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The Radiology Of Headache
Headache
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The Radiology of Headache Jordan M. Prager, MD, * and David]. Mikulis, MDt
The radiographic evaluation of headache may be effectively directed by clinical presentation into two categories: acute and chronic. In general, acute headache is best evaluated by CT because of its greater sensitivity to early hemorrhage. Also, it is easier to obtain a CT examination in the critically ill patient who requires monitoring equipment. Chronic headache is best evaluated by MRI because of its greater sensitivity to vascular disease, tumor, infection and posttraumatic changes. Magnetic resonance imaging has excellent resolution in the posterior fossa, a region that is poorly visualized by CT, and more accurately demonstrates gliosis and demyelination. Similarly, MRI may detect changes not seen by CT in patients with trigeminal neuropathy, migraine, and temporomandibular joint dysfunction. Note that the various etiologies of acute headaches may also present with chronic symptomatology. This article is not an exhaustive listing of the causes of headache. Headaches with radiologic findings or controversial clinical associations are included. A brief introduction to magnetic resonance imaging (MRI) is included as an appendix. ACUTE HEADACHE Subarachnoid Hemorrhage The sudden onset of severe head pain should lead the examiner to be suspicious of subarachnoid hemorrhage from aneurysm. Intraparenchymal hemorrhage may also be considered, either primary or secondary into a preexisting lesion. Computed tomography is the examination of choice in either case. Acute hemorrhage is well visualized on CT as an area of high attenuation, whether in the brain parenchyma or in between the meninges (Fig. 1). The only exception is when the patient is severely anemic. In this case, the blood may be isodense to brain or, rarely, hypodense. Limitations to visualization exist when the hemorrhage is contiguous with bone owing to artifact. This is especially prominent in the posterior fossa. Widening the window setting may help in these cases. Subarachnoid hemorrhage from aneurysm is usually seen on CT, and, when *Assistant Professor, Department of Radiology, Section of Neuroradiology, The University of Chicago, Chicago, Illinois t Assistant Professor, Department of Radiology, Section of Neuroradiology, The Massachusetts General Hospital; and Instructor, Harvard Medical School, Boston, Massachusetts
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demonstrated, lumbar puncture is usually not necessary. If early surgery is considered, angiography would be the next step. The aneurysm that has bled is not usually seen on CT because of overlying blood or intraluminal clot. The location of the hemorrhage, however, may indicate the site of the aneurysm. If the dominant location of the hemorrhage is in the anterior interhemispheric fissure, an anterior cerebral artery aneurysm is suspected. There is a caveat in that the apex of the aneurysm may be directed towards the contralateral side, so that when the aneurysm leaks, much of the blood will be directed to the opposite side. In this situation, a right-sided aneurysm will show dominant hemorrhage on the left side. Dominant hemorrhage in the sylvian fissure or suprasellar cistern indicates aneurysm of the middle cerebral artery or internal carotid artery, respectively (see Fig. 1). Prior to hemorrhage (Fig. 2), an aneurysm may be seen as a focal region of high attenuation, perhaps with calcification and contrast enhancement. If the aneurysm has already thrombosed, it may show decreased attenuation. A negative CT scan should still be followed by lumbar puncture when clinical suspicion is high and when positive by angiography. At the time of angiography, all major vessels must be studied because of the 20% incidence of multiple aneurysms. Because aneurysm may present without hemorrhage in acute or chronic fashion, a patient with an appropriate history should still receive an aggressive workup. Magnetic resonance imaging may be a helpful screening tool for aneurysm, but angiography remains the gold standard. On MRI, an aneurysm shows a combination of high and low intensities from slow- and rapid-flowing blood and clot. Parenchymal Hemorrhage Intraparenchymal hemorrhage may be primary, from amyloid angiopathy or hypertension in the older population (Fig. 3). Drug abuse, particularly use of cocaine or amphetamines, may cause hemorrhage in the younger population. Secondary intraparenchymal hemorrhage is due to an underlying lesion such as metastatic tumor or arteriovenous malformation. Computed tomography is the initial examination of choice because of its sensitivity to hemorrhage and the ease of performing the examination. Computed tomography is very sensitive to hemorrhage but not specific. If the patient is stable, further diagnostic evaluation with MRI and possibly angiography may be of value to narrow the differential possibilities. Hypertensive hemorrhage occurs in the putamen, thalamus, and, less frequently, the dentate nucleus in the cerebellum. Lobar hemorrhage in the elderly is frequently due to amyloidosis. Multiple lesions, severe edema, and enhancing tissue surrounding a hemorrhage suggest metastatic disease. The metastatic lesions most likely to bleed include lung, kidney, melanoma, and choriocarcinoma. Gliomas may bleed occasionally, and certain blood dyscrasias may be predisposed to hemorrhage. Vascular Malformations Various vascular malformations, the high-flow arteriovenous malformation or low-flow cavernous hemangioma, and venous angioma may hemorrhage as well. Of these, the most likely to bleed is arteriovenous malformation (Fig. 4). The enlarged arteries and veins may be seen on either CT or MRI; MRI shows more distinct definition and characterization. Magnetic resonance imaging will show prominent low-intensity (dark) serpiginous regions of flow void as well as focal increasedintensity (bright) regions from slow flow and methemoglobin (subacute hemorrhage). Old hemorrhage (hemosiderin and ferritin) is seen as a low-intensity region on long TR images surrounding the lesion. If there have been discrete episodes of hemorrhage, there may be several rings of low-intensity signal. Acute (Text continued on page 531)
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Figure 1. Subarachnoid hemorrhage from aneurysm. Computed tomography scans show high-attenuation, subarachnoid blood predominantly in the suprasellar cistern on the left side (A, arrow) and extending to the ambient cistern, interhemispheric fissure, sylvian fissure (B, arrow), and deep cortical sulci (C, arrows). D, Left common carotid angiogram shows aneurysm of internal carotid artery (arrows) at the superior junction of the posterior communicating artery. This is referred to as a posterior communicating artery aneurysm.
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Figure 2. Aneurysm presenting as headache. A, Computed tomography scan shows increased attenuation mass with a partially calcified rim. T,-weighted sagittal (B) and T2weighted axial (C) magnetic resonance images show aneurysm (arrow) with mixed signal intensity within from How and clot. D, Left carotid angiogram shows large aneurysm of the supraclinoid carotid artery.
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Figure 3. Acute intraparenchymal hemorrhage in a 70-year-old man. A, Computed tomography scan shows large increased-attenuation mass in right frontoparietal region. The area of decreased attenuation surrounding the mass represents edema, and the area of increased attenuation refers to hematoma. T j (B), first-echo long TR (C), and T2(D) magnetic resonance images at a lower level show decreased-attenuation central region as a result of deoxyhemoglobin. The surrounding bright signal on both T j and T2 images is caused by methemoglohin, and on T2, the outer bright rim is from edema. A thin, dark rim is already beginning to develop on the long TR images (arrows) as a result offerritin and hemosiderin. Note the magnetic susceptibility effect on the long TR sequence. The first-echo dark gray hematoma and rim becomes black on the second echo.
