Chapter 02: Introduction To Brain Imaging *complete*

Front 1

What does midline shift mean in general?

Back 1

Any shift of midline structures is presumed to represent a mass lesion on the side from which the midline is displaced. For practical purposes, there are no “sucking” brain wounds that draw the midline toward themselves.

Front 2

How can you quickly rule out subfalcine herniation?

Back 2

If the interventricular septum and third ventricle are located in the midline, no subfalcine herniation is present

Front 3

What things could loss of sulcation represent?

Back 3

Loss of sulci may result from compression owing to mass or opacification of CSF following subarachnoid hemorrhage or, less commonly, meningitis or spreading of a CSF-borne tumor.

Front 4

What does medial displacement of the sulci mean?

Back 4

Significant medial displacement of the sulci may represent compression resulting from an extracerebral fluid collection, such as a subdural or epidural hematoma. Because these may be bilateral and similar in density to the brain, care needs to be taken in evaluating the periphery of the brain.

Front 5

Describe the six corners of the suprasellar cistern:

Back 5

The five corners of the pentagon are the interhemispheric fissure anteriorly, the sylvian cisterns anterolaterally, and the ambient cisterns posterolaterally. The sixth point of the Jewish star is in the interpeduncular fossa posteriorly.

Front 6

What would cause asymmetry of the suprasellar cistern?

Back 6

Significant asymmetry may be a result of uncal herniation.

Front 7

What would cause asymmetry of the quadrigeminal plate cistern?

Back 7

Any asymmetry must be suspect, and abnormality of this cistern may represent rotation of the brain stem resulting from transtentorial herniation.

Front 8

How can you tell ventricular enlargement from hydrocephalus from atrophy?

Back 8

Hydrocephalus is distinguished from enlargement of the ventricular system as the result of atrophy by a discrepancy in the degree of ventricular and sulcal enlargement, and by a characteristic pattern of frontal horn and temporal horn enlargement and a round appearance of the anterior portion of the third ventricle.

Front 9

What five questions do you need to quickly ask yourself while reading an ER head scan?

Back 9

If a radiologist can give the right answers to these five questions, there is no neurosurgical emergency.

1. Is the middle of the brain in the middle of the head?
2. Do the two sides of the brain look alike?
3. Can you see the smile and the pentagon or Jewish star?
4. Is the fourth ventricle in the midline and more or less symmetrical?
5. Are the lateral ventricles huge, with effaced sulci?

Front 10

What special considerations should be made when ruling out a stroke?

Back 10

It is important to note that thrombolysis candidates require close scrutiny of the basal ganglia and cortex for signs of early ischemia. When stroke triage is performed, specialized imaging techniques such as perfusion CT and CT angiography (CTA) supplement the initial screening CT. In an increasing number of centers, MR stroke triage is performed, provided that the clinical suspicion of intracranial hemorrhage is very low and no contraindication to MR is known.

Front 11

Where should the cerebellar tonsils lie in relation to the foramen mangum?

Back 11

The cerebellar tonsils should project no more than 3 mm below a line drawn between the anterior and posterior lips of the foramen magnum.

Front 12

Which modality is used for acute neuro-problems, which for less acute problems?

Back 12

That is, if the onset of neurologic symptoms referable to the brain was within 48 hours, start with a CT. If the problem is older than 3 days, start with an MR.

Front 13

When should you give contrast in general?

Back 13

If the CT or MR suggests tumor, give contrast. Don’t use intravenous iodinated contrast for CT in the acute setting unless brain abscess or tumor is a strong consideration or if needed for your stroke triage protocol. Give gadolinium for MR whenever there is a clinical finding that suggests a specific neurologic localization, a seizure, or a strong history of cancer or infectious disease.

Front 14

What is MR spectroscopy?

Back 14

Proton MR spectroscopy shows the distribution of brain metabolites based upon the chemical shift of the protons within them, which is a property determined by the chemical environment of the protons in question.

Front 15

What three substances are routinely analyzed in MR spectroscopy?

