Brain and Its Coverings - Radiology Technique Selection

TECHNIQUE SELECTION

The primary goal of a radiologic examination is to provide useful information for disease management. Radiologic stud-ies can provide a diagnosis or can give information about dis-ease extent or response to treatment. In the present medical climate, it has also become imperative that radiologic workups be performed efficiently and in a cost-effective man-ner. This requirement presents a problem for clinicians trying to decide which test to order in a given clinical situation.

The major strengths and weaknesses of neuroradiologic examinations have been discussed earlier. The following brief discussion concerns the appropriate ordering of examinations in clinical situations. Several points should be emphasized. First, although a recommended modality may clearly be superior to another in evaluating a particular neurologic condition, the choice of examination is not always obvious before the diagnosis is established. For example, in patients with nonfocal headache, MR scans are more sensi-tive than CT scans for detecting most intracranial abnormal-ities. However, if the headache is produced by subarachnoid hemorrhage, CT would be a much better examination than MR imaging, because subarachnoid hemorrhage is nearly in-visible on MR images. Choice of examinations may also be limited by what is locally available. If MR imaging is unavail-able, or if the MR scanner is of poor quality or the interpret-ing radiologist is inadequately trained in MR image interpretation, then CT would be an excellent examination for evaluating most neurologic disorders.

Next, it is important to realize that the least expensive ex-amination is not always the best first choice, even in this cost-conscious age. For example, most suspected skull fractures should be evaluated with CT scanning and not with plain films, despite the significant cost differential, because what is really important in management decisions is not the fracture itself but the potential underlying brain injury. Some neuro-logic diseases require multiple radiologic studies for accurate evaluation. Complex partial seizures refractory to medical management frequently require multiple examinations to lo-calize the seizure focus prior to temporal lobectomy. Such a workup normally includes MR imaging and ictal/interictal SPECT and/or PET scanning of the brain, as well as a cerebral arteriogram to identify cerebral dominance.

Finally, certain examinations are contraindicated in cer-tain patients, and an alternative test must suffice. Patients with ferromagnetic cerebral aneurysm clips or pacemakers should not undergo MR imaging. Patients with a strong his-tory of allergic reaction to iodinated contrast media should not routinely undergo contrast-enhanced CT scanning, un-less they are pretreated with anti-inflammatory agents (ie, steroids). MR scanning is frequently unsuccessful in claustro-phobic or uncooperative patients unless they are sedated.

Congenital Anomalies

Congenital anomalies of the brain are best evaluated by MR imaging. MR imaging is the best examination for demon-strating intracranial anatomy. It provides excellent discrimi-nation between gray matter and white matter, superb views of the posterior fossa and craniocervical junction, and, most importantly, the ability to view the brain in any plane. MR imaging has, for all practical purposes, completely replaced CT for this indication. The one exception is in evaluation of osseous structures including various craniofacial anom-alies and in suspected premature fusion of the cranial sutures.

Craniocerebral Trauma

CT is the preferred modality for studying practically all acute head injuries. Examination times are short, intracranial hem-orrhage is well demonstrated, and skull fractures are readily apparent. Unstable patients can also be easily monitored. In-travenous administration of contrast agents is unnecessary in the usual trauma setting. CTA and occasionally MRA areutilized with increasing frequency to assess for vascular in-jury associated with blunt or penetrating trauma. CTA is typ-ically the first-line evaluation for dissection or laceration, particularly when a displaced fracture crosses a vascular fora-men or in the case of penetrating vessel injury. Occasionally, cerebral arteriography is performed to look for carotid or vertebral artery injury, particularly when CTA or MRA are inconclusive or when there is an anticipated endovascular treatment of the injured vessel.

Although MR imaging is not routinely performed in the acute trauma setting, it may sometimes be helpful in patients with neurologic deficits unexplained by a head CT examina-tion. For example, traumatic brainstem hemorrhages are often difficult to see on CT scans but are usually quite obvi-ous on MR images. MR imaging is also useful in demonstrat-ing tiny shear lesions within the brain in diffuse axonal injury and in assessing the brain in remote head trauma.

Intracranial Hemorrhage

The best examination to perform in most cases of suspected acute intracranial hemorrhage is a head CT scan. CT scans can be obtained quickly, allowing rapid initiation of treat-ment, and they are very good at demonstrating all types of intracranial hemorrhage, including subarachnoid blood. Be-cause most nontraumatic subarachnoid hemorrhage (SAH) is secondary to a ruptured cerebral aneurysm, CTA is now performed routinely following a conventional head CT demonstrating SAH. In most cases, the CTA is adequate for aneurysm detection and characterization prior to surgical or endovascular treatment. MR imaging takes much longer to perform in a potentially unstable patient, and subarachnoid hemorrhage may be difficult to see. However, MR imaging is more useful in the subacute or chronic setting, especially be-cause it gives information about when a hemorrhagic event occurred. This information might be useful in such settings as nonaccidental head trauma (eg, child abuse). MR imaging is also very sensitive to petechial hemorrhage that frequently accompanies a cerebral infarction and could potentially help to identify an underlying cause for an intracranial hemor-rhage (eg, tumor, arteriovenous malformation, occluded dural sinus). Cerebral arteriography is generally reserved when the etiology of hemorrhage is not discernable by CTA/MRA, when it is necessary to evaluate the flow dynamics of a vascular lesion or for planning endovascular treatment.