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Figure 4. Arteriovenous malformation. A, Contrast-enhanced computed tomography (CECT) shows a nonspecific enhancing mass without significant edema. Band C, Magnetic resonance images show How voids in a serpiginous configuration. D, T2 magnetic resonance image shows variable signal within blood vessels from slow and rapid How. A draining vein is seen coursing towards the vein of Galen. This appearance is diagnostic of an arteriovenous malformation. E, Angiogram shows feeding vessels from anterior cerebral, middle cerebral, and posterior cerebral arteries with large draining veins.
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hemorrhage (deoxyhemoglobin) is shown as a low-intensity region on long TR images and a hypointense to isointense region on Tcweighted images. Cavernous hemangiomas do not have prominent arteries or veins, but they do have a characteristic appearance on MRI secondary to a combination of subacute and chronic hemorrhage (Fig. 5). There is a bright central region from methemoglobin and a single rim or several rings of decreased intensity from hemosiderin and ferritin. These lesions tend to have repeated small hemorrhages. Cavernous hemangiomas are frequently seen on MRI but have not yet been clinically well characterized, so their relationship to headache is not clear. Finally, venous angiomas may hemorrhage infrequently. The anomalous vein courses through the white matter to the brain surface or centrally to the deep venous system. These lesions are easily seen as linear flow void on MRI or as a linear area of increased attenuation on contrast-enhanced CT. Angiography shows a cluster of medullary veins converging on a large central draining vein (Fig. 6). Obstructive Hydrocephalus The rapid development of hydrocephalus may also cause acute headache. This may be due to a lesion that obstructs cerebrospinal fluid pathways, such as a colloid cyst in the third ventricle. A mass in the pineal region or posterior fossa may obstruct the aqueduct or fourth ventricle, respectively. Congenital causes such as aqueducted stenosis or insufficiency may also suddenly cause symptomatic hydrocephalus (Fig. 7). Computed tomography is adequate to diagnose hydrocephalus and to suggest the appropriate diagnosis. Magnetic resonance imaging is frequently obtained later for further characterization when the patient is stable. CHRONIC HEADACHE Migraine The most frequently encountered headaches in clinical practice are due to either migraine or muscle-contraction headache. In migraine, an early CT study showed an increased incidence of atrophy in 58% of a test group of 53 patients. 2 More recent MRI studies lO show incidence of atrophy of 35% and increased signal foci on long TR images in the white matter (Fig. 8), known as white-matter foci (WMF). These WMF are separate from the ventricles and are brighter than cerebrospinal fluid on the intermediate and long TR images. In a study by Prager et at'2 involving 100 patients with severe chronic headache, 77 patients had migraine. In 40 patients less than 41 years of age, 25% had WMF. In 34 patients who were 41 to 60 years of age, 68% had WMF. All three patients older than 60 years of age had WMF. A study involving 74 patients with classical migraine found WMF in 26% of patients from age 9 to 39.10 The incidence of WMF is well documented in migraine, 10. 12. 15 but some questions remain regarding the incidence of WMF in the normal population less than 60 years old. The only documentation of this incidence comes indirectly from studies that have a control population directed towards other disease processes with WMF. One such study was recently completed by Bowen et al,3 who examined 50 normal patients with an age range of 31 to 51 and found WMF similar to those seen in the migraine studies in 6% of the patients. An explanation for the presence of WMF and atrophy is not immediately apparent. In the study by Prager et al,12 a history of vasoactive medication was also examined, and it was found that approximately 50% of these patients had WMF. There is no pathologic literature that looks specifically at the lesions of migraine patients with WMF. Kirkpatrick and Hayman 9 studied 12 patients pathologically for correlation with WMF in patients 52 to 72 years of age. They found eight
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Figure 5. Large cavernous hemangioma. A, Contrast-enhanced computed tomography shows nonspecific enhancing mass in right thalamus with surrounding decreased attenuation and mass effect on the third ventricle. B, T} magnetic resonance image reveals a mass in the thalamus that contains mottled high and medium signal intensities with a dark rim. First-echo long TR (C) and T2 magnetic resonance images (D) show the magnetic susceptibility effect of the rim, which represents ferritin and hemosiderin. This appearance is characteristic of cavernous hemangioma.
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patients with zones of atrophic perivascular demyelination. This was believed to be due to mild vascular insufficiency causing perivascular fibrous gliosis. Perhaps the process that leads to the appearance of WMF in normal individuals over 60 years of age4-6 is occurring at an accelerated rate in the migraine patient because of the inherent vascular instability seen in these patients. The most serious complication of migraine is stroke. Acute infarction is seen earlier with MRI than with eT, and it may be demonstrated as early as 6 hours after the onset of symptoms. Finally, there have been reports of transient contrast enhancement on eT during migraine with complete resolution on follow-up studies. " 11 In such a case, hyperemia or focal damage to the blood-brain barrier from vascular, inflammatory, or neoplastic etiology may be considered. Magnetic resonance imaging should be sufficient in most cases to rule out the possibilities that need immediate attention. Trigeminal Neuropathy Pain related to the trigeminal nerve, with or without other symptoms, is a common clinical problem. Localization along the trigeminal nerve has classically been deduced from symptoms and signs. However, a recent paper by Hutchins et aF compared clinical localization with MRI and eT. They found clinical localization to be inaccurate and MRI to be most sensitive. Seventy-six patients were studied, and other than the combination of pain and numbness usually seen with peripheral lesions, and trismus seen with lesions of the masticator space, clinical characteristics were not useful for localization. Because of this lack of clinical specificity, the peripheral and central portions of the nerve must be examined in a patient with trigeminal neuropathy. Twenty of Hutchins' patients had tic douloureaux (trigeminal neuralgia or tic), which was studied by MRI. Three had positive pathologic findings; the diagnoses were multiple sclerosis, arteriovenous malformation, and maxillary sinusitis. Two patients had vascular compression. In a separate study, Hutchins et alB reviewed the MRI studies of 29 patients with tic douloureaux and 30 patients with unrelated disorders, looking specifically for vascular contact with the trigeminal nerve. Fiftyseven per cent of symptomatic patients without multiple sclerosis had vascular contact at the root-entry zone. In the 30 symptomatic patients, there was a 27% incidence of vascular contact. The study found no correlation between the degree of vessel contact and symptoms; therefore, actual nerve compression was not a sensitive indicator for vascular decompression. Magnetic resonance imaging was suggested for patients with atypical tic or tic not responsive to standard therapy to rule out masses, multiple sclerosis (15% in the series), or gross vascular abnormality (Fig. 9). Temporomandibular Joint Dysfunction Temporomandibular joint dysfunction may present with varied symptoms, including pain in the temporomandibular joint region or in the face, neck, or head. There may be jaw clicking or abnormal movements of the jaw. The first radiologiC procedure is often standard polytomography to detect osseous cortical abnormalities, fracture, changes with the mouth open, and the relationship of the mandible to the skull base. Magnetic resonance imaging may also be the first examination or may follow eT if the findings are not diagnostic or the patient does not respond to therapy. An early study'6 comparing eT and MRI found MRI superior for viewing the soft tissues. Within a very short time, eT has been relegated to an adjunctive role to view the bony compartments in the region. '4 According to Rao et al,'3 on sagittal MRI, a posterior band of the temporomandibular joint disk at the apex of the condylar head constitutes normal disk. Forward deviation of the posterior band relative to the apex of the condylar head indicates anterior displacement of the disk
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Figure 6. Venous angioma with hemorrhage. A and B, Noncontrast and contrast-enhanced computed tomography images show high-attenuation blood in right frontal region with some moderate enhancement in B. T j (C), first-echo long TR (D), and T, (E) magnetic resonance images show bright central regions surrounded by a magnetic susceptibility signal void that represent subacute, acute, and chronic hemorrhage. F and G, Angiogram in venous phase shows typical caput medusae appearance of radiating medullary veins (arrows) with central large vein (arrow) extending to cortex.
