenous enhancement. The mass effect and amount of peritumoral edema are variable. Cystic meningiomas are not rare, and multiple tumors may be present, especially in cats. A classic sign that indicates the presence of a meningioma is the appearance of a dural tail sign, which represents a portion of the mass dissecting within the meninges and the most external surface of the neural tissue.

Gliomas

This neoplasia is of intra-axial location. It grows from within the brain parenchyma and is often characterized by ill-defined margins. Gliomas exhibit more variation than meningiomas when it relates to margination and opacity from surrounding brain parenchyma. They can be iso- to hyperdense on a T2-weighted protocol. Enhancement is also variable. However, it tends to be of heterogenous characteristics due to the often-found ischemia, necrosis, hemorrhage, and/or lack of blood supply within the process. Ring-like enhancement is not uncommon. It also tends to show a more defined and exuberant secondary production of perilesional edema than the edema noted with meningiomas. Differences exist between well-differentiated gliomas and poorly differentiated glial tumors. Gliomas with a differentiated cell type tend to be hyperintense on the T2-weighted MRI protocols and with little peritumoral edema. Poorly differentiated glial tumors are often of higher intensity with large heterogeneously enhancing characteristics, larger peritumoral edema and mass effects than differentiated gliomas.

Final Disclaimer Note

Even though advanced imaging techniques such as MRI can give a glimpse to many pathological conditions, even at the molecular level, the appearance or abnormal imaging findings are nonspecific. In other words, the appearance of many non-neoplastic processes may resemble that of neoplastic conditions. Therefore, histopathology is required (always) for a definitive diagnosis.