Aneurysms

Although cerebral arteriography has traditionally been con-sidered the “gold standard” for cerebral aneurysm evaluation, CTA has supplanted catheter arteriography as the first-line imaging modality for aneurysm detection. The current liter-ature varies slightly; however, CTA is reported to have excel-lent sensitivity (greater than 95% for aneurysms measuring4 mm or larger) as well as high specificity. In most cases, CTA is adequate for surgical or endovascular treatment planning. If CTA fails to identify a suspected aneurysm following SAH, cerebral arteriography will typically be performed. Cerebral arteriography not only allows aneurysm identification, but also provides other critical preoperative information such as aneurysm orientation, presence of vasospasm, location of adjacent vessels, and collateral intracranial circulation. Ar-teriography also helps to determine which aneurysm has bled when more than one aneurysm is present. As men-tioned previously, interventional neuroradiologists can treat aneurysms, usually in nonsurgical patients, by placing thrombosing material (ie, coils) within the aneurysm itself via an endovascular approach.

Although most patients with symptomatic cerebral aneurysms present with subarachnoid hemorrhage, some aneurysms act like intracranial masses. These situations usu-ally warrant evaluation by MR imaging as a first examination. The same is sometimes true with posterior communicating ar-tery aneurysms (which can produce symptoms related to the adjacent third cranial nerve) or with aneurysms arising from the internal carotid artery as it courses through the cavernous sinus (which can affect any of the cranial nerves that lie within this structure, including cranial nerves III, IV, V, or VI).

Vascular Malformations

Patients with a vascular malformation (eg, arteriovenous malformation, cavernous angioma, venous angioma, or cap-illary telangiectasia) often seek medical attention after an in-tracranial hemorrhage or a seizure. In this setting, the first test that should be performed is either a CT examination (to look for intracranial hemorrhage) or MR imaging. Although an intracranial hemorrhage is usually very obvious on a CT scan, the vascular malformation itself may be difficult, if not impossible, to see unless intravenous contrast material is ad-ministered. MR imaging, on the other hand, is quite sensitive for detecting vascular malformations, whether they have bled or not. The choice of the initial examination for evaluation of a vascular malformation can be difficult. Usually, patients undergo noncontrast head CT scanning to look for intracra-nial hemorrhage when they come to the emergency depart-ment. This is usually followed by CTA, particularly if an arteriovenous malformation (AVM) is suspected. Otherwise, the head CT is followed by gadolinium-enhanced MR imag-ing to further characterize the CT findings. If a true high-flow arteriovenous malformation is suspected, either clinically or from a cross-sectional imaging study, then cerebral arteriog-raphy is performed. In contrast to cerebral aneurysms, catheter angiography is still performed routinely to evaluate AVMs. This is done because catheter angiography provides details of flow dynamics within the AVM and demonstrates certain anatomic features that are necessary to elucidate prior to initiation of treatment. As spatial resolution and dynamicsequences improve, CT or MR angiography may someday re-place conventional arteriography in the workup of these le-sions, as with aneurysms.

Infarction

Today, most patients with suspected cerebral infarction un-dergo CT scanning in the acute setting, even though infarc-tions are demonstrated earlier and are more obvious on MR imaging. So why is CT usually performed first? The answer is that clinicians who manage stroke patients are not so inter-ested in seeing the infarct itself. Infarct location is usually sus-pected from the physical examination, and acute infarcts may not even be visible on CT scans for 12 to 24 hours after onset of stroke symptoms. Clinicians are very interested, though, to know if a stroke is secondary to something besides an infarct (eg, intracranial hemorrhage, brain tumor), or if an infarct is hemorrhagic, because thrombolytic agents would be con-traindicated in this setting. CT can quickly answer both of these questions. MR imaging, specifically diffusion-weighted imaging, can sensitively detect acute infarctions and is typically ordered in cases of high clinical suspicion, when the initial CT study is nondiagnostic or when brainstem or posterior fossa infarcts are suspected.