(Illustration continued on opposite page)
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Figure 6 (Continued).
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Figure 7. Aqueductal stenosis. A and B, Sagittal and axial T J magnetic resonance images show large lateral and third ventricles. The aqueduct is small, and the fourth ventricle is normal. Note the large infundibular recess of the third ventricle on both images (arrow).
(Fig. 10). Further classification is based on any change with the mouth open. To make other diagnoses in the area, such as perforation or adhesions, other modalities, including arthrography or an arthroscopy, may be necessary. Degenerative joint disease may be seen with all of the aforementioned modalities as well as CT. Rao et al conclude that, in most cases, the findings on MRI are sufficient to decide between conservative or surgical therapy. Craniocerebral Trauma A history of trauma is important in the patient with chronic headache. Computed tomography or MRI may show nonspecific tissue loss; CT will better show bony changes. In addition to these changes, MRI may also show more specific change related to old hemorrhagic shear injury. On follow-up studies, these patients may show magnetic susceptibility on the long TR images. Magnetic susceptibility due to iron deposition in the tissues is seen as decreased intensity on the T2 weighted images (Fig. 11). Shear injury is particularly well seen at the gray-white junction and in the deep hemispheric white matter. Other manifestations of old trauma are white matter foci, bright on long TR images from gliosis. Chronic subdural hematoma may frequently present as headache. In the elderly, no history of trauma may be given, despite the presence of a prominent collection. A subacute subdural hematoma, approximately 1 to 4 weeks in duration, may be isodense on CT. On MRI, this is the time when a subdural hematoma is most obvious, owing to the presence of methemoglobin, which appears bright on both Tc and T 2 -weighted images. The chronic subdural hematoma is well seen on both CT and MRI, but there is greater characterization on MRI. Neoplastic, Inflammatory, Congenital, and Idiopathic Causes of Headache Central nervous system neoplasm is another obvious source of headache, and it is the sole presenting complaint in about one third of these cases. In general, the
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Figure 8. Migraine in a 44-year-old patient. First-echo long TR (A) and T, (B) magnetic resonance images. White-matter foci are seen bilaterally (arrows). The white-matter foci appear similar to those seen in multiple sclerosis, but the subcortical location without periventricular lesions makes multiple sclerosis less likely. C and D, Magnetic resonance image in coronal plane in a different patient shows typical location and appearance of whitematter foci (arrows).
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Figure 9. Trigeminal neuralgia in a patient with multiple sclerosis. First-echo long TR (A) and T 2 (B) magnetic resonance images show multiple bright foci that are confluent in areas in the pons and cerebellum (arrows). First-echo long TR (C) and T2 (D) magnetic resonance images in the same patient at the superior aspect of the ventricles show multiple bright foci typical of multiple sclerosis. These lesions are typically oval-shaped and oriented perpendicular to the ventricles in the white matter.
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Figure 10. Temporomandibular joint dysfunction. A, T,-weighted sagittal magnetic resonance image shows bow-tie-shaped disk. The posterior aspect of the disk (arrow) is in normal position directly above the mandibular condyle. B, T,weighted sagittal magnetic resonance image shows marked anterior displacement of the disk (arrow). (Courtesy of Louis Kircos, MD, University of Chicago, Chicago, Illinois.)
mass must be of considerable size to provide traction on or to displace a large blood vessel and produce pain. Frequently, by this time, other symptoms and signs, in addition to a chronic progressive headache, will suggest the correct diagnosis. Magnetic resonance imaging is superior for tumor detection and characterization, but eT is sufficient to rule out lesions that demand immediate therapy. Inflammatory causes of headache include infectious and idiopathic etiologies. In cases of acute and chronic meningitis and encephalitis, MRI is superior to eT for detection of abnormality. Gadolinium enhancement of the meninges is well seen over the convexities or at the base of the brain in meningitis. In encephalitis, minimal increase in brain water is evident on the T2-weighted images. In sinusitis, MRI and eT play complementary roles. Magnetic resonance imaging is superior for characterizing the extent of soft-tissue involvement; eT is superior for evaluation of pony changes. Temporal arteritis may be a very elusive diagnosis. In the appropriate clinical
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Figure 11. Shear injury. Gradient echo sequence sensitive for magnetic susceptibility. Numerous signal-void lesions (black areas) are seen scattered in the cortex, gray-white junction, and periventricular white matter. These areas represent ferritin and hemosiderin at the sites of old shear injury.
setting, the sedimentation rate may be normal and biopsy may miss the pathologic area. Angiography may be helpful and show typical changes of arteritis, which include irregular regions of narrowing and dilatation in the superficial temporal artery or other neighboring vessels. Sinus thrombosis may develop contiguous to areas of infection, such as the transverse sinus from mastoiditis or cavernous sinus from sphenoid sinus inflammation. Magnetic resonance imaging or CT may be suggestive, and angiography may be necessary for diagnosis. Congenital causes for headache include abnormalities at the skull base and posterior fossa. Examples include the Chiari malformation and the Dandy-Walker syndrome and variant. Magnetic resonance imaging provides excellent demonstration of these abnormalities. SUMMARY The patient who presents with a severe and acute headache should be evaluated radiographically with CT. The key diagnosis to make in this situation is hemorrhage, either subarachnoid or intraparenchymal. Computed tomography is more sensitive to acute hemorrhage than is MRI. When the patient is stable, MRI frequently contributes information to narrow the diagnostic possibilities, because vascular malformations and certain parenchymallesions have a characteristic appearance on MRI. Hydrocephalus may also present acutely and is easily seen on CT or MRI. In a patient with chronic headaches, MRI is generally most informative. The migraine patient may show WMF and atrophy. The patient with trigeminal neuropathy may demonstrate central or peripheral lesions. In temporomandibular joint dysfunction, conventional tomography and MRI are frequently used. Magnetic resonance imaging shows excellent detail of the disk and surrounding soft tissues, whereas tomography better demonstrates bony changes. When a history of trauma is present, MRI may show a subacute subdural hematoma. These collections are easily seen on MRI, even when isodense on CT. Evidence of old shear injury is also well seen on MRI. Finally, neoplastic, inflammatory, congenital, and idiopathic sources of head-
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Figure 12. Subacute subdural hematoma. A, Computed tomography shows bilateral isodense predominantly temporal, nonspecific abnormalities. There is no midline shift to suggest mass effect. T, coronal (B), first-echo long TR (C), and T, (D) magnetic resonance images show variable, predominantly high-intensity bilateral extra-axial collection on all sequences, which is consistent with chronic subdural hematoma with episodes of rebleeding. There are several compartments. The darker components seen on the long TR images are due to deoxyhemoglobin and cellular debris (arrows) from recent bleeding episodes. The bright signal is due to methemoglobin. When there is continued oozing, the bright signal from methemoglobin may be seen for months. (Courtesy ofWilliam A. Wagle, MD, Albany Medical College, Albany, New York.)
