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NeuroAids Vol. 2, Issue 7 (August 1999)        


Imaging of Neuroaids

K. E. Sakaie1 and R. G. Gonzalez1

1 Massachusetts General Hospital, NMR Center, Bldg. 149
13th Street, Charlestown, MA 02129, USA
E-mail: kens@nmr.mgh.harvard.edu , rggonzalez@partners.org

Keywords: AIDS, HIV, MRS, CT, MR spectroscopy, MRI, SPECT, imaging, AIDS dementia complex.

Authors Correction: The authors have requested that Figure 1 of the original manuscript published on 8/27/99 be moved to its current location (Figure 8). This correction took effect on 8/30/99.--
The Editors

 


Abstract
Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

Up to 90% of those infected by the human immunodeficiency virus will have central nervous system (CNS) involvement (1). This may include a subclinical CNS HIV infection, subtle motor cognitive deficits, or a progressive, severe dementia. The patient who is immunocompromised by the virus may also have CNS infestation by parasites, fungi, uncommon bacteria, other viruses, and neoplasms uncommon in the immunocompetent population. CNS injury by HIV and its complications produce neuropathological, physiologic, and metabolic abnormalities that are detectable noninvasively by modern neuroimaging methods. Modern structural imaging involving CT and MRI plays a critical role in the clinical evaluation and treatment of HIV+ patients with new onset neurological symptoms. The advanced functional and metabolic imaging probes - Magnetic Resonance Spectroscopy (MRS), dynamic contrast functional MRI (fMRI), and Single Photon Emission Computed Tomography (SPECT) - may contribute diagnostic specificity to structural findings and are providing insight into the pathobiology of HIV related dementia. For discussion of Positron Emission Tomography (PET), another advanced probe, the reader is referred to the literature (2). Here we review the effects of HIV on the adult brain as revealed by clinical and advanced neuroimaging.

Computed Tomography & Magnetic Resonance Imaging
Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

Structural neuroimaging with magnetic resonance imaging (MRI) and computed tomography (CT) of HIV encephalopathy is usually normal until later stages of the disease.
It has been established that CNS infection by HIV may occur concurrently or shortly after acute systemic HIV infection. During this acute stage, structural neuroimaging studies are usually normal even if the infected individual has clinical signs of CNS involvement, such as acute meningoencephalitis accompanied by abnormal quantities of immune response factors in the cerebral spinal fluid (CSF). Rarely, patients who present a severe encephalopathy will demonstrate multifocal abnormalities as seen in T2 weighted MRI sequences. The abnormalities may be reversed with antiretroviral therapy (3). Some individuals may remain neurologically asymptomatic for years while a few progress to AIDS dementia within months after infection, developing severe neurologic complications.

HIV infection of the CNS produces a progressive cognitive-motor disorder often called the AIDS Dementia Complex (ADC). Damage to neural tissues occurs mainly in the subcortical gray matter, especially the basal ganglia, which is thought to produce a progressive subcortical dementia that may be detected in early stages by neuropsychological testing. Mononuclear cell infiltrates, multinucleated giant cells, reactive astrocytosis, abnormalities of the white matter and vacuolar changes in the infected tissues are observed histopathologically. In the early stages of CNS HIV infection, MR and CT imaging studies do not usually demonstrate brain abnormalities.

As the HIV-induced brain injuries progress, cortical atrophy ensues, and white matter disease due to HIV oligodendroglial damage occurs. MR and/or CT detect these changes as atrophy and white matter disease. HIV leukoencephalopathy tends to involve the periventricular white matter and the centrum semiovale. On T2 weighted MRI sequences, the affected white matter displays increased signal (Figure 1). There is no associated mass effect and no contrast enhancement (4)(5).

Figure 1

Figure 1 (Enlarge): MRI of a patient with AIDS dementia. T2 weighted image at the level of the lateral ventricles displays a diffuse white matter abnormality consisting of increased signal. The cortical sulci are prominent given the age of the subject, indicating atrophy.
Magnetic Resonance Spectroscopy
Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

Magnetic resonance spectroscopy (MRS) demonstrates neurochemical abnormalities early in the course of CNS HIV infection.
Long one of the essential assays in analytic chemistry, MRS is emerging as a powerful tool for noninvasive study of in vivo pathology. MRS can be performed with a clinical MRI scanner, yielding quantitative measurements of metabolite concentrations in 5 to 10 minutes. Such measurements provide biochemical information not available from analyses of blood, urine, and cerebrospinal fluid. MRS also augments structural measurements (MRI, CT) and neuropsychological tests.

