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INTRODUCTION

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Neuroimaging is vitally important for the modern neurohospitalist. This chapter summarizes the various imaging modalities available, demonstrates a systematic way of reading a CT head scan, and reviews the key principles of MRI imaging and which sequences are useful for investigating which pathologies. The chapter also uses case-based illustrations to demonstrate which and when imaging modality is most useful.

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Imaging plays a fundamental role in the job of the modern neurohospitalist. Where history and examination allow the localization of a lesion and the formulation of the most likely disease process, it is commonly imaging that confirms the causative pathology. In the current hospital setting, the neurohospitalist regularly attends acute and emergency neurological presentations. Hence, it is essential that they have a good understanding of what tests are available, which is the most appropriate for a given situation, and how to interpret both common and important conditions. The aim of this chapter is to describe the imaging tools available to the modern neurohospitalist and use clinical vignettes to demonstrate how, why, and when they are most useful.

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THE PLAIN COMPUTED TOMOGRAPHY (CT) SCAN

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What is a CT scan and how does it work?

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The CT scan uses a motorized x-ray tube that rotates around the patient, transmitting narrow beams of x-ray through the patient, which are picked up by detectors and relayed to a computer. The computer uses sophisticated mathematical techniques to construct a high-resolution 2D image for a given “slice” of the patient. Tissues differ with regards to their ability to block the proportion of x-rays that pass through them, also known as “attenuation.” As the attenuation of x-rays is dependent on individual tissue density, different structures can be seen (Figure 10-1A).1 The attenuation signal is measured in Hounsfield unit (HU) after Nobel laureate Sir Godfrey Hounsfield, the co-inventor of the CT scanner;2 the scale is arbitrary with air being –1000, water 0, and cortical bone +1000 (Table 10-1).1

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Figure 10-1

Axial (A), sagittal (B), and coronal (C) CT Head. CT images can be rendered by computer processing into 3-D images (D).

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Table 10-1.

The Density of Different Tissues on CT

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In modern (helical) CT scanners, the X-ray tube and detectors rotate continuously around the patient and can scan a larger volume of the patient in a shorter period of time. Continually moving the subject in the horizontal plane during image acquisition gives rise to a series of ...

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