ache may be demonstrated by either MRI or eT, depending on presentation. MRI will generally show superior characterization. APPENDIX
Introduction to Magnetic Resonance Imaging The MRI scanner employs a strong magnetic field and radiowaves (radiofrequency pulses) to "excite" protons and cause them to "resonate." The resonating
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protons dissipate this excess energy ("relax") over time and eventually return to their pre-excitation state. During this relaxation process, the protons themselves emit radiowaves that are detected and "read," yielding information regarding proton location, density, movement, and rate of relaxation. The MRI scanner receives the radiowave signal and is able to produce cross-sectional images of the body displaying these variations of proton behavior in tissues. The extraordinary images obtained with MRI are the result of greater contrast resolution compared to CT. This translates into greater differences in image brightness between adjacent tissues. These images are generated via measurement of the two primary forms of proton relaxation-Tj and T 2-processes that occur simultaneously. T j Relaxation: T j relaxation (simply called T j) is the rate at which excited protons return to the equilibrium or relaxed state following excitation by a radiofrequency pulse. T2 Relaxation: Immediately after the application of a radiofrequency pulse, all the protons are resonating "in phase" with each other. The protons soon lose this phase coherence because of slight differences in the resonant frequency of each proton due to the local magnetic field variations present in all tissues. T2 relaxation (or T 2) therefore represents the rate of loss of phase coherence between the resonating protons. The MR image is therefore a map of signal intensities in which the signal intensity is dependent on T j, T 2, and proton density (density of protons within the tissue being imaged). In practice, one can manipulate these Y1RI parameters to emphasize T j relaxation (Tj-weighted image), T2 relaxation (T2 -weighted image), or the overall density of protons in the tissue (proton-density image). A short TR (time between radiofrequence pulses) favors T j, and a long TR favors T z . With a longer TR, two radiowaves may be sampled. The first echo produces the proton-density image, and the second echo produces the Tz-weighted image. For the purposes of image interpretation, cerebrospinal fluid signal intensity can be used to determine whether an image is T,- or Tz-weighted: Cerebrospinal fluid is dark on T, (only air, bone, and blood flow are darker) and bright on T z. One of the most important concepts to understand regarding image analysis is that most lesions have the same signal characteristics as cerebrospinal fluid-dark (low signal) on T j and bright (high signal) on T z• Another indispensable concept is that there are only a few entities that have high signal on Tj-weighted sequences. Fat, flow (can actually have low, high, or intermediate signal), methemoglobin (a breakdown product of blood in a hemorrhagic lesion), gadolinium (an injectable MRI contrast agent), and fluid containing appreciable amounts of protein are bright on T,. Complex signal patterns are seen with stationary blood extravasated into tissues and with flowing fluids. Detection of Hemorrhage The appearance of extravasated blood on MRI is quite complex and depends on tissue oxygenation, age of hemorrhage, integrity of red blood cell membrane, and the presence of hemoglobin breakdown products. In the acute phase, a parenchymal hematoma can have signal characteristics very similar to that of normal brain. The diagnosis of acute hemorrhage, including subarachnoid hemorrhage, is difficult to make by MRI; CT maintains superiority in this area. Subacute blood appears bright on T, and has a variable signal on T2 depending on whether intact (hemorrhage appears dark) or lysed (hemorrhage appears bright) red blood cells are present. The blood products in chronic hemorrhage, i.e., hemosiderin and ferritin, which are frequently seen in old contusions and shear injury, demonstrate significantly reduced signal characteristics owing to "magnetic susceptibility" effects. Iron in the tissues causes disorganization of magnetic fields, which results in loss of
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signal. This magnetic susceptibility effect is much more apparent on T 2 -weighted imagcs. Limitations of Magnetic Resonance Imaging The advantages of \iRI are obvious in terms of lesion detection and anatomic characterization. Furthermore, no ionizing radiation is involved. The only major limitations include detection of hyperacute hemorrhage, detection of calcium (calcium returns no signal; therefore, dystrophic calcifications seen in various tumors can be missed), longer scan times compared to eT, and difficulty in monitoring sick patients (monitors or life-support devices may not function in the presence of strong magnetic fields). There arc contraindications to MRI including ferromagnetic aneurysm clips (new clips arc no longer ferromagnetic), cardiac pacemakers, certain cochlear implants, permanent neurostimulators (TENS units), known ocular metallic foreign bodies, and certain prosthetic heart valves and metallic foreign bodies if they are present near sensitive structures (e.g., the eye). The advantages and disadvantages of MRI are usually weighed carefully by the clinician and the radiologist when they are selecting the appropriate imaging study. ACKNOWLEDGMENT The authors wish to thank Dr. Ruth Ramsey for her review of the manuscript.