In MRS, one detects a signal resulting from the interaction between atomic nuclei and the local magnetic field. The focus here is on signals from protons in hydrogen, although a number of other nuclei, notably in carbon and phosphorus, are studied in vivo. Each type of molecule has different magnetic properties, allowing one to distinguish signals from protons within different types of molecules. For example, figure 2 depicts brain spectra with distinct peaks from neurochemicals commonly studied by MRS: myoinositol (myo), choline (Cho), creatine (Cr), and N-acetyl aspartate (NAA). The intensity of each peak indicates the concentration of the molecule. The spectra show quantitative differences between HIV-positive and HIV-negative subjects. Other molecules of interest include lactate (LAC), glutamate (GLU), glutamine (GLN), and lipids. The figure also shows that MRS can localize the signal, allowing comparison of metabolite changes among different tissue types. ADC patients display significant differences in metabolite changes among deep gray, white, and cortical gray matter.

Figure 2

Figure 2 (Enlarge): Characteristic 1H MR spectra from the white matter region of a healthy and an HIV+ subject suffering from AIDS dementia complex (ADC). The spectra are normalized using creatine (Cr) as an internal reference. There is a significant increase in myoinositol (myo) and choline (Cho) with a decrease in N-acetyl aspartate (NAA). The basal ganglia exhibit similar trends, while the cortical gray matter does not show significant changes.
Trends in metabolite concentration reflect the cellular processes underlying injury. NAA, found in neurons and axons (6), is used as a neuronal marker (5). A reduction in the NAA signal reflects neuronal loss or injury, as seen in many brain pathologies including neurodegenerative diseases such as ADC (7)(8)(9)(10)(11)(12). The correlation between NAA signal reduction and neuronal loss or injury is supported by quantitative postmortem neuropathology studies (13)(14).

The Cho peak comprises signal from several compounds: phosphocholine, glycerophosphocholine, and, to a lesser degree, free choline. As astrocytes contain more Cho than neurons, astrogliosis can increase the levels of Cho with respect to NAA. Pathologic disturbances in cell membrane turnover may also lead to an elevation of Cho levels. Elevated Cho is found throughout the brain in advanced ADC (7)(8)(10)-- possibly reflecting astrogliosis found histologically.

Trends in metabolite levels measured with MRS correlate with and can exceed the sensitivity of other indicators (15). Changes in the NAA signal indicate neuronal injury even when MRI, CT, and psychological exams appear normal (16)(17). Cross sectional (Figure 3) (18) and longitudinal (Figure 4) (5) studies of HIV+ patients with cognitive abnormalities find progressive decreases in NAA and increases in Cho and myo as dementia worsens.

Figure 3

Figure 3 (Enlarge): Characteristic 1H MR spectra from the right frontal white matter region of a healthy control subject and HIV-cognitive motor complex (HIV-CMC) patients with minor cognitive motor disorder (AIDS dementia complex (ADC) stage 0.5), mild dementia (ADC stage 1), and moderate dementia (ADC stage 2). Progressive increases in creatine (Cr), choline-containing compounds (Cho), and myoinositol (MI) are observed in the HIV-CMC patients with increasing severity of dementia, whereas the concentration of N-acetyl compounds (NA) remain normal until moderate dementia (from (18))

Figure 4

Figure 4 (Enlarge): Serial brain MR spectra (a-c) of an HIV seropositive patient through an interval of 10 months. The NAA/Cr ratios steadily decreased through the study period. This finding suggests decreasing cerebral neuronal mass. It correlates with the progressive cognitive decline in this patient who expired 3 weeks after the last MRS when he manifested ADC Stage 4. The serial brain MRIs showed no morphologic change through the study period (from (5)).