Back 15

In practice, three normal metabolites are the most interesting: choline, which is a marker for cell membranes and hence a marker for cellular density; N-acetyl aspartate (NAA), which is a compound found only in neurons and therefore a marker of neuronal density; and creatine, which is evenly distributed in many types of cells and serves as a reference standard.

Front 16

What should you think of if the choline peak is sky-high?

Back 16

If the choline peak is sky high, think of a meningioma.

Front 17

What substance is considered a tumor marker?

Back 17

Choline may be considered a tumor marker. It's a marker for cell membranes and hence a marker for cellular density.

Front 18

What would cause a decrease in the NAA-to-creatine ratio?

Back 18

A decrease in the NAA-to-creatine ratio is seen in a variety of conditions that are associated with neuronal death. Focally decreased NAA is seen in mesial temporal sclerosis and infarcts. Global depletion of NAA can be seen in multiple sclerosis and dementing diseases such as Alzheimer's disease (AD).

Front 19

What does Alzheimer's look like on MR spectroscopy?

Back 19

Global depletion of NAA can be seen in dementing diseases such as Alzheimer's disease (AD), which also demonstrates elevated myoinositol.

Front 20

What should you think of if you see markedly elevated NAA on MR spectroscopy?

Back 20

Markedly elevated NAA levels are seen in Canavan's disease as a result of a specific defect in the enzyme that metabolizes it. The NAA accumulates, producing a distinct spectroscopic pattern.

Front 21

How is the choline peak on MR spectroscopy used for tumor grading?

Back 21

Since the prognosis of a primary brain tumor is determined by the highest histologic grade of tissue within it, and histologic grade correlates with choline-to-creatine ratio, biopsy of the site with the highest choline-to-creatine ratio is likely to reflect the histologic grade of the tumor. Biopsy targeted by MR spectroscopy will better reflect the true nature of the lesion.

Front 22

What is seen on MR spectroscopy in ischemia?

Back 22

A characteristic doublet peak of lactic acid can help make the diagnosis of ischemia. This has been useful in infants with suspected hypoxemic ischemic encephalopathy. This may also aid in diagnosis of mitochondrial encephalopathies.

Front 23

In diffusion weighted imaging, what is bright, the restricted on unrestricted water?

Back 23

The more restricted the movement of water, the brighter it will be on DWI sequences.

Front 24

What does a stroke look like on DWI?

Back 24

In stroke, ischemic areas tend to swell following osmosis of free water into the dying cells, and these areas become bright on DWI as a result of the increased ratio of intracellular to extracellular water. This change on DWI precedes changes on T2 and FLAIR, making DWI a key sequence in the early detection of stroke.

Front 25

Describe diffusion weighted imaging:

Back 25

This technique exploits the phenomenon of diffusion, which is related to Brownian motion at the molecular level. DWI takes advantage of the fact that intracellular water molecules are much more limited in their movement than extracellular ones, because they quickly bump into the cell membrane that contains them.

Front 26

What does CSF look like on DWI? How can this be helpful?

Back 26

CSF contains the least restricted water in the brain and will be dark on DWI. Low signal on DWI therefore distinguishes arachnoid cysts from intracranial epidermoid cysts.

Front 27

What do tumor, trauma, and infection look like on DWI?

Back 27

Tumor, trauma, and infection can have an ambiguous appearance on DWI, as both intracellular and extracellular water may increase. Fortunately, the T2 effects of extracellular edema can be accounted for and “subtracted” out using apparent diffusion coefficient (ADC) maps.

Front 28

Describe tensor diffusion imaging (TDI):

Back 28

The diffusion phenomenon has also been exploited to map white matter tracts for surgical treatment planning and other purposes. This tool, tensor diffusion imaging (TDI), exploits the fact that within elongated cell processes such as axons, water can diffuse more freely “down the tube” than “sideways,” allowing for “tractography.”

Front 29

Describe how MR perfusion scans are performed and what findings suggest ischemia:

Back 29

Most MR perfusion scans rely on a first-pass bolus gadolinium injection, during which the brain is imaged sequentially. Because the gadolinium is paramagnetic, the signal on highly T2*-weighted images is decreased in a manner proportional to perfusion. The abnormally perfused brain does not demonstrate this flow-related phenomenon as much or as soon. In the acute stroke patient, a delay of the time to peak that is greater than 6 seconds strongly suggests ischemia.