REFERENCES 1. Alvarez-Cermeno IC, Gobernado JM, Freije R, et al: Cranial computed tomography in pediatric migraine. Pediatr Radiol 14:195-197, 1984 2. du Boulay GH, Ruiz IS, Rose FC. et al: CT changes associated with migraine. AJNR 4:472-473, 1983 3. Bowen BC. Barker WW, Loewenstein DA, et al: MR signal abnormalities in memory disorder and dementia. AJNR 11:283-290, 1990 4. George AE, de Leon MJ, Kalnin A, et al: Leuko-encephalopathy in normal and pathologic aging. 2. MRI of brain lucencies. AJNR 7:567-570, 1986 5. Gerard G, Weisberg LA: MRI periventricuiar lesions in adults. Neurology 36:998-1001, 1986 6. Hendrie HC, Farlow MR, Austrom MG, et al: Foci of increased T2 signal intensity on brain MR scans of healthy elderly subjects. AJNR 10:703-707,1989 7. Hutchins LG, Harnsberger HR, Hardin CW, et al: The radiological assessment of trigeminal neuropathy. AJNR 10:1031-1038, 1989 8. Hutchins LG, Harnsberger HR, Jacobs JM, et al: Trigeminal neuralgia (tic douloureux): MR imaging assessment. Radiology 175:837-841, 1990 9. Kirkpatrick JB, Hayman LA: White-matter lesions in MR imaging of clinically healthy brains of elderly subjects: Possible pathologic basis. Radiology 162:509-511, 1987 10. Kuhn MJ, Shekar PC: A comparative study of magnetic resonance imaging and computed tomography in the evaluation of migraine. Comput Med Imag Graphics 14(2):149-152, 1990 11. Muller J-p, Destee A, Lozes G, et al: Transient cortical contrast enhancement on CT scan in migraine. Headache 27:578-579, 1987 12. Prager J, Mikulis D, Davis K, et al: MRI findings in 100 patients with severe headache [abstract]. In Programs and Abstracts of Society for Magnetic Resonance in Medicine, August, 1990 13. Rao VM, Farole A, Karasick 0: Temporomandibular joint dysfunction: Correlation of MR imaging, arthrography, and arthroscopy. Radiology 17:663-667, 1990 14. Schellhas KP: Temporomandibular joint injuries. Radiology 173:211-216, 1989
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15. Soges LJ, Cacayorin EO, Petro CR, et al: Migraine: Evaluation by MR. AJNR 9:425429, 1988 16. Westesson P, Katzberg RW, Tallents RH, et al: CT and MR of the temporomandibular joint: Comparison with autopsy specimens. AJR 148:1165-1171, 1987
Address reprint requests to Jordan M. Prager, MD Department of Radiology University of Chicago Box 429 5841 S. Maryland Avenue Chicago, IL 60637
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The Radiology of Headache Jordan M. Prager, MD, * and David]. Mikulis, MDt
The radiographic evaluation of headache may be effectively directed by clinical presentation into two categories: acute and chronic. In general, acute headache is best evaluated by CT because of its greater sensitivity to early hemorrhage. Also, it is easier to obtain a CT examination in the critically ill patient who requires monitoring equipment. Chronic headache is best evaluated by MRI because of its greater sensitivity to vascular disease, tumor, infection and posttraumatic changes. Magnetic resonance imaging has excellent resolution in the posterior fossa, a region that is poorly visualized by CT, and more accurately demonstrates gliosis and demyelination. Similarly, MRI may detect changes not seen by CT in patients with trigeminal neuropathy, migraine, and temporomandibular joint dysfunction. Note that the various etiologies of acute headaches may also present with chronic symptomatology. This article is not an exhaustive listing of the causes of headache. Headaches with radiologic findings or controversial clinical associations are included. A brief introduction to magnetic resonance imaging (MRI) is included as an appendix. ACUTE HEADACHE Subarachnoid Hemorrhage The sudden onset of severe head pain should lead the examiner to be suspicious of subarachnoid hemorrhage from aneurysm. Intraparenchymal hemorrhage may also be considered, either primary or secondary into a preexisting lesion. Computed tomography is the examination of choice in either case. Acute hemorrhage is well visualized on CT as an area of high attenuation, whether in the brain parenchyma or in between the meninges (Fig. 1). The only exception is when the patient is severely anemic. In this case, the blood may be isodense to brain or, rarely, hypodense. Limitations to visualization exist when the hemorrhage is contiguous with bone owing to artifact. This is especially prominent in the posterior fossa. Widening the window setting may help in these cases. Subarachnoid hemorrhage from aneurysm is usually seen on CT, and, when *Assistant Professor, Department of Radiology, Section of Neuroradiology, The University of Chicago, Chicago, Illinois t Assistant Professor, Department of Radiology, Section of Neuroradiology, The Massachusetts General Hospital; and Instructor, Harvard Medical School, Boston, Massachusetts
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demonstrated, lumbar puncture is usually not necessary. If early surgery is considered, angiography would be the next step. The aneurysm that has bled is not usually seen on CT because of overlying blood or intraluminal clot. The location of the hemorrhage, however, may indicate the site of the aneurysm. If the dominant location of the hemorrhage is in the anterior interhemispheric fissure, an anterior cerebral artery aneurysm is suspected. There is a caveat in that the apex of the aneurysm may be directed towards the contralateral side, so that when the aneurysm leaks, much of the blood will be directed to the opposite side. In this situation, a right-sided aneurysm will show dominant hemorrhage on the left side. Dominant hemorrhage in the sylvian fissure or suprasellar cistern indicates aneurysm of the middle cerebral artery or internal carotid artery, respectively (see Fig. 1). Prior to hemorrhage (Fig. 2), an aneurysm may be seen as a focal region of high attenuation, perhaps with calcification and contrast enhancement. If the aneurysm has already thrombosed, it may show decreased attenuation. A negative CT scan should still be followed by lumbar puncture when clinical suspicion is high and when positive by angiography. At the time of angiography, all major vessels must be studied because of the 20% incidence of multiple aneurysms. Because aneurysm may present without hemorrhage in acute or chronic fashion, a patient with an appropriate history should still receive an aggressive workup. Magnetic resonance imaging may be a helpful screening tool for aneurysm, but angiography remains the gold standard. On MRI, an aneurysm shows a combination of high and low intensities from slow- and rapid-flowing blood and clot. Parenchymal Hemorrhage Intraparenchymal hemorrhage may be primary, from amyloid angiopathy or hypertension in the older population (Fig. 3). Drug abuse, particularly use of cocaine or amphetamines, may cause hemorrhage in the younger population. Secondary intraparenchymal hemorrhage is due to an underlying lesion such as metastatic tumor or arteriovenous malformation. Computed tomography is the initial examination of choice because of its sensitivity to hemorrhage and the ease of performing the examination. Computed tomography is very sensitive to hemorrhage but not specific. If the patient is stable, further diagnostic evaluation with MRI and possibly angiography may be of value to narrow the differential possibilities. Hypertensive hemorrhage occurs in the putamen, thalamus, and, less frequently, the dentate nucleus in the cerebellum. Lobar hemorrhage in the elderly is frequently due to amyloidosis. Multiple lesions, severe edema, and enhancing tissue surrounding a hemorrhage suggest metastatic disease. The metastatic lesions most likely to bleed include lung, kidney, melanoma, and choriocarcinoma. Gliomas may bleed occasionally, and certain blood dyscrasias may be predisposed to hemorrhage. Vascular Malformations Various vascular malformations, the high-flow arteriovenous malformation or low-flow cavernous hemangioma, and venous angioma may hemorrhage as well. Of these, the most likely to bleed is arteriovenous malformation (Fig. 4). The enlarged arteries and veins may be seen on either CT or MRI; MRI shows more distinct definition and characterization. Magnetic resonance imaging will show prominent low-intensity (dark) serpiginous regions of flow void as well as focal increasedintensity (bright) regions from slow flow and methemoglobin (subacute hemorrhage). Old hemorrhage (hemosiderin and ferritin) is seen as a low-intensity region on long TR images surrounding the lesion. If there have been discrete episodes of hemorrhage, there may be several rings of low-intensity signal. Acute (Text continued on page 531)
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Figure 1. Subarachnoid hemorrhage from aneurysm. Computed tomography scans show high-attenuation, subarachnoid blood predominantly in the suprasellar cistern on the left side (A, arrow) and extending to the ambient cistern, interhemispheric fissure, sylvian fissure (B, arrow), and deep cortical sulci (C, arrows). D, Left common carotid angiogram shows aneurysm of internal carotid artery (arrows) at the superior junction of the posterior communicating artery. This is referred to as a posterior communicating artery aneurysm.