The drop in NAA also correlates with CD4 counts (Figure 5) (18) and the degree of encephalopathy (18)(19). Elevation in myo and Cho occurs earlier and to a greater extent in the basal ganglia and white matter than in the cortex (Figure 3), a result that agrees with the clinical finding that ADC is a subcortical dementia. Furthermore, myo and Cho elevation is found in HIV positive patients before the onset of ADC (9). Treatment with zidovudine reverses the trends among the metabolites, indicating that the cellular injury and gliosis is, to a certain extent, reversible (17)(20). MRS may therefore provide means for early diagnosis and evaluation of treatment.

Figure 5

Figure 5 (Enlarge): Relation between clinical measurements of CD4 count and on frontal white matter N-acetyl compounds (NAA) (from (18)).

Functional MRI
Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

Dynamic contrast functional MRI demonstrates abnormally increased cerebral blood volume in gray matter and multifocal cortical cerebral blood flow deficiencies.
The term functional MRI (fMRI) encompasses 2 general types of experiments. The first exploits regional hemodynamic changes accompanying brain activation. This method employs endogenous contrast incurred by hemodynamic changes and has been used to study basic brain functional activities such as vision, audition, speech, and memory. There are no reports of use of this method in the study of HIV related neurological disease. The other type, dynamic contrast fMRI, requires the use of an injectable contrast agent. The latter method has been used for the study of neuroAIDS.

Dynamic imaging after the first pass of a bolus-injection of intravascular paramagnetic contrast agents is possible on time scales comparable to vascular mean transit times. The passage of contrast agent through the cerebral capillary network is monitored by taking a series of T2* weighted images. The resultant signal intensity-time curves obtained from analysis of these images are converted to concentration-time curves. These curves may be used to calculate relative cerebral blood volume and flow (rCBV and rCBF).

Using dynamic contrast fMRI, Tracey et al. (21), found a generalized increase in rCBV in gray matter (more notably in deep gray) but not in white matter regions of HIV infected patients when compared to controls (Figure 7). An increase in rCBV apparently corresponds to worsening CDC (Centers for Disease Control) criteria and ADC stage, especially in the deep gray matter structures. It is not surprising that the differences in rCBV are more striking in the basal ganglia region in view of histopathologic involvement of this area in people infected with HIV. The results are also consistent with the PET findings of hypermetabolism of the deep gray matter (2). White matter generally exhibits pathologic abnormalities, but rCBV appears normal - a finding which may reflect a small change in rCBV of white matter undetectable by this method or that the underlying physiologic changes resulting in increased rCBV do not occur in white matter. In one interesting case, CBV images from an AIDS patient were obtained before and after 6 months of antiretroviral treatment (Figure 6). Along with significant clinical improvement (ADC 2 to ADC 0), this patient experienced a normalization of his CBV.

Figure 6

Figure 6 (Enlarge): Representative dynamic contrast functional MRI images of normalized cerebral blood volumes at the level of the basal ganglia. The scale goes from white to yellow to red, which represents low to moderate to high relative blood volume. (a) Dynamic cerebral blood volume (CBV) image from a healthy control subject (left) and an HIV+ patient (right) with moderate dementia (ADC Stage 2). Increase in the cortical and deep gray dynamic CBV in the patient with dementia is apparent. (b) Functional MRI images from the same patient obtained before (left) and after (right) 6 months of treatment with zidovudine. Improvement in dementia (ADC Stage 2 to 0) with treatment was accompanied by a reduction in the cortical and deep gray matter dynamic CBV (from (21).

Chang and her colleagues have recently reported a CBF analysis using dynamic contrast fMRI (22). They found that HIV+ subjects had areas of decreased CBF in the inferolateral frontal cortices and increased CBF in the posterior inferior white matter (22). The authors concluded that these findings were similar to those described by investigators employing SPECT.

Single Photon Emission Computed Tomography
Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

Perfusion deficits are observed in HIV encephalopathy with single photon emission computed tomography (SPECT).
SPECT measures brain perfusion using radiopharmeceuticals such as HMPAO Tc-99m-hexamethylpropyleneamine oxine (HMPAO) and Iodine-123-N-isopropyl-p-iodoamphetamine-hydroacetate (IMP). Imaging of gamma rays from radioactive decay can localize perfusion defects. Such defects can appear even when structural CT and MRI scans appear normal and hence provide early clues about ADC. SPECT also improves the accuracy of diagnosis, as differences in uptake patterns among radiopharmeceuticals can vary with the severity of AIDS encephalopathy (23).