Front 30

Describe how CT perfusion scans are performed and what findings suggest ischemia:

Back 30

CT perfusion relies on the principle that perfused areas of the brain will attenuate the x-ray beam more than the ischemic brain during an iodinated contrast injection. This is because more of the contrast agent will reach the normal brain sooner than it will reach the abnormal brain. Sequential scans are performed, and the time to peak enhancement and other parameters can be calculated. Delayed arrival of contrast and transit of contrast documents ischemia, and other parameters may predict infarct.

Front 31

How is MR perfusion used in managing brain tumors?

Back 31

MR perfusion techniques also play an important role in the management of primary brain tumors by predicting the most malignant portion of the tumor, which determines the biologic nature of the lesion and the patient's prognosis. Increased relative cerebral blood volume within a tumor appears to correlate with tumor angiogenesis and hence tumor grade. Areas of increasing abnormality on perfusion-weighted MR examinations correlate well with areas of increasing malignancy. Biopsy and treatment guided by these images promise to improve prognosis and outcome in patients with astrocytoma and other brain tumors.

Front 32

How does functional MR work?

Back 32

Functional MR imaging (FMRI) refers to studies of the brain using blood oxygen level–dependent imaging (BOLD). These images rely upon the fact that deoxyhemoglobin produces changes in magnetic susceptibility that are proportional to the metabolic activity in a given brain structure. Some controversy persists as to the exact nature of the BOLD signal change, but it is known to correlate well with neuronal activity. By comparing images captured during sensory stimulation, motor activity, or a higher cortical task with those obtained while the patient is in a resting or control condition, one can create images highlighting the area or areas of the brain that are responsible for the brain function in question.

Front 33

Which would be worse to miss: extracerebral hematomas or intracerebral hematoms?

Back 33

The most important abnormalities to be detected are extracerebral hematomas. These lesions produce devastating neurologic symptoms that can be completely reversed if treated early. Intracerebral contusions are of secondary interest because they are more difficult to treat surgically, and the results of treatment are less encouraging.

Front 34

What do most noncontrast head CT's performed for ischemia look like?

Back 34

The majority of strokes are bland infarcts, and in the acute phase the CT scan is normal or nearly normal. In these patients we search for evidence of hemorrhage. If no hemorrhage is seen, a bland infarct is presumed to be present but, as yet, occult to CT scanning. The absence of hemorrhage visible on CT allows the clinician to perform anticoagulation or thrombolytic therapy to prevent progression or even reverse the neurologic deficit.

Front 35

So what if you see an intracerebral hematoma in a stroke pt on non-contrast head CT? What's the etiology?

Back 35

A cerebral hematoma presenting as a stroke suggests hypertensive encephalopathy or amyloid angiopathy, depending upon the distribution of the lesion and the age of the patient.

Front 36

What findings on non-enhanced head CT (aside from hemorrhage) would prevent use of thrombolytics?

Back 36

Recent developments in stroke therapy require further attention to the examination of patients considered for acute thrombolysis, because hemorrhagic complications are more common when early signs of large infarcts are present on the initial CT or, by inference, MR. Loss of gray/white distinction, low attenuation in the basal ganglia, and poor definition of the insula on CT may contraindicate thrombolytic therapy.

Front 37

How can CT perfusion be used to distinguish irreversible ischemia from the reversible ischemic prenumbra?

Back 37

CT techniques rely on the usually valid inference that visible parenchymal changes are irreversible, and that, conversely, some areas of diminished blood flow might be saved if the plain CT appears normal. A CT perfusion study demonstrating asymmetry corresponding to clinical symptoms may thus define an “ischemic penumbra” if one mentally subtracts the abnormal plain CT volume from the abnormally perfused volume of brain. One can compare multiple perfusion parameter maps to refine this assessment. Relative cerebral blood volume appears to correlate with infarct, allowing a mismatch between perfusion time and volume to suggest the ischemic penumbra.

Front 38

How can MR distinguish between irreversible ischemia and reversible ischemia?

