mrs y học

• 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 – h

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, 1-8cm 3

– smaller voxal, lesser S/N ratio

– 2-D MRS for mapping of metabolite

Concentration of normal metabolites in

brain varies with patient’s age

• Noticeable during first 3 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

• 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

• 3.56 ppm

• Decreased myoinositol

– lithium use in mania

– development of diabetic neuropathy

• Elevated myoinositol and decreased NAA

– Alzheimer disease

• Significant in head and neck carcinoma

• Glutamate and Glutamine (Glx)

– ↑in high grade astrocytoma and

meningioma, necrotic process

• 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

• Typically no NAA, markedly↑Cho (up to 300 times normal),↑lactate & alanine

• Atypical and malignant meningioma may show NAA resonance,

indistinguishable from astrocytoma

• 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

• Very low CD4 lymphocyte, abn MRI

– ↑Cho

• HIV (+) newborn

– ↑NAA/Cr, Cho/Cr

Degenerative disorders of the elderly

– normal NAA, Cho and Cr,↑lactate

– superimposed dementia, more↑lactate

Degenerative disorders in children

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 min 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,

Neurofibromatosis type I

Hamartomas vs astrocytoma

• 43% hamartomas

– areas of increased T2 SI in brain

– no change in size for 3 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