Advances in MR Imaging of Leukodystrophies

Leukodystrophies are hereditary disorders of white matter that impair brain that is initially normally formed and developed (1, 2). They can affect brain myelin throughout life. The disorders are commonly progressive in nature and ultimately fatal. First manifestations are often cognitive deterioration and neuropsychological problems. Motor and balance difficulties occur, as do visual abnormalities. Classic leukodystrophies lead to a vegetative state or death within months to years. In general, the earlier the onset of symptoms, the more progressive the disease course. Most leukodystrophies are monogenetic disorders. The mutant gene often encodes an enzyme or protein that maintains neuronal and/or glial health and is responsible for regulation of brain metabolism. Many enzymes involved play a role in lipid metabolism. The inability to degrade or synthesize substrate leads to an upstream excess or downstream lack of vital lipids. This can cause a wide range of pathology, from inflammatory demyelination to axonal degeneration and microglial activation. Hence, the often cited prominent demyelination is only one manifestation in leukodystrophies, and myelinforming oligodendrocytes are not the only cells affected in these disorders. Yet, there are some common characteristics that distinguish the pathology in leukodystrophies from that of other disorders. MRI has played a seminal role in visualization of the lesion pattern. (3). The demyelinating lesions are usually confluent and symmetric. Many of the classic disorders, such as X-linked adrenoleukodystrophy (X-ALD), metachromatic leukodystrophy (MLD) and Krabbe, show relative sparing of subcortical fibers. Yet again, other disorders have early involvement of the U fibers and other unique characteristics, such as cystic rarefaction and degeneration. Hereditary disorders that do not show demyelination but rather hypomyelination have come to increasing attention (4). Common neurological features in hypomyelinating disorders are developmental delay, nystagmus, cerebellar ataxia and spasticity. One of the better characterized hypomyelinating disorders is Pelizaeus Merzbacher disease (PMD). Beyond this classic disorder, a multitude of other hypomyelinating disorders exist. Only about half of the patients with evidence of hypomyelination on MRI come to a definitive diagnosis. Yet, specific clinical and MRI features can be pathognomonic and lead to diagnosis.


Introduction
Leukodystrophies are hereditary disorders of white matter that impair brain that is initially normally formed and developed (1,2).They can affect brain myelin throughout life.The disorders are commonly progressive in nature and ultimately fatal.First manifestations are often cognitive deterioration and neuropsychological problems.Motor and balance difficulties occur, as do visual abnormalities.Classic leukodystrophies lead to a vegetative state or death within months to years.In general, the earlier the onset of symptoms, the more progressive the disease course.Most leukodystrophies are monogenetic disorders.The mutant gene often encodes an enzyme or protein that maintains neuronal and/or glial health and is responsible for regulation of brain metabolism.Many enzymes involved play a role in lipid metabolism.The inability to degrade or synthesize substrate leads to an upstream excess or downstream lack of vital lipids.This can cause a wide range of pathology, from inflammatory demyelination to axonal degeneration and microglial activation.Hence, the often cited prominent demyelination is only one manifestation in leukodystrophies, and myelinforming oligodendrocytes are not the only cells affected in these disorders.Yet, there are some common characteristics that distinguish the pathology in leukodystrophies from that of other disorders.MRI has played a seminal role in visualization of the lesion pattern.(3).The demyelinating lesions are usually confluent and symmetric.Many of the classic disorders, such as X-linked adrenoleukodystrophy (X-ALD), metachromatic leukodystrophy (MLD) and Krabbe, show relative sparing of subcortical fibers.Yet again, other disorders have early involvement of the U fibers and other unique characteristics, such as cystic rarefaction and degeneration.Hereditary disorders that do not show demyelination but rather hypomyelination have come to increasing attention (4).Common neurological features in hypomyelinating disorders are developmental delay, nystagmus, cerebellar ataxia and spasticity.One of the better characterized hypomyelinating disorders is Pelizaeus Merzbacher disease (PMD).Beyond this classic disorder, a multitude of other hypomyelinating disorders exist.Only about half of the patients with evidence of hypomyelination on MRI come to a definitive diagnosis.Yet, specific clinical and MRI features can be pathognomonic and lead to diagnosis.degeneration suggested by the MRI.Regions of relative sparing include the U-fibers, corpus callosum, internal capsule, and the anterior commissure.The cerebellar white matter and brainstem show variable degrees of involvement but do not undergo cystic degeneration.