The underlying cause of most cerebral infarctions is thromboembolism related to atherosclerosis. A CT/CTA or MR/MRA (including DW and PW MR imaging) study may provide a positive imaging diagnosis of brain infarction, re-veal the extent and location of vessel occlusion, demonstrate the volume and severity of ischemic tissue, and predict final infarct size and clinical prognosis. CT and MR perfusion can identify areas of completed infarct (ie, infarct core) and po-tentially salvageable surrounding brain parenchyma at risk of infarction (ie, ischemic penumbra). Ultrasonography and cerebral arteriography can also be performed in the setting of stroke or transient ischemic attack to identify vascular stenoses or occlusions; these examinations are usually re-served for patients who might be candidates for carotid en-darterectomy. Functional examinations (SPECT and PET) have also been used in patients with strokelike symptoms to identify regions of the brain at risk for infarction. These stud-ies are not widely available and therefore do not enter into the imaging algorithm for most stroke patients.

In most medical centers, MR imaging is performed to as-sess brain tumor response to treatment. Anatomic imaging is often supplemented with some type of physiologic imaging including MR perfusion, MR spectroscopy, and PET scan-ning. Perfusion MRI, MRS, and PET scanning can frequently differentiate recurrent tumor from postradiation tissue necrosis, which can mimic tumor on an MR or a CT scan. MR perfusion imaging also provides functional information regarding the vascular density (ie, neovascularity) of a tumor, which may help to predict tumor grade or help guide a potential biopsy site.

Today, cerebral arteriography is rarely performed for brain tumor evaluation except to map the blood supply of very vascular tumors (ie, juvenile angiofibromas, paragan-gliomas) preoperatively. Such lesions can also be embolized prior to surgery in order to minimize intraoperative blood loss by injecting various materials into feeding vessels to occlude them.

Infection

Intracranial infections are best evaluated by contrast-en-hanced MR imaging. Abscesses, cerebritis, subdural empye-mas, and other infectious or inflammatory processes are all very well demonstrated. MR imaging is especially useful in as-sessing patients with acquired immunodeficiency syndrome (AIDS). Not only does it allow identification of secondary in-fections (eg, toxoplasmosis, cryptococcosis, progressive multi-focal leukoencephalopathy), but it is also exquisitely sensitive to the white matter changes produced by the human immun-odeficiency virus itself. CT scanning is less sensitive than MR imaging in the detection of intracranial infections and should be reserved for patients in whom MR imaging is con-traindicated. Cerebral arteriography is only useful in one par-ticular situation, suspected vasculitis. Involvement of brain arteries and arterioles in this condition requires arteriogra-phy for diagnostic confirmation.

Brain Tumors and Tumor-like Conditions

The best examination to order in the setting of suspected brain tumor is a contrast-enhanced MR scan. This is true for primary neoplasms as well as for metastatic disease. MR im-aging is especially useful in identifying tumors of the pitu-itary region, brainstem, and posterior fossa, including the cerebellopontine angle.

Although MR imaging is the preferred examination for in-tracranial neoplasms, it is occasionally supplemented by a CT scan, which can give important pretreatment information not provided by MR images. For example, CT can demonstrate tumor calcification, occasionally a useful factor in differenti-ating between types of neoplasms. Also, CT is very useful in identifying bone destruction in skull-base lesions.

Infection

Inherited and Acquired Metabolic, White Matter, and Neurodegenerative Diseases

As with suspected intracranial infections, this large and di-verse group of diseases is best evaluated with MR imaging, which sensitively detects white matter abnormalities. In fact, one of the very first clear indications for MR imaging was in the workup of suspected multiple sclerosis. Although brain abnormalities in these conditions may be quite obvious on MR imaging, there is one problem: many of these conditions appear very similar, and an exact diagnosis may not be possible.

In patients with dementia and suspected neurodegenerative disease, PET imaging is currently the procedure of choice for diagnostic evaluation.

Seizure and Epilepsy

Seizure is a common clinical indication for imaging the brain, particularly in the emergency setting. CT is the best modality to screen for multiple underlying causes of seizure including hemorrhage, mass lesion, or vascular malforma-tion. CT is also very useful to assess for secondary trauma that may occur during a seizure. MRI is often subsequently performed depending on various factors including the pa-tient’s age, clinical presentation, and type of seizure, or in the case of epilepsy. MRI is superior to CT in evaluating fine cerebral anatomy because of its excellent soft-tissue contrast and the absence of beam hardening artifact, as well as its mul-tiplanar capability. Particular MR protocols are utilized to discriminate the hippocampal structures and to detect other epileptogenic foci, including various cortical malformations, neoplasms, and vascular malformations.

In the case of medically refractory epilepsy, patients may pursue surgery for more definitive treatment. During surgi-cal planning, additional functional imaging performed in-cludes ictal SPECT and interictal PET. These studies help confirm a suspected epileptogenic focus, which demon-strates increased activity during or immediately following a seizure (SPECT) versus decreased metabolic activity be-tween seizures (PET). Cerebral arteriography is often per-formed prior to epilepsy surgery in order to establish cerebral dominance by intracarotid sodium amytal injection (Wada test). Following catheter injection of amytal into the internal carotid artery, function within the corresponding cerebral hemisphere is temporarily depressed, allowing for neurological testing of memory and language in the con-tralateral hemisphere.