Back 38

Highly T2-weighted sequences are used to exclude hemorrhage, diffusion-weighted imaging defines infarcted tissue, and perfusion scans show areas of diminished blood flow. By subtracting the volume of abnormal diffusion from the volume of abnormal perfusion, the area of “diffusion-perfusion mismatch” representing the penumbra of potentially salvageable brain is defined. MRA defines the vascular lesion directly.

Front 39

How should seizure patients be imaged?

Back 39

Seizure patients present interesting problems for the radiologist. If it is the patient's first seizure, an intracranial tumor, infection, or other acute process must be excluded. For this reason, contrast-enhanced MR or contrast-enhanced CT is the preferred approach. If the patient is in the immediate postictal state, or if a residual neurologic deficit is present at the time of imaging, a noncontrast CT scan should be obtained as the first study.
If the seizure disorder is chronic, and particularly if it is refractory to medical therapy, then a detailed MR examination, including high-resolution coronal images of the medial temporal lobes and other clinically suspected abnormal brain structures, is performed.

Front 40

What modality should be used for imaging infection or cancer?

Back 40

In any patient in whom infectious disease or cancer is a consideration, contrast-enhanced MR is the preferred study. Parenchymal tumor or metastatic disease will be demonstrated with this study, and contrast-enhanced MR has the advantage of depicting meningeal disease much better than any other imaging modality.

Front 41

How should headache patients be imaged?

Back 41

Patients with severe acute headaches should be imaged with noncontrast head CT. Acute severe headaches may be the result of subarachnoid hemorrhage, acute hydrocephalus, or an enlarging intracranial mass. The chronic headache patient is generally evaluated by MR scanning. If the headache is not accompanied by local neurologic symptoms, a noncontrast MR scan is usually sufficient. However, if the headache is associated with focal neurologic complaints, then gadolinium-enhanced MR scanning is indicated. When chronic headache is the sole presenting complaint, the yield of imaging is low.

Front 42

How are chronically demented patients imaged?

Back 42

The chronic dementia patient is generally studied by noncontrast MR as a screening examination for large frontal masses, hydrocephalus, and other treatable abnormalities that may cause a clinical picture that is indistinguishable from AD. MR may also demonstrate small-vessel ischemic changes in the cerebral white matter and small infarcts, which also may clinically mimic AD. If these findings are not present, and the clinical picture is correct, the clinician may offer a diagnosis of AD. PET studies may play a role in assessing prognosis and guiding therapy, especially in the clinical setting of mild cognitive impairment.

Front 43

How should acutely demented/delirious patients be imaged?

Back 43

The comatose or acutely confused patient should be imaged to detect an intracranial hemorrhage. These patients are studied urgently with noncontrast CT. However, the majority of patients who present in this manner will not have an acute structural lesion of the brain. Many will be comatose owing to metabolic abnormalities of the brain. An acute infarct may be present, but this may be invisible on CT, particularly in the brainstem.

Front 44

What are some general signs of an intracranial mass on neuroimaging?

Back 44

The normal midline structures may be shifted contralateral to the mass. The sulci adjacent to the mass may be effaced, since the CSF in the sulci is displaced by the mass. Similarly, ipsilateral ventricular structures may be compressed by a mass, rendering the ipsilateral ventricle smaller than the contralateral ventricle.

Front 45

Can atrophy cause midline shift?

Back 45

Shift ipsilateral to an atrophic lesion is very unusual and is only seen commonly in congenital hemiatrophy. Even if a complete hemispherectomy is performed, shift of the midline toward the side of the hemispherectomy defect is almost always a sign of mass in the remaining cerebral hemisphere or an extra axial mass compressing it.

Front 46

What are some general signs of atrophy on neuroimaging?

Back 46

An atrophic lesion is recognized by widening of the ipsilateral sulci or enlargement of the ventricle adjacent to the lesion.

Front 47

What are some possible signs of Alzheimer's on neuroimaging?

Back 47

It has been recently suggested that specific neuroradiologic features of AD exist, such as focal atrophy of the hippocampal regions of the medial temporal lobe, but this has yet to be confirmed prospectively with sufficient reliability. PET scanning may sometimes be useful in this setting.