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Figure 2. Aneurysm presenting as headache. A, Computed tomography scan shows increased attenuation mass with a partially calcified rim. T,-weighted sagittal (B) and T2weighted axial (C) magnetic resonance images show aneurysm (arrow) with mixed signal intensity within from How and clot. D, Left carotid angiogram shows large aneurysm of the supraclinoid carotid artery.
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Figure 3. Acute intraparenchymal hemorrhage in a 70-year-old man. A, Computed tomography scan shows large increased-attenuation mass in right frontoparietal region. The area of decreased attenuation surrounding the mass represents edema, and the area of increased attenuation refers to hematoma. T j (B), first-echo long TR (C), and T2(D) magnetic resonance images at a lower level show decreased-attenuation central region as a result of deoxyhemoglobin. The surrounding bright signal on both T j and T2 images is caused by methemoglohin, and on T2, the outer bright rim is from edema. A thin, dark rim is already beginning to develop on the long TR images (arrows) as a result offerritin and hemosiderin. Note the magnetic susceptibility effect on the long TR sequence. The first-echo dark gray hematoma and rim becomes black on the second echo.
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Figure 4. Arteriovenous malformation. A, Contrast-enhanced computed tomography (CECT) shows a nonspecific enhancing mass without significant edema. Band C, Magnetic resonance images show How voids in a serpiginous configuration. D, T2 magnetic resonance image shows variable signal within blood vessels from slow and rapid How. A draining vein is seen coursing towards the vein of Galen. This appearance is diagnostic of an arteriovenous malformation. E, Angiogram shows feeding vessels from anterior cerebral, middle cerebral, and posterior cerebral arteries with large draining veins.
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hemorrhage (deoxyhemoglobin) is shown as a low-intensity region on long TR images and a hypointense to isointense region on Tcweighted images. Cavernous hemangiomas do not have prominent arteries or veins, but they do have a characteristic appearance on MRI secondary to a combination of subacute and chronic hemorrhage (Fig. 5). There is a bright central region from methemoglobin and a single rim or several rings of decreased intensity from hemosiderin and ferritin. These lesions tend to have repeated small hemorrhages. Cavernous hemangiomas are frequently seen on MRI but have not yet been clinically well characterized, so their relationship to headache is not clear. Finally, venous angiomas may hemorrhage infrequently. The anomalous vein courses through the white matter to the brain surface or centrally to the deep venous system. These lesions are easily seen as linear flow void on MRI or as a linear area of increased attenuation on contrast-enhanced CT. Angiography shows a cluster of medullary veins converging on a large central draining vein (Fig. 6). Obstructive Hydrocephalus The rapid development of hydrocephalus may also cause acute headache. This may be due to a lesion that obstructs cerebrospinal fluid pathways, such as a colloid cyst in the third ventricle. A mass in the pineal region or posterior fossa may obstruct the aqueduct or fourth ventricle, respectively. Congenital causes such as aqueducted stenosis or insufficiency may also suddenly cause symptomatic hydrocephalus (Fig. 7). Computed tomography is adequate to diagnose hydrocephalus and to suggest the appropriate diagnosis. Magnetic resonance imaging is frequently obtained later for further characterization when the patient is stable. CHRONIC HEADACHE Migraine The most frequently encountered headaches in clinical practice are due to either migraine or muscle-contraction headache. In migraine, an early CT study showed an increased incidence of atrophy in 58% of a test group of 53 patients. 2 More recent MRI studies lO show incidence of atrophy of 35% and increased signal foci on long TR images in the white matter (Fig. 8), known as white-matter foci (WMF). These WMF are separate from the ventricles and are brighter than cerebrospinal fluid on the intermediate and long TR images. In a study by Prager et at'2 involving 100 patients with severe chronic headache, 77 patients had migraine. In 40 patients less than 41 years of age, 25% had WMF. In 34 patients who were 41 to 60 years of age, 68% had WMF. All three patients older than 60 years of age had WMF. A study involving 74 patients with classical migraine found WMF in 26% of patients from age 9 to 39.10 The incidence of WMF is well documented in migraine, 10. 12. 15 but some questions remain regarding the incidence of WMF in the normal population less than 60 years old. The only documentation of this incidence comes indirectly from studies that have a control population directed towards other disease processes with WMF. One such study was recently completed by Bowen et al,3 who examined 50 normal patients with an age range of 31 to 51 and found WMF similar to those seen in the migraine studies in 6% of the patients. An explanation for the presence of WMF and atrophy is not immediately apparent. In the study by Prager et al,12 a history of vasoactive medication was also examined, and it was found that approximately 50% of these patients had WMF. There is no pathologic literature that looks specifically at the lesions of migraine patients with WMF. Kirkpatrick and Hayman 9 studied 12 patients pathologically for correlation with WMF in patients 52 to 72 years of age. They found eight
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Figure 5. Large cavernous hemangioma. A, Contrast-enhanced computed tomography shows nonspecific enhancing mass in right thalamus with surrounding decreased attenuation and mass effect on the third ventricle. B, T} magnetic resonance image reveals a mass in the thalamus that contains mottled high and medium signal intensities with a dark rim. First-echo long TR (C) and T2 magnetic resonance images (D) show the magnetic susceptibility effect of the rim, which represents ferritin and hemosiderin. This appearance is characteristic of cavernous hemangioma.
