الفهرس 
Concepts of MRS
Chemical Shift.Only 14 pages are availabe for public view
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Abstract
Intracranial cystic mass lesions include true cysts lined by epithelial, ependymal or meningothelial cells, dermoid and epidermoid cysts, parasitic cysts (cysticercosis, hydatid cysts) or may be pseudocystic neoplastic or inflammatory lesions (abscesses) secondary to accumulation of necrotic, intercellular mixed or proteinaceous material.
The differential diagnosis of various intracranial cystic lesions is sometimes difficult on the basis of CT or MRI findings. Proton MRS and DWI are useful as additional diagnostic modalities in their differentiation. Combination of DWI with calculated ADC values and metabolite spectrum acquired by MRS added more information to MRI in the differentiation of intracranial cystic mass lesions. Magnetic Resonance Spectroscopy allows in-vivo exploration of the metabolites involved in the physiopathology of the lesion. The principal metabolites that can be analyzed are: N-acetyl aspartate (NAA), choline-containing compounds (Cho), creatine/phosphocreatine (Cr), glutamate/glutamine (Glx), Myo-inositol (mI), lactate (Lac) and lipids (Lip). It detects resonance peaks of brain metabolites such as Choline, Creatine, NAA, Myoinositol, Glutamate, and lactate.
Multi-voxel technique is much more preferred than the single one as it takes less time, covers the side of the lesion, areas outside of the lesion and can evaluate very small lesions avoiding contamination of the voxel by the surrounding normal brain parenchyma, CSF or fat which may occur with single voxel technique. The important metabolites to be commented in the long TE are Cho, NAA, Cr and sometimes the MI, as well as, lipid/lactate peaks, however, it is better to comment on MI and lipid/lactate peaks in the short one. Other metabolites seen on short TE are alanine (mainly seen in meningioma), succinate, aspartate, acetate and others (seen in abscesses) and Glx seen in non-neoplastic lesions.
Diffusion-weighted imaging provides a novel way to evaluate the diffusion properties of the molecular motion of water within tissues (Brownian motion) providing information about tissue integrity. It is a quick and practical technique in emergency radiology and is unaffected by size of the lesion that may be required for in vivo PMRS examination.
It has high sensitivity and specificity for the differentiation of brain abscess from other non-abscess intracranial cystic mass lesions. The criterion for the diagnosis of benign cysts is the hypointensity on DWI, with the ADC comparable to that of the CSF. All arachnoid cysts show hypointensity on DWI with PMRS lactate peak arachnoid cyst which is very minimal, and all of the epidermoid cysts show hyperintensity and a high lactate peak around 1.3 ppm, and there is a virtual absence of NAA and choline. No lipid peak should be seen.
The contents of the lesions are predominantly responsible for the difference in ADC of the cystic cavities of abscesses from tumor cysts. Diffusivity of water molecule is affected in the organized abscess environment containing microorganisms, macromolecules, and intact inflammatory cells. The diagnosis of the brain abscess was concluded when the cavity show hyperintensity on DWI with the ADC values lower than (0.9 to 0.13 ×10ˉ³ mm²/s). Tumor cysts show hypointensity on DWI with an ADC value of (2.2 ± 0.9 ×10ˉ³mm²/s).
The specific spectrum of the abscess cavity shows elevation of cytosolic amino acids and lactate with additional acetate, alanine, and succinate, and this spectrum is significantly different from the spectra of cystic or necrotic brain tumors.
Proton MR Spectra obtained from cystic and necrotic tumors typically show: decreased NAA as a marker for neuronal integrity, diminished Creatine, involved in cellular energetic and osmotic balance and increased Choline, involved in cell membrane turnover. Lactate and mobile lipids could be evident in aggressive tumors reflecting increased anaerobic metabolism and cellular necrosis respectively.
With higher WHO tumors grading, gliomas exhibit significantly increased Choline and Lipid formation.
Proton MR Spectroscopy differentiates recurrent gliomas from radiation necrosis. As tumor recurrence show increased Choline peak and increased Cho/Cr ratio, while radiation necrosis shows absent or depressed Choline peak. DWI shows radiation necrosis as heterogeneous intensity whereas recurrent tumor appears as predominantly hyperintensity. Mean ADC values of the recurrent tumor 1.18 × 10ˉ³ while that of radiation necrosis is 1.4×10ˉ³ mm²/s showing that the ADC values of the recurrent tumor is significantly lower than those of radiation necrosis.