Fig. 2.3. CT scan demonstrating extensive subarachnoid hematoma with the lower density aneurysm lumen visible (arrow).

The ability of CT to detect subtle calcification and show excellent bone detail highlights its use as an adjunct to MRI. Detection of calcification within a tumor may aid the differential diagnosis. Subtle calcification may not be apparent on an MR scan but is obvious on a CT scan, which may lead to the diagnosis of tuberous sclerosis in the investigation of epilepsy. Skull base detail is very well shown on a CT scan and is complementary to MRI in fully defining complex lesions of this region. This ability to define bone detail is also very useful in spinal imaging, particularly in trauma, to define and classify fractures. Reconstruction of data into sagittal and coronal planes is easily achieved and adds essential information on alignment and extent of abnormality.

Magnetic Resonance Imaging

The principles of nuclear magnetic resonance were first described in the 1930s by C.J. Gorter and used extensively as spectroscopy to study physical and chemical properties of matter. It was not until the late 1970s that images of human anatomy were produced and the tech nique became known as "magnetic resonance imaging". Extensive development has enabled MRI to become the investigation of choice for most neuroradiological imaging.

The quantum mechanics and mathematics that attempt to explain MRI are beyond this author and are not required for image interpretation. An understanding of the simplified principles and the various sequences produced is necessary [2, 8].

From a practical view of image interpretation, we need to know what is white, black or gray on a particular MR sequence. These are summarized in Table 2.2. Fat, very proteinaceous tissues, certain degradation products of hemorrhage, and gadolinium influence free protons to produce high signal on T1 weighting. Gray and white matter will be intermediate signal, but white will be slightly higher signal because of its increased fat content. Brain edema and most pathologies will be intermediate signal, i.e. grey, and CSF will be black.

On T2 weighting, CSF is high signal and gray matter is higher signal than white matter. Air and cortical bone are very low signal owing to the small amount of free protons in these. Arterial blood flow and certain venous flow will present no signal (flow void) on standard spinecho sequences. Most pathologies will be high signal, as are certain hemorrhagic breakdown products. So most tumors will be high signal on T2 weighting and low on T1, although atypical patterns of signal can help to characterize certain tumors (Fig. 2.4).

Routine scanning with MRI usually involves T1- and T2-weighted sequences in at least two planes. The weighting can be gained by various scanning techniques, including conventional spin echo (CSE), fast spin echo (FSE) and gradient echo (GE). Acquisitions can be acquired in two- or three-dimensional modes. Very fast scanning is possible with techniques such as echo-planar imaging (EPI) or halfFourier acquisition single-shot turbo spin-echo ("HASTE"), although they may be limited by artifact and poor signal-to-noise ratio. Special sequences can be used to suppress fat, such as in "STIR" (short tau inversion recovery), which is useful for skull base and orbital imaging, and to suppress CSF, as in "FLAIR" (fluid-attenuated inversion recovery), which increases the con-spicuity of lesions at brain-CSF interfaces. More recently, specialized applications of MR scan-

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Essentials of Human Physiology

Essentials of Human Physiology

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