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.

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.

What is a CT scan and how does it work?

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

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 longitudinal slices. The raw data are used to generate a series of images at given intervals of distance of the scanned body part in the axial, sagittal, or coronal planes (Figure 10-1A-C). Further post-acquisition processing can generate detailed three-dimensional images (Figure 10-1D).

What makes a CT head so useful?

An additional advantage of a CT scan is that the “window setting” can be optimized to accentuate the tissue of interest. The “window level” (WL) is the shade of gray (in HU), which is arbitrarily set as the midpoint in the range. The “window width” (WW) is the range of HU in which the image is viewed. For example, a bony window setting (WL 500 WW3000) increases the window range to highlight the contrast between bony tissue and brain tissue, thus making skull fractures more obvious (Figure 10-2A); conversely, a WL 40 WW80 is best to view brain tissue and WL50 WW 175 best visualizes fresh blood. (Figure 10-2B and C).³

For the neurohospitalist, the CT head scan is often the first modality of choice due to its wide availability, speed, and sensitivity for many pathological processes, in particular for acute hemorrhage (Box 10-1 and 10-2).⁴

How do I know it is not artifact?

It is important for the neurohospitalist to understand some of the most common CT artifacts, as they can obscure important findings and mimic important pathological processes. Volume averaging occurs when a CT voxel contains tissues of widely different densities (such as bone and brain parenchyma) producing beam attenuation proportional to the average values of these tissues. Commonly in the brain, with brain and bone included in the same voxel, this average density may have the appearance of blood. This artifact is now less common with a reduction in voxel volume on the latest generations of CT scanners. Beam hardening is another common artifact that appears as streaks and shadows adjacent to areas of high density—this is commonly encountered adjacent to the petrous temporal bone, resulting in obscuration of the brain parenchyma in the posterior fossa. Artifacts resulting from metal or patient motion are common.

A Systematic Way to Read a CT Head Scan (“Blood in my Ventricles and Cortex makes my Hindbrain feel Bony”)

A common reason that radiological findings are not seen is that a systematic approach is not employed when reviewing the image. This is all too common for physicians who, unlike radiologists, are often not taught a systematic reviewing system. We advocate using an acronym that is memorable and serves to ensure that all features are reviewed (Box 10-3 and 10-4). As you gain experience, the clinical history will direct you to a particular portion of the scan making your review more targeted. It is however important to be systematic and consistent in your approach. By being systematic and thorough every time, it is unlikely you will miss a significant finding.

What is the differential diagnosis for his condition?

The patient is a previously well hypertensive and diabetic on warfarin with atrial fibrillation and has evidence of soft tissue trauma. The likely cause of his insult is vascular but there are many possibilities such as cardioembolic stroke (note history of atrial fibrillation), hypertensive intra-cranial hemorrhage (ICH), or a traumatic extra-axial hemorrhage; the latter two would be exacerbated by warfarin. The management of these conditions is very different, and a clear diagnosis must be confirmed urgently.

In this situation where the patient was clinically stable but may have changed rapidly, a CT head scan was able to quickly identify the cause of the condition with minimal risk and continue to monitor the underlying pathology.

The Contrast CT

What is contrast, and why is it useful?

The Contrast CT uses intravenously injected iodine-based contrast agents that are radio-opaque. Contrast enhancement can be divided into two phases—the first is the intravascular phase that lasts as long as sufficient contrast is present in the vascular lumen (arteries, veins, or capillaries). The second phase is the interstitial phase whereby the contrast medium crosses a disrupted blood–brain barrier and leaks out of the damaged vessels into the interstitium with resultant enhancement of the surrounding parenchyma—this can be seen in acute inflammation (such as MS), tumor, infection, and ischemia. It is the presence and pattern of contrast enhancement that often helps differentiate between these different pathologies. For example, ischemia and tumor can have similar low-density appearance on the noncontrast CT, and differentiation between them is greatly facilitated with contrast (Figure 10-4A and B). The vascular and interstitial phases can be seen with both CT and MRI. Iodine-based contrast agents carry a risk of nephrotoxicity and should be used with caution in patients with renal impairment.