Front 48

What are some reversible causes of cerebral atrophy?

Back 48

Patients with Addison's disease or other causes of dehydration or abnormal fluid balance may occasionally present with a CT picture of atrophy. With treatment, a more normal appearance of the brain can be restored. Nutritional causes of reversible cerebral atrophy exist in anorexia nervosa and bulimia. The relative contribution of dehydration and starvation in these conditions is difficult to determine. Alcoholism may also occasionally result in reversible “cerebral atrophy.” Although the neurotoxic effects of alcohol are not reversible, it has been hypothesized that the accompanying nutritional deficiencies may be corrected, restoring a more normal appearance to the brain on imaging studies.

Front 49

What's the first question to answer once we find a mass?

Back 49

Is the mass intra-axial, within the brain and expanding it, or extra-axial, outside the brain and compressing it?

Front 50

What kinds of things cause intra-axial masses?

Back 50

Intra-axial masses are, most commonly, metastases, intracranial hemorrhages, primary intracranial tumors such as glioblastoma, and brain abscesses.

Front 51

What kinds of things cause extra-axial masses?

Back 51

Extra-axial masses are, most commonly, subdural or epidural hematomas, meningiomas, neuromas, and dermoid or epidermoid cysts.

Front 52

How can you tell an intra-axial mass from an extra-axial mass on CT?

Back 52

Extra-axial masses generally possess a broad dural surface. In contrast, intra-axial masses are surrounded completely by brain. In the posterior fossa, the most reliable sign of an extra-axial mass is widening of the ipsilateral subarachnoid space. The cerebellum and brainstem are displaced away from the bony margins of the calvarium by the mass. In contrast, intra-axial masses demonstrate a narrow ipsilateral subarachnoid space. In the supratentorial compartment, we evaluate a mass somewhat differently. With an intra-axial mass, the gyri are expanded and the CSF spaces are compressed. The CSF spaces adjacent to an extra-axial mass, on the other hand, become larger as we approach the mass.

Front 53

How can you distinguish intra-axial from extra-axial lesions on MR?

Back 53

With the multiplanar capability of MR we are frequently able to visualize direct displacement of the brain away from the dura by an extra-axial mass. When gadolinium is administered, extra-axial masses frequently show dural enhancement, whereas this is less common with intra-axial masses. Extra-axial masses tend to enhance homogeneously, e.g., meningioma or neuroma, or not at all, e.g., extracerebral hematomas and cysts. Intra-axial lesions tend to enhance in a ringlike or irregular fashion. In general, intra-axial masses have more surrounding edema than extra-axial masses of the same size.

Front 54

What should you think of if you multiple infarcts?

Back 54

If multiple infarcts are seen, they may represent border zone infarcts resulting from global hypoperfusion or they may be a result of a cardiac source of emboli.With multiple border zone infarcts, global hypoperfusion because of cardiac arrest must be suspected. If the deep gray matter structures bilaterally are involved, pure anoxia owing to carbon monoxide poisoning or respiratory arrest should be considered. These pure patterns are somewhat idealized, because hypoxemia and ischemia are frequently associated.

Front 55

What should you think if you see ischemia involving the deep gray structures bilaterally?

Back 55

If the deep gray matter structures bilaterally are involved, pure anoxia owing to carbon monoxide poisoning or respiratory arrest should be considered. These pure patterns are somewhat idealized, because hypoxemia and ischemia are frequently associated.

Front 56

Where does herpes encephalitis tend to localize?

Back 56

The most common locations for involvement are (1) the medial temporal lobes adjacent to the trigeminal ganglia and (2) the orbital frontal regions adjacent to the olfactory bulbs.

Front 57

What does enhancement of a lesion tell you about it (in general)?

Back 57

Enhancement of the brain parenchyma means that the blood-brain barrier has broken down and that the process is biologically active. In the astrocytoma tumor line, an increase in enhancement correlates with higher tumor grade. However, enhancement does not imply malignancy. Infarcts, hemorrhages, abscesses, and encephalitis all can demonstrate contrast enhancement. However, in these nonneoplastic processes, enhancement appears only in the acute phase and resolves with time.