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patients with zones of atrophic perivascular demyelination. This was believed to be due to mild vascular insufficiency causing perivascular fibrous gliosis. Perhaps the process that leads to the appearance of WMF in normal individuals over 60 years of age4-6 is occurring at an accelerated rate in the migraine patient because of the inherent vascular instability seen in these patients. The most serious complication of migraine is stroke. Acute infarction is seen earlier with MRI than with eT, and it may be demonstrated as early as 6 hours after the onset of symptoms. Finally, there have been reports of transient contrast enhancement on eT during migraine with complete resolution on follow-up studies. " 11 In such a case, hyperemia or focal damage to the blood-brain barrier from vascular, inflammatory, or neoplastic etiology may be considered. Magnetic resonance imaging should be sufficient in most cases to rule out the possibilities that need immediate attention. Trigeminal Neuropathy Pain related to the trigeminal nerve, with or without other symptoms, is a common clinical problem. Localization along the trigeminal nerve has classically been deduced from symptoms and signs. However, a recent paper by Hutchins et aF compared clinical localization with MRI and eT. They found clinical localization to be inaccurate and MRI to be most sensitive. Seventy-six patients were studied, and other than the combination of pain and numbness usually seen with peripheral lesions, and trismus seen with lesions of the masticator space, clinical characteristics were not useful for localization. Because of this lack of clinical specificity, the peripheral and central portions of the nerve must be examined in a patient with trigeminal neuropathy. Twenty of Hutchins' patients had tic douloureaux (trigeminal neuralgia or tic), which was studied by MRI. Three had positive pathologic findings; the diagnoses were multiple sclerosis, arteriovenous malformation, and maxillary sinusitis. Two patients had vascular compression. In a separate study, Hutchins et alB reviewed the MRI studies of 29 patients with tic douloureaux and 30 patients with unrelated disorders, looking specifically for vascular contact with the trigeminal nerve. Fiftyseven per cent of symptomatic patients without multiple sclerosis had vascular contact at the root-entry zone. In the 30 symptomatic patients, there was a 27% incidence of vascular contact. The study found no correlation between the degree of vessel contact and symptoms; therefore, actual nerve compression was not a sensitive indicator for vascular decompression. Magnetic resonance imaging was suggested for patients with atypical tic or tic not responsive to standard therapy to rule out masses, multiple sclerosis (15% in the series), or gross vascular abnormality (Fig. 9). Temporomandibular Joint Dysfunction Temporomandibular joint dysfunction may present with varied symptoms, including pain in the temporomandibular joint region or in the face, neck, or head. There may be jaw clicking or abnormal movements of the jaw. The first radiologiC procedure is often standard polytomography to detect osseous cortical abnormalities, fracture, changes with the mouth open, and the relationship of the mandible to the skull base. Magnetic resonance imaging may also be the first examination or may follow eT if the findings are not diagnostic or the patient does not respond to therapy. An early study'6 comparing eT and MRI found MRI superior for viewing the soft tissues. Within a very short time, eT has been relegated to an adjunctive role to view the bony compartments in the region. '4 According to Rao et al,'3 on sagittal MRI, a posterior band of the temporomandibular joint disk at the apex of the condylar head constitutes normal disk. Forward deviation of the posterior band relative to the apex of the condylar head indicates anterior displacement of the disk
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Figure 6. Venous angioma with hemorrhage. A and B, Noncontrast and contrast-enhanced computed tomography images show high-attenuation blood in right frontal region with some moderate enhancement in B. T j (C), first-echo long TR (D), and T, (E) magnetic resonance images show bright central regions surrounded by a magnetic susceptibility signal void that represent subacute, acute, and chronic hemorrhage. F and G, Angiogram in venous phase shows typical caput medusae appearance of radiating medullary veins (arrows) with central large vein (arrow) extending to cortex.
(Illustration continued on opposite page)
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Figure 6 (Continued).
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Figure 7. Aqueductal stenosis. A and B, Sagittal and axial T J magnetic resonance images show large lateral and third ventricles. The aqueduct is small, and the fourth ventricle is normal. Note the large infundibular recess of the third ventricle on both images (arrow).
(Fig. 10). Further classification is based on any change with the mouth open. To make other diagnoses in the area, such as perforation or adhesions, other modalities, including arthrography or an arthroscopy, may be necessary. Degenerative joint disease may be seen with all of the aforementioned modalities as well as CT. Rao et al conclude that, in most cases, the findings on MRI are sufficient to decide between conservative or surgical therapy. Craniocerebral Trauma A history of trauma is important in the patient with chronic headache. Computed tomography or MRI may show nonspecific tissue loss; CT will better show bony changes. In addition to these changes, MRI may also show more specific change related to old hemorrhagic shear injury. On follow-up studies, these patients may show magnetic susceptibility on the long TR images. Magnetic susceptibility due to iron deposition in the tissues is seen as decreased intensity on the T2 weighted images (Fig. 11). Shear injury is particularly well seen at the gray-white junction and in the deep hemispheric white matter. Other manifestations of old trauma are white matter foci, bright on long TR images from gliosis. Chronic subdural hematoma may frequently present as headache. In the elderly, no history of trauma may be given, despite the presence of a prominent collection. A subacute subdural hematoma, approximately 1 to 4 weeks in duration, may be isodense on CT. On MRI, this is the time when a subdural hematoma is most obvious, owing to the presence of methemoglobin, which appears bright on both Tc and T 2 -weighted images. The chronic subdural hematoma is well seen on both CT and MRI, but there is greater characterization on MRI. Neoplastic, Inflammatory, Congenital, and Idiopathic Causes of Headache Central nervous system neoplasm is another obvious source of headache, and it is the sole presenting complaint in about one third of these cases. In general, the
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Figure 8. Migraine in a 44-year-old patient. First-echo long TR (A) and T, (B) magnetic resonance images. White-matter foci are seen bilaterally (arrows). The white-matter foci appear similar to those seen in multiple sclerosis, but the subcortical location without periventricular lesions makes multiple sclerosis less likely. C and D, Magnetic resonance image in coronal plane in a different patient shows typical location and appearance of whitematter foci (arrows).
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Figure 9. Trigeminal neuralgia in a patient with multiple sclerosis. First-echo long TR (A) and T 2 (B) magnetic resonance images show multiple bright foci that are confluent in areas in the pons and cerebellum (arrows). First-echo long TR (C) and T2 (D) magnetic resonance images in the same patient at the superior aspect of the ventricles show multiple bright foci typical of multiple sclerosis. These lesions are typically oval-shaped and oriented perpendicular to the ventricles in the white matter.
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Figure 10. Temporomandibular joint dysfunction. A, T,-weighted sagittal magnetic resonance image shows bow-tie-shaped disk. The posterior aspect of the disk (arrow) is in normal position directly above the mandibular condyle. B, T,weighted sagittal magnetic resonance image shows marked anterior displacement of the disk (arrow). (Courtesy of Louis Kircos, MD, University of Chicago, Chicago, Illinois.)
mass must be of considerable size to provide traction on or to displace a large blood vessel and produce pain. Frequently, by this time, other symptoms and signs, in addition to a chronic progressive headache, will suggest the correct diagnosis. Magnetic resonance imaging is superior for tumor detection and characterization, but eT is sufficient to rule out lesions that demand immediate therapy. Inflammatory causes of headache include infectious and idiopathic etiologies. In cases of acute and chronic meningitis and encephalitis, MRI is superior to eT for detection of abnormality. Gadolinium enhancement of the meninges is well seen over the convexities or at the base of the brain in meningitis. In encephalitis, minimal increase in brain water is evident on the T2-weighted images. In sinusitis, MRI and eT play complementary roles. Magnetic resonance imaging is superior for characterizing the extent of soft-tissue involvement; eT is superior for evaluation of pony changes. Temporal arteritis may be a very elusive diagnosis. In the appropriate clinical
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Figure 11. Shear injury. Gradient echo sequence sensitive for magnetic susceptibility. Numerous signal-void lesions (black areas) are seen scattered in the cortex, gray-white junction, and periventricular white matter. These areas represent ferritin and hemosiderin at the sites of old shear injury.