Secondary Complications
Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

Unusual brain infections in the immunocompromised HIV patient are commonly observed with MRI and CT
Bacterial meningitis and pyogenic abscesses are infrequent in HIV+ patients with AIDS, except in those with a history of being IV drug abusers (24). The cellular immunodeficiency in HIV+ patients eventually causes a variety of CNS infections caused by opportunistic organisms such as Toxoplasma gondii (25) or viral agents such as cytomegalovirus (CMV), the JC virus (a papovavirus which causes progressive multifocal leukoencephalopathy), and herpes type I (26). Also found are fungal infections (cryptococcosis, aspergillosis, candidiasis) (27), mycobacterial infections (28), nocardiosis, and neurosyphilis (29). We will focus on two of the most clinically relevant infections, toxoplasmosis and progressive multifocal leukoencephalopathy.

Toxoplasma encephalitis is the most common cause of intracranial mass lesion in AIDS patients. In AIDS patients, toxoplasma encephalitis is a reactivation infection. The clinical presentation of toxoplasma encephalitis is nonspecific and indistinguishable from manifestations of other CNS infection processes or primary brain lymphoma (fever, headaches, focal neurological deficits, seizures, etc.). On CT and MRI, toxoplasma encephalitis may appear as a focal or multiple ring and/or nodular enhancing lesion(s) surrounded by variable degrees of vasogenic edema (Figure 7). The lesion(s) tend to be located superficially (at the cortico-medullary junction) and/or deeply located (basal ganglia, thalamus). Occasionally lesions may be encountered in the brainstem or cerebellum.

Figure 7

Figure 7 (Enlarge): MRI of AIDS patient with toxoplasmosis. T1 weighted image obtained after the administration of intravenous contrast demonstrates numerous enhancing lesions throughout the brain.

Progressive multifocal leukoencephalopathy (PML), caused by JC virus infection of oligodendrocytes, leads to demyelination. Once the diagnosis is made, the prognosis is ominous, with death usually occuring within 9 months to 1 year. The incidence of PML in the AIDS population has been estimated to be 1-4%. MR is the modality of choice for the evaluation of this entity. Characteristic MR findings include areas of decreased T1 and increased T2 signal affecting white matter, usually in asymmetric distribution, but with predilection for the parietal and occipital lobes (Figure 8). The white matter lesion often extends to the subcortical U fibers with characteristic scalloped margins. There is no associated mass effect; indeed, there is often evidence of tissue loss. Contrast enhancement is uncommon. On CT, areas of abnormal decreased white matter density without mass effect or enhancement may be seen. Cerebellum, brainstem, basal ganglia, thalamus, and external capsule involvement by PML, although infrequent, can be seen.

Figure 8

Figure 8 (Enlarge): MRI of progressive multifocal leukoencephalopathy. T2 weighted MRI at the level of the lateral ventricles reveals a subcortical, hyperintense signal abnormality in the right hemisphere. (As per imaging convention, the right hemisphere appears on the left side of the image). The lesion appears restricted to the white matter and does not exhibit mass effect. The lesion did not enhance with contrast administration. The appearance is most consistent with progressive multifocal leukoencephalopathy.

Primary central nervous system lymphoma often resembles brain infections on structural imaging
The most common malignant neoplasms associated with AIDS are (1) primary CNS lymphoma, which occurs in up to 6% of the neurological symptomatic AIDS patients, (2) metastatic lymphoma, which may result in leptomeningeal and epidural spread, and (3) Kaposi sarcoma, which rarely affects the CNS. In AIDS patients, primary CNS lymphoma is the second most common cause of a focal CNS mass lesion after toxoplasma encephalitis. Clinically, it is difficult to accurately differentiate between primary brain lymphoma and other intracranial masses, particularly toxoplasma encephalitis. On CT and MR, lymphoma may appear as focal ring and/or nodular enhancing mass lesion(s) with surrounding edema (Figure 9). The lesions may be single or multiple, superficial (cortico-medullary junction) and/or deep (basal ganglia, thalamus, corpus callosum). The posterior fossa may also be involved (cerebellum, pons, midbrain). Rapid progression of the mass(es) on serial imaging studies also favor lymphoma. However, CT and MR findings are usually not pathognomonic, and lymphoma cannot be differentiated with certainty from toxoplasma encephalitis or other infectious process. In AIDS patients, primary CNS lymphoma resembles toxoplasmosis in 50-80% of the cases.