Fig. 1. Lesion Patterns on Conventional MRI in Leukodystrophies and Hypomyelinating Disorders
Brainstem lesion patterns can also often provide critical clues to the diagnosis.Leukoencephalopathy with Brainstem and Spinal Cord Involvement and Elevated White Matter Lactate (LBSL) has recently been described and shows distinct involvement of the pyramidal tracts, medial lemniscus, mesencephalic trigeminal tracts, corticobulbar tracts, and superior cerebellar peduncles.(22).Recognition of the distinct lesion pattern led to the identification of the responsible gene DARS2, which encodes mitochondrial aspartyl-tRNA synthetase.In hypomyelinating disorders, the boundaries between gray and white matter often appear "blurred" (4).The T2 hypointensity of the white matter is milder in hypomyelination than in demyelination and other white matter pathology.Overall, the brain MRI in hypomyelination looks like that of a young child, with less well distinguished gray and white matter.As opposed to "delayed myelination," the pattern on brain MRI is unchanged, as in myelination is "stuck" on two MRIs 6-12 months apart in a child older than one year of age.While at first glance, hypomyelinating disorders may all have a similar MRI appearance, there are a group of disorders for which the MRI scan can provide clues to the diagnosis (see Table 2).In these individual disorders attention to the deep gray matter structures will often reveal a characteristic "signature" (4).Advanced magnetic resonance (MR) imaging techniques, such as proton MR spectroscopic and diffusion tensor (DT) MR imaging, permit the investigation of changes in metabolite levels and water diffusion parameters in leukodystrophy patients.Both metabolite measures and water diffusion parameters offer an opportunity to assess the degree of axonal loss and demyelination in the leukodystrophies.

Proton MR spectroscopy
Magnetic resonance spectroscopy (MRS) offers the unique ability to measure metabolite levels in vivo in a non-invasive manner (23,24,25).These metabolite quantifications can be used to identify disease, measure the severity of an injury, or monitor a patient's response to treatment.Table 3 shows the most well characterized metabolite abnormalities detected by MRS in leukodystrophies.The resonances seen in the brain by MRS are typically low weight molecules (see Figure 2).In the normal brain, the most prominent peak arises from N-acetylaspartate (NAA) at 2.0 ppm.The other major peaks include creatine (Cr) and phosphocreatine (phospho-Cr), which are observed at 3.0 and 3.3 ppm, respectively, as well as choline containing compounds.1H MR spectra acquired with short echo times are characterized by additional resonances from myo-inositol (MI) at 3.5 ppm, and glutamate and glutamine, which overlap with each other so that they are often referred to as Glx, at ~2.5 ppm.Under normal conditions, the lactate concentration is very low in the adult brain.This resonance (observed as a doublet) occurs at 1.32 ppm.NAA within the adult brain is found exclusively in neurons, serving as a marker of neuronal density and viability and reported to be decreased in a number of neurological disorders.

