mrs y học

Clinical Applications of Proton MR Spectroscopy AJNR 17:1-15, Jan 1996 • Chemical shift – resonance frequency of a nucleus depends on chemical environment, producing a small but perceptible change in Larmor resonance • Proton MRS – high nature abundance – high nuclear magnetic sensitivity • Phosphorus 31 MRS • Resonance frequency/ chemical shift location – parts per million (ppm) from main magnetic resonance frequency of system • Height or area under the peak – relative concentration of protons • Width of peak at half-height – proportional to 1/T2 • Shimming the field to resonance of water, assure homogeneity of the field • Spatial localization methods – point-resolved surface coil spectroscopy (PRESS) – stimulated-echo method (STEAM) • Suppressing the signal from water – chemical shift selective excitation (CHESS) • Voxal – localized single volume, 1-8cm3 – smaller voxal, lesser S/N ratio – 2-D MRS for mapping of metabolite concentration • Long TE – signal from most metabolites lost except choline, creatine, N-acetyl aspartate, lactate • Short TE – myoinositol, glutamate, glutamine Concentration of normal metabolites in brain varies with patient’s age • Noticeable during first years • ↑NAA/Cr and↓Cho/Cr as brain matures • Neuronal maturation and increase in number of axons, dendrites and synapses N-acetyl aspartate (NAA) • • • • • Resonates at 2.0ppm, largest peak Neuronal marker Decrease with many insults to brain Not present in tumors outside the CNS Canavan disease – the only disease in which NAA increase Choline (Cho) • 3.2ppm • Glycerophosphocholine, phosphochoine, phosphatidylcholine • Phospholipid metabolism of cell membrane • Increased membrane synthesis and/or an increased number of cells Creatine (Cr) • • • • 3.02 ppm Maintain energy-dependent system Increased in hypometabolic states The peak remains fairly stable even in face of disease, used as a control value Lactate • 1.32 ppm • Two distinct resonant peaks, doublet • Carbohydrate catabolism take place normal cellular oxidative respiration mechanism • TE: 272 ms, lactate projects above baseline • TE: 136 ms, inverted doublet Astrocytomas • Significant↓NAA (40-70% of normal), moderate ↓Cr,↑Cho • Proton MRS can’t distinguish between different histologic grades • Presence of lactate and↑Cho peak: higher grade ? • Different from infection – ↓Cho in infection Meningiomas • Typically no NAA, markedly↑Cho (up to 300 times normal),↑lactate & alanine • Atypical and malignant meningioma may show NAA resonance, indistinguishable from astrocytoma Metastases • Nonspecific • Moderate to marked↓NAA,↓Cr,↑Cho • Some meta contain lipid resonance ( esp breast ca.), also present in high grade astrocytoma with necrosis Radiation injury • Damage to vascular endothelium, resulting in ischemia and necrosis • ↑lactate appreciable before MRI abn • ↓NAA, Cho and Cr • Broad peak 0-2.0 ppm (tissues necrosis) • Tumor recurrence:↑Cho HIV infection • Marked metabolic alteration in only mild AIDS-related dementia, correlated with severity of disease • ↓NAA, NAA/Cho, NAA/Cr • Very low CD4 lymphocyte, abn MRI – ↑Cho • HIV (+) newborn – ↑NAA/Cr, Cho/Cr Degenerative disorders of the elderly • Alzheimer disease – – – – biochemically,↓cortical acetylcholine early diagnosis:↓NAA,↑myoinositol other dementia, no↑myoinositol Cho level ?? • Parkinson disease – normal NAA, Cho and Cr,↑lactate – superimposed dementia, more↑lactate Degenerative disorders in children • Nonspecific↓NAA,↑lactate • Adrenoleukodystrophy – ↓NAA/Cr,↑Cho/Cr,↑lactate & glx • Canavan – deficiency in aspartoacyclase (breakdown NAA) – marked↑NAA • MELAS – ↑ lactate Hepatic encephalopathy • Cirrhosis and other chronic liver disorders • Mood/ behavior change, tremor, dysarthria, dementia, myelopathy • ↑glutamine/ glutamate • ↓Cho and myoinositol Cerebral ischemia • ↑lactate in initial 24 hrs • ↓NAA as early as 4th day • Chronic infarction – ↓NAA, Cr, Cho, but no lactate • Experimentally,↑lactate detected after only 2-3 after ischemia • Age-related WM hyperintensity onT2 – normal NAA, Cr,↑Cho, no lactate Hippocampal sclerosis • Pathologically identified in 65% of temporal lobe sclerosis • ↓NAA/Cho,↑or normal Cho/Cr, 　↑ lactate • Acutely swollen hippocampus with normal NAA, Cho, Cr but ↑lactate – ischemia, predispose to hippocampal sclerosis Neurofibromatosis type I Hamartomas vs astrocytoma • 43% hamartomas – areas of increased T2 SI in brain – no change in size for yrs, no mass effect, no enhancement – MRS similar to normal brain • 6-15% low grade astrocytoma – MRS compatible with tumor Multiple sclerosis • Chronic MS – NAA decreased (axonal loss) – free lipids, 0.9-1.6 ppm (disintegration of myelin) • Acute plaques, NAA normal ... small but perceptible change in Larmor resonance • Proton MRS – high nature abundance – high nuclear magnetic sensitivity • Phosphorus 31 MRS • Resonance frequency/ chemical shift location – parts... brain – no change in size for yrs, no mass effect, no enhancement – MRS similar to normal brain • 6-15% low grade astrocytoma – MRS compatible with tumor Multiple sclerosis • Chronic MS – NAA decreased... necrotic process Astrocytomas • Significant↓NAA (40-70% of normal), moderate ↓Cr,↑Cho • Proton MRS can’t distinguish between different histologic grades • Presence of lactate and↑Cho peak: higher