CT Imaging in Specific Diseases

There are some diseases in which CT head scans are particularly useful, and it is important to be aware of their common radiological features. Awareness of these common pathologies will help you maintain a high index of suspicion when applying “Blood in my Ventricles and Cortex makes my Hindbrain feel Bony.”

Intracranial Hemorrhage

When intracranial hemorrhage occurs, the CT head scan rapidly answers three important questions: where is the bleeding, how old is it, and is there mass effect or midline shift?

Intracranial bleeding can occur in only a limited number of locations; it can be extradural, subdural, sub-arachnoid, or parenchymal. Extradural hemorrhages are usually traumatic with the source of bleeding usually a torn meningeal artery, most commonly the middle meningeal. Acutely arterial blood fills in the space between the dura and the skull, forming a characteristic elliptical-shaped clot often associated with mass effect on the adjacent brain tissue (Figure 10-3D). The cranial sutures limit the size of extradural hematomas. Conversely, subdural hemorrhage occurs due to stretching and tearing of bridging cortical veins beneath the dura. The resulting low-pressure hemorrhage slowly forms a clot, which has a longer “crescent” shape than an extradural hemorrhage (Figure 10-3A). Subdural hematomas spread more diffusely over the affected hemisphere and are limited by dural reflections (such as the falx cerebri) and not by sutures. Subarachnoid hemorrhage may be traumatic, but can also be due to rupture of berry aneurysms or an arteriovenous malformation (AVM). CT brain scans are more than 90% sensitive for detecting subarachnoid hemorrhage.⁴ The bleeding is commonly seen in the basal cisterns or in the Sylvian fissure and can follow the surface “gyral” markings of the brain (Figure 10-3E). Typically, the patient will present with an acute “thunderclap” headache, which should raise suspicion when reading the scan. Parenchymal hemorrhage is usually arterial (but is sometimes venous after a venous thrombosis) and is most commonly due to hypertension, but there are a myriad of other possible causes including an underlying tumor, cavernous malformation, AVM, or cerebral amyloid angiopathy (CAA).⁴ They can be seen in any part of the brain and cause varying degrees of mass effect. Parenchymal hemorrhage in the basal ganglia is characteristic of hypertension (Figure 10-3F).

When blood travels in arteries and veins, it is relatively isodense compared with brain tissue at up to 40HU. When blood “extravasates” in CSF spaces in acute hemorrhage, it can be difficult to see (Figure 10-3D); this is why a CT scan is not 100% sensitive for detecting subarachnoid hemorrhage. As the hemorrhage converts into a clot, it becomes denser, resulting in an increase in x-ray attenuation compared with the surrounding brain tissue (Figure 10-3F, Box 10-5). Over time the blood cells in the clot lyse and it gradually liquefies becoming progressively less dense (Box 10-5).

Ischemic Stroke

With the advent of IV thrombolysis as an effective treatment within 4.5 hours after the onset of an ischemic stroke⁵ the CT head scan has become an essential tool for the neurohospitalist in the assessment of acute stroke patients. While the primary role of CT scanning in acute stroke is to exclude an ICH that would be a contraindication to IV thrombolysis, there are many early or subtle features of acute stroke such as a hyperdense vessel, loss of the insular ribbon, and subtle loss of gray-white matter differentiation and these signs should be actively sought when applying “Blood in my Ventricles and Cortex makes my Hindbrain feel Bony” (Box 10-6). It is however important to note that the features of acute ischemic stroke may not appear until 24 hours post event and the absence of early features should not dissuade from thrombolysis (Box 10-6). If the noncontrast CT is negative for ICH, CT angiography and CT perfusion can be performed at the same time to further assess for ischemia and the state of the intra- and extracranial vessels. Dedicated imaging and management of stroke is discussed in depth in Chapter 13.

Spinal Trauma

The one area of acute neurological care, which is not usually the remit of the neurohospitalist, is trauma; a trauma specialist should perform the “clearance” of a cervical spine or the assessment of cranio-facial trauma. There are occasions in which a trauma history may be censored (such as an elderly patient with confusion and collapse) and significant spinal injury may not be immediately obvious. The CT-spine is the most sensitive test for acute spinal injuries (far more sensitive than plain c-spine x-rays)⁶ (Figure 10-5A and B) and a low threshold should be employed for performing this when there are unexplained upper-motor limb signs in patients with limited history.