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NAA is the source of acetyl in myelin membrane biosynthesis (26) and is coupled to lipid metabolism and energy generation (27).Creatine serves as a marker for energy-dependent systems in cells and it tends to be low in processes that have low metabolism, such as necrosis and infarction.As Cr and phospho-Cr are in equilibrium, the Cr peak is thought to remain stable in size, despite bioenergetic abnormalities that occur with multiple pathologies.Consequently, the Cr resonance is often used as an internal standard.
The choline (Cho) resonance arises from signals of several soluble components that resonate at 3.2 ppm.This resonance contains contributions primarily from glycerophosphocholine (GPC), phosphocholine (PCho), and Cho.Changes in this resonance are commonly seen with diseases that have alterations in membrane turnover and in inflammatory and gliotic processes (28,29).The function of MI is not fully understood, although it is believed to be an essential requirement for cell growth, an osmolyte, and a storage form for glucose (30).MI is primarily located in glia, and an increase in MI is commonly thought to be a marker of gliosis (31).Lactate is produced by anaerobic metabolism, and increased lactate has been found during hypoxia (32), mitochondrial diseases (33,34), seizures (35), and in the first hours after birth (36,37).
In the developing brain, Cho and MI are the dominant peaks in the MR spectrum.Their levels are high compared to Cr.In contrast, NAA levels are low in newborns and increase with age, while Cho and MI decrease with age.During the first 6 months of life, these metabolic changes are most rapid, leveling off at about 30 months of age (50).These changes are crucial, as both hypomyelination and delayed myelination affect changes of these metabolites.In X-ALD, proton MRS sometimes shows metabolite abnormalities beyond the margins of disease depicted by conventional MR imaging (38).The white matter lesion in children with X-ALD shows reduced NAA/Cr and increased Cho/Cr, MI/Cr and Glx/Cr (30).Spectroscopic changes in normal appearing white matter (NAWM) that precede disease progression in patients with X-ALD have been described (39).The changes are an increase in choline and a decrease in NAA.They occur in areas where subsequent lesion progression is observed, but not in the remainder of the brain.These areas may represent a zone of impending or beginning demyelination.Adrenomyeloneuropathy (AMN) is the adult variant of X-ALD.The disease pathology is usually limited to spinal cord and peripheral nerves ("'pure AMN") but shows cerebral involvement on histopathology.MRS studies showed reduced global NAA/Cho and NAA/Cr compared to controls.These changes are most prominent in internal capsule and parieto-occipital white matter.Decreased ratios of NAA in the absence of Cho/Cr elevation suggest prominent axonal involvement (40).Furthermore, Dubey et al demonstrated that the Expanded Disability Status Scale (EDSS) score inversely correlated with global NAA/Cr, suggesting a potential role of axonal injury in clinical disability in pure AMN.(40).Brain involvement demonstrable by MRI is rare in female subjects heterozygous for X-ALD, including those who have clinical evidence of spinal cord involvement.Nevertheless, NAA levels are reduced in the corticospinal projection fibers in female subjects with normal results on MRI, suggesting axonal dysfunction (41).Canavan disease is caused by a deficiency in aspartoacylase (ASPA), an enzyme involved in the process of degrading NAA to aspartate and acetate.Deficiency leads to the accumulation of NAA, which impairs normal myelination and results in spongiform degeneration of the brain (42,43).The elevations in NAA can be detected by MR spectroscopy in vivo (Figure 2), a diagnostic clue that can then be confirmed by urine measurement of NAA.The distinctly higher NAA peak can even be detected in the newborn although a radiologist only familiar with MR spectra in adults may not recognize the elevation in the newborn as pathologic.
LBSL is a disorder clinically characterized by slowly progressive signs of pyramidal, cerebellar and dorsal column dysfunction.LBSL shows a very distinct MRI pattern, with selective involvement of cerebral and cerebellar white matter and brainstem and spinal tracks, while U-fibers are spared (44).In LBSL, MR spectroscopy characteristically reveals decreased NAA and increased lactate, Cho and MI in the white matter, suggesting axonal damage and gliosis (45).Lately, mutations in the DARS2 gene, which encodes mitochondrial aspartyl-tRNA synthetase, have been identified as the underlying defect.Cerebrotendinous xanthomatosis (CTX) is a rare but treatable disorder characterized by a defect in the metabolic pathway of cholesterol (46).Symptoms in infancy include diarrhea, cataracts and psychomotor retardation.In adulthood, the spectrum of neurologic dysfunction includes mental retardation leading to dementia, psychiatric symptoms, premature retinal aging, and epileptic seizures.The most distinctive MR imaging abnormalities are bilateral T2 hyperintensities in the dentate nuclei and adjacent cerebellar white matter (47).An MRS study showed a reduction in NAA levels and the presence of a lactate peak (see Figure 2).NAA decreases are attributed to neuroaxonal damage due to neurotoxic deposition of cholesterol (48).A recent case report described the presence of abnormal lipid peaks at 0.9 and 1.3 ppm in the cerebellar hemisphere (49).These peaks can either be attributed to membrane breakdown or they may serve as surrogate markers of major lipid storage, with a potential role in monitoring therapeutic response.In addition, one patient had an increase in MI concentration, pointing to gliosis and astrocytic proliferation (50).
The MR spectra of patients with 4H-Syndrome, a rare form of hypomyelinating leukodystrophy, reveals low Cho/Cr and NAA/Cr, while a prominent MI peak can be observed (51,52).Low Cho levels are indicative of hypomyelination due to decreased membrane synthesis and turnover.In a new syndrome characterized by Hypomyelination with Atrophy of the Basal Ganglia and Cerebellum (H-ABC), MI and Cr levels are found to be elevated in the cerebral white matter, while NAA and choline levels are normal (53).These findings suggest that neither axonal loss nor active demyelination occurs in the setting of gliosis.In Pelizaeus Merzbacher disease, another hypomyelinating disorder, there have been discrepant reports on metabolite abnormalities detected by MRS.In part, these findings may be explained by the concurrent pathophysiologic processes of hypomyelination, gliosis, and neuronal loss over time (54).