setting, the sedimentation rate may be normal and biopsy may miss the pathologic area. Angiography may be helpful and show typical changes of arteritis, which include irregular regions of narrowing and dilatation in the superficial temporal artery or other neighboring vessels. Sinus thrombosis may develop contiguous to areas of infection, such as the transverse sinus from mastoiditis or cavernous sinus from sphenoid sinus inflammation. Magnetic resonance imaging or CT may be suggestive, and angiography may be necessary for diagnosis. Congenital causes for headache include abnormalities at the skull base and posterior fossa. Examples include the Chiari malformation and the Dandy-Walker syndrome and variant. Magnetic resonance imaging provides excellent demonstration of these abnormalities. SUMMARY The patient who presents with a severe and acute headache should be evaluated radiographically with CT. The key diagnosis to make in this situation is hemorrhage, either subarachnoid or intraparenchymal. Computed tomography is more sensitive to acute hemorrhage than is MRI. When the patient is stable, MRI frequently contributes information to narrow the diagnostic possibilities, because vascular malformations and certain parenchymallesions have a characteristic appearance on MRI. Hydrocephalus may also present acutely and is easily seen on CT or MRI. In a patient with chronic headaches, MRI is generally most informative. The migraine patient may show WMF and atrophy. The patient with trigeminal neuropathy may demonstrate central or peripheral lesions. In temporomandibular joint dysfunction, conventional tomography and MRI are frequently used. Magnetic resonance imaging shows excellent detail of the disk and surrounding soft tissues, whereas tomography better demonstrates bony changes. When a history of trauma is present, MRI may show a subacute subdural hematoma. These collections are easily seen on MRI, even when isodense on CT. Evidence of old shear injury is also well seen on MRI. Finally, neoplastic, inflammatory, congenital, and idiopathic sources of head-
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Figure 12. Subacute subdural hematoma. A, Computed tomography shows bilateral isodense predominantly temporal, nonspecific abnormalities. There is no midline shift to suggest mass effect. T, coronal (B), first-echo long TR (C), and T, (D) magnetic resonance images show variable, predominantly high-intensity bilateral extra-axial collection on all sequences, which is consistent with chronic subdural hematoma with episodes of rebleeding. There are several compartments. The darker components seen on the long TR images are due to deoxyhemoglobin and cellular debris (arrows) from recent bleeding episodes. The bright signal is due to methemoglobin. When there is continued oozing, the bright signal from methemoglobin may be seen for months. (Courtesy ofWilliam A. Wagle, MD, Albany Medical College, Albany, New York.)
ache may be demonstrated by either MRI or eT, depending on presentation. MRI will generally show superior characterization. APPENDIX
Introduction to Magnetic Resonance Imaging The MRI scanner employs a strong magnetic field and radiowaves (radiofrequency pulses) to "excite" protons and cause them to "resonate." The resonating
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protons dissipate this excess energy ("relax") over time and eventually return to their pre-excitation state. During this relaxation process, the protons themselves emit radiowaves that are detected and "read," yielding information regarding proton location, density, movement, and rate of relaxation. The MRI scanner receives the radiowave signal and is able to produce cross-sectional images of the body displaying these variations of proton behavior in tissues. The extraordinary images obtained with MRI are the result of greater contrast resolution compared to CT. This translates into greater differences in image brightness between adjacent tissues. These images are generated via measurement of the two primary forms of proton relaxation-Tj and T 2-processes that occur simultaneously. T j Relaxation: T j relaxation (simply called T j) is the rate at which excited protons return to the equilibrium or relaxed state following excitation by a radiofrequency pulse. T2 Relaxation: Immediately after the application of a radiofrequency pulse, all the protons are resonating "in phase" with each other. The protons soon lose this phase coherence because of slight differences in the resonant frequency of each proton due to the local magnetic field variations present in all tissues. T2 relaxation (or T 2) therefore represents the rate of loss of phase coherence between the resonating protons. The MR image is therefore a map of signal intensities in which the signal intensity is dependent on T j, T 2, and proton density (density of protons within the tissue being imaged). In practice, one can manipulate these Y1RI parameters to emphasize T j relaxation (Tj-weighted image), T2 relaxation (T2 -weighted image), or the overall density of protons in the tissue (proton-density image). A short TR (time between radiofrequence pulses) favors T j, and a long TR favors T z . With a longer TR, two radiowaves may be sampled. The first echo produces the proton-density image, and the second echo produces the Tz-weighted image. For the purposes of image interpretation, cerebrospinal fluid signal intensity can be used to determine whether an image is T,- or Tz-weighted: Cerebrospinal fluid is dark on T, (only air, bone, and blood flow are darker) and bright on T z. One of the most important concepts to understand regarding image analysis is that most lesions have the same signal characteristics as cerebrospinal fluid-dark (low signal) on T j and bright (high signal) on T z• Another indispensable concept is that there are only a few entities that have high signal on Tj-weighted sequences. Fat, flow (can actually have low, high, or intermediate signal), methemoglobin (a breakdown product of blood in a hemorrhagic lesion), gadolinium (an injectable MRI contrast agent), and fluid containing appreciable amounts of protein are bright on T,. Complex signal patterns are seen with stationary blood extravasated into tissues and with flowing fluids. Detection of Hemorrhage The appearance of extravasated blood on MRI is quite complex and depends on tissue oxygenation, age of hemorrhage, integrity of red blood cell membrane, and the presence of hemoglobin breakdown products. In the acute phase, a parenchymal hematoma can have signal characteristics very similar to that of normal brain. The diagnosis of acute hemorrhage, including subarachnoid hemorrhage, is difficult to make by MRI; CT maintains superiority in this area. Subacute blood appears bright on T, and has a variable signal on T2 depending on whether intact (hemorrhage appears dark) or lysed (hemorrhage appears bright) red blood cells are present. The blood products in chronic hemorrhage, i.e., hemosiderin and ferritin, which are frequently seen in old contusions and shear injury, demonstrate significantly reduced signal characteristics owing to "magnetic susceptibility" effects. Iron in the tissues causes disorganization of magnetic fields, which results in loss of
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signal. This magnetic susceptibility effect is much more apparent on T 2 -weighted imagcs. Limitations of Magnetic Resonance Imaging The advantages of \iRI are obvious in terms of lesion detection and anatomic characterization. Furthermore, no ionizing radiation is involved. The only major limitations include detection of hyperacute hemorrhage, detection of calcium (calcium returns no signal; therefore, dystrophic calcifications seen in various tumors can be missed), longer scan times compared to eT, and difficulty in monitoring sick patients (monitors or life-support devices may not function in the presence of strong magnetic fields). There arc contraindications to MRI including ferromagnetic aneurysm clips (new clips arc no longer ferromagnetic), cardiac pacemakers, certain cochlear implants, permanent neurostimulators (TENS units), known ocular metallic foreign bodies, and certain prosthetic heart valves and metallic foreign bodies if they are present near sensitive structures (e.g., the eye). The advantages and disadvantages of MRI are usually weighed carefully by the clinician and the radiologist when they are selecting the appropriate imaging study. ACKNOWLEDGMENT The authors wish to thank Dr. Ruth Ramsey for her review of the manuscript.
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15. Soges LJ, Cacayorin EO, Petro CR, et al: Migraine: Evaluation by MR. AJNR 9:425429, 1988 16. Westesson P, Katzberg RW, Tallents RH, et al: CT and MR of the temporomandibular joint: Comparison with autopsy specimens. AJR 148:1165-1171, 1987
Address reprint requests to Jordan M. Prager, MD Department of Radiology University of Chicago Box 429 5841 S. Maryland Avenue Chicago, IL 60637