Figure 9

Figure 9 (Enlarge): MRI of AIDS patient with primary CNS lymphoma. T1 weighted image obtained after the administration of intravenous contrast demonstrates a ring-enhancing lesion in the medial left temporal lobe.

Functional MRI and SPECT may aid in differentiating lymphoma from toxoplasmosis
Employing dynamic contrast fMRI, Ernst and colleagues found increased rCBV in regions with active lymphoma (Figure 10) but reduced rCBV in the central regions of toxoplasmosis lesions (Figure 11). rCBV is reduced in the edema surrounding lesions of both types (30). The rise within active lymphoma probably results from hypervascularity within the tumor, and the drop within toxoplasmosis lesions probably reflects lack of vasculature. The drop within the edema probably reflects vasoconstriction with increased interstitial pressure.

Figure 10

Figure 10 (Enlarge): Primary brain lymphoma lesion (arrows) in a 50-year old man with AIDS. Left: rCBV map from perfusion MR imaging. Right: Gadolinium-enhanced T1-weighted image. The lymphoma lesion shows areas of greatly increased rCBV (arrows, left image). In this case, the maximum rCBV was 270% of that of the contralateral brain region.

Figure 11

Figure 11 (Enlarge): Toxoplasmosis lesion (arrows) in a 63-year old man with AIDS. Left: rCBV map from perfusion MR imaging. Right: Gadolinium-enhanced T1-weighted MR image. In contrast to figure 9, the rCBV is consistently reduced in the central regions and in the enhancing rim of the toxoplasmosis lesion, as well as in the surrounding edema. In this case, the maximum rCBV was 94% of that in the contralateral region (from (30)).

Different uptake patterns of radiopharmeceuticals as detected by SPECT may help differentiate pathologies. For example, Tl-201 can accurately distinguish primary cerebral lymphoma from infection processes, particularly cerebral toxoplasma encephalitis (31)(32)(33)(34). Uptake is due to an active cell membrane pump in the active, growing neoplastic tissue. Active lymphoma is considered when the intracranial Tl-201 SPECT activity is greater than the activity of the contralateral scalp (Figure 12). Non-lymphoma lesions do not display increased activity.

Figure 12

Figure 12 (Enlarge): Thallium SPECT imaging of primary CNS lymphoma. In this patient with AIDS, the contrast enhanced CT scan (left image) reveals a ring enhancing lesion with extensive surrounding edema. The appearance is consistent with toxoplasma abscess or CNS lymphoma. The thallium SPECT scan (right image) demonstrates intense uptake of radiotracer by the lesion, indicating that lymphoma. This diagnosis was subsequently confirmed by biopsy. (Images courtesy of A. Ruiz and J. Donovan-Post, U. Miami).


Acknowledgements

The authors thank P.L. Lee (MGH) for providing figure 3, L. Chang (UCLA) for use of figures 4 and 6, H.E. Gendelman (U. Nebraska) and Arnold Publishers for figure 5, T. Ernst (UCLA) for figures 10 and 11, and A. Ruiz and J. Donovan-Post (U. Miami) for providing figure 12.

Symbol Definition

T2, T2*, and T1 are parameters which influence the size of the MRI signal. The values of these parameters, which depend on the rate and intensity of fluctuations in magnetic fields at the atomic level, often differ among tissue types. For example, lipids have a shorter T2 than cerebrospinal fluid. MRI takes advantage of these differences to enhance the image contrast among tissues.

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Abstract CT & MRI MRS fMRI SPECT Secondary Complications References

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