Diffusion tensor imaging
While conventional MRI and MR spectroscopy show greatest utility in the diagnoses of leukodystrophies, other techniques, such as diffusion tensor imaging, are starting to be applied to leukodystrophies as well.Diffusion tensor (DT) MRI measures diffusivity of free water molecules in the brain.From the diffusion tensor, one can calculate indices that describe features of the orientationally averaged water diffusivity (isotropic part) and the water molecule displacements affected by the orientation of a regularly ordered structure in the tissue (anisotropic diffusion).Compared to gray matter, in which diffusion shows less directional dependence, white matter apparent diffusion is very anisotropic.(55).This property, termed fractional anisotropy (FA), depends on the orientation of the white matter tracks and on the degree and integrity of myelination (56)(57)(58)(59).In X-ALD, DT imaging may reflect the different abnormal white matter (AWM) zones in patients with X-ALD (60).We and others have observed that FA decreases and the isotropic apparent diffusion coefficient (iADC) increases over the zones toward the center of the lesion.The decrease in mean FA indicates the loss of an ordered structure governing the directionality of water molecule displacement.The increase in mean iADC suggests an increase in free water and a decrease in structures that restrict water diffusion.
In X-ALD, a strong correlation exists between NAA levels and FA, reinforcing the concept that both reflect axonal integrity.However, proton MR spectroscopic imaging reveals a low NAA level in regions with normal MR and DT imaging findings (61).In contrast, DT imaging showed no abnormalities outside the lesion on T2-weighted images.Further, the membrane turnover and cell accumulation associated with beginning demyelination, recognized in the enhanced choline and creatine signal intensity on proton MR spectroscopic imaging, did not have an effect on diffusion parameters.This suggests that proton MR spectroscopic imaging may have a higher sensitivity than both conventional MR and DT imaging in the early detection of abnormalities related to demyelination or axonal loss in X-ALD patients.However, in other instances, DTI has proven utility over proton MRSI.In pure AMN patients, DTI-based three-dimensional fiber tracking has shown occult tract-specific cerebral microstructural abnormalities in patients who had a normal conventional brain magnetic resonance image (62).This advance in MR imaging demonstrates that the corticospinal tract abnormalities in AMN reflect a centripetal extension of the spinal cord long-tract distal axonopathy.DTI anisotropy has also proved useful in Krabbe disease (63,64).In particular, measurements of white matter metabolites may help assess disease progression and determine optimal candidates for treatment options.Patients with Krabbe disease who underwent stem cell transplantation within the 1st month of life showed substantially smaller decreases in anisotropy ratios than those who were treated later.These findings correlate well with global assessments of disease progression as recognized by neurodevelopmental evaluations and conventional MR imaging.DT MR imaging studies of cerebral white matter development in human premature and term infants demonstrated that, in general, the apparent diffusion coefficient decreases while relative anisotropy increases with brain maturation.The most prominent regional difference at term is the increased relative anisotropy in the internal capsule, indicative of high directionality of diffusion, which could in part be related to myelination.Axonal diameter increases before and during myelination.This diameter change could also contribute in an important way to the diminished water diffusion perpendicular to the orientation of the fiber and thereby contribute to the increase in relative anisotropy.As anisotropy precedes myelination changes seen on conventional imaging, it is expected that quantitative analysis of diffusion parameters would add to the field of hypomyelinating disorders.Yet, little or no data is currently available in this group of disorders.Clearly, more systematic research is needed in regards to hypomyelination and DTI.

Recent applications of advanced MR technology
One major technological advance has been imaging at higher field strength.Magnetic resonance imaging at 4 and 7T allows for better visualization of lesion architecture, white matter tracts, and gray-white matter distinction compared with 1.5T.The field of proton MR spectroscopy has also benefited from higher field strength (65,66).Better spectral resolution results from improved signal-to-noise ratio and chemical-shift dispersion, and this in turn leads to more reliable detection of metabolites such as myoinositol and glutamine.Using 7T MRSI, decreases in NAA in the cortex of X-ALD patients were detected, which appears greater in male hemizygotes than in female heterozygotes and most pronounced with the occurrence of white matter lesions in males.(66).Although the cytoarchitecture of the cerebral cortex generally appears normal in X-ALD, scattered neuronal loss can be seen in gray matter during a pathologic examination.Both ratios of myoinositol and choline to creatine were found to be higher in normal appearing white matter of adult ALD patients with brain lesions compared to those without lesions.Yet, the interpretation of 7T MRSI data also poses challenges.Voxels close to the scalp show poor water and lipid suppression.Also, the quantification of spectral data in the presence of substantial radiofrequency excitation field (B1) variations is difficult.Therefore, focus has shifted on using adiabatic pulses to compensate for radiofrequency inhomogeneity and reduce the chemical shift displacement error (67,68).
Overcoming some of the shortcomings of DTI, novel methods now exist to map complex fiber architectures of white matter and other brain tissues.Diffusion spectrum imaging (DSI) allows resolution of regions of 3 way fiber crossings (69-71).On DTI this was not possible as fiber crossing led to decreases in FA, making it difficult to distinguish pathological changes from normal fiber crossings.The years ahead will likely bring more studies employing DSI in leukodystrophy patients.
Advances have also been in the development of ex vivo MRI and novel magnetic resonance contrast agents for imaging of autopsy specimen.In metachromatic leukodystrophy, postmortem studies have demonstrated the pathological correlate of the "tigroid stripes" characteristically seen on conventional imaging (72).Through direct correlation of postmortem MR imaging on 1 cm thick blocks with neuropathology staining, the authors were able to show that perivascular clusters of glial cells containing lipid material corresponded to the stripes on MRI.Using contrast agents ex vivo, the corresponding substrate of imaging can be further elucidated.One such example is luxol fast blue that displays a binding affinity for myelinated constituents of the brain (73).The specificity of luxol fast blue for lipid constituents results in an increase in longitudinal and transverse relaxation rates of tissue dependent on myelination status.The relaxation rates of white matter increase sufficiently to permit T1-weighted images of ex vivo samples that are similar in contrast behavior to T1-weighted in vivo imaging.The contrast increases in MR images of LFB-stained ex vivo brain tissues enhancing delineation between upper lamina and the more myelinated lower lamina.Other advances in MR technology have brought great practical benefits.Sedation and anesthesia represent risks in advanced brain disease of LD patients.Using new techniques, such as propeller MRI, it has become possible to oversample k space and, thereby, compensate for motion and allow follow-up MR imaging without sedation (74,75).As an www.intechopen.com Neuroimaging -Clinical Applications 570 alternative to these retrospective motion-correction techniques, it is also possible to prospectively correct motion in structural imaging and single-voxel spectroscopy using image-based navigators (76)(77)(78).In patients with more advanced leukodystrophies, these advances may allow for imaging without sedation and thereby give insight into the more advanced stages of disease.Overall, not one advanced imaging technology is expected to bring about a breakthrough in the leukodystrophies.Rather, the multimodal approach with coregistration of high resolution imaging with advanced spectroscopic and diffusion imaging will lead to new pathophysiological insights in the years to come.Different diseases, varying phenotypes, and stages within the disease will require varying imaging modalities.

Conclusions
MRI has allowed for much progress in the field of leukodystrophies.Prior to arrival of MRI, the specific vulnerability of brain white matter was not well understood.Today, MRI has helped define disorders through the recognition of specific lesion patterns and their evolution over time.This has also led to identification of novel leukodystrophies and the genes underlying these disorders.Even in previously well characterized disorders, MRI patterns have shed light on disease mechanisms.The understanding of the pathology and molecular basis of leukodystrophies has in turn allowed for new insight into the significance of MRI changes and elucidated the capabilities of MR techniques.Brain MRI today is a valuable tool in monitoring disease progression and the success of therapeutic interventions in leukodystrophies.Advances in new techniques encourage a multimodal approach employing a variety of sequences sensitive to different brain tissue characteristics.Together, these techniques will be able to provide clues to the early stages of disease -insight not gained by pathology in the past.

Fig. 2 .
Fig. 2. The spectra of a normal brain are shown at both long echo time (top), TE = 250-280 ms, and short echo time (bottom), TE = 20-50 ms.The spectra for the disease states are as follows: the Canavan spectra are acquired at long echo time, and the ALD and CTX spectra are acquired at short echo time.

Table 2 .
Characteristics on Brain MRI in Hypomyelinating Disorders

Table 3 .
Brain Metabolites Abnormalities on Proton MR Spectroscopy www.intechopen.comAdvances in MR Imaging of Leukodystrophies