MR Spectroscopy of Brain

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             Magnetic resonance spectroscopy is used to measure the levels of different metabolites in body tissues. The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain, as well as to provide information on tumor metabolism.

 
                   
                   
 

What is MR Spectrum?

        The "spectrum" is a graph of the relative concentrations of these molecules based on their discrete radio-frequency signal, which is shifted away (chemical shift) from the water signal the MRI uses. A high peak means that the molecule at that RF location exists in greater concentration than an adjacent low peak. To be precise, it is the area under the peak that is relevant. Applying appropriate calibrations, the concentration of a given metabolite is calculated from the peak area. The peak locations correspond to specific molecules such as glucose, creatine, lactate etc, however peaks from non-identical molecules may overlap. In practice, the peak position identifies the brain metabolite. The combination of peak locations (chemical shift compared to water) and heights provides a biochemical fingerprint of the volume of interest (VOI).

 
   
 

MR Spectroscopy of the brain

An MR spectroscopy exam is used to obtain Spectrum of the brain. MR spectroscopy is used to characterize biochemical components of normal and abnormal brain tissue. The most common nuclei that are used are 1H (proton), 23Na (sodium), 31P (phosphorus). Proton spectroscopy is easier to perform and provides much higher signal-to-noise than either sodium or phosphorus. Proton MRS can be performed within 10-15 minutes and can be added on to conventional MR imaging protocols. It can be used to serially monitor biochemical changes in tumors, stroke, infarcts, epilepsy, metabolic disorders, infections, and neurodegenerative diseases. MR spectroscopy may be a useful adjunct to conventional imaging to distinguish recurrent tumor from treatment-related change in the brain such as inflammation or dead cells. A common clinical problem is distinguishing tumor recurrence from radiation effects several months following surgery and radiation therapy. 

The MR spectra do not come labeled with diagnoses. 
They require interpretation and should always be correlated with the MR images before making a final diagnosis.

 
                      
 

CLINICAL APPLICATIONS

  • Brain Tumors
    • MRS can be used to determine the degree of malignancy.
    • As a general rule, as malignancy increases, NAA and creatine decrease, and choline, lactate, and lipids increase.
    • NAA decreases as tumor growth displaces or destroys neurons.
    • Very malig-nant tumors have high metabolic activity and deplete the energy stores, resulting in reduced creatine.
    • Very hypercellular tumors with rapid growth elevate the choline levels.
    • Lipids are found in necrotic portions of tumors, and lactate appears when tumors outgrow their blood supply and start utilizing anaerobic glycolysis.
    • Elevated choline is a marker for recurrent tumor.
    • Radiation change generally exhibits low NAA, creatine, and choline on spectroscopy.
    • If radiation necrosis is present, the spectrum may reveal elevated lipids and lactate.
    • As mentioned above, one key feature of gliomas is elevated choline beyond the margin of enhancement due to infiltration of tumor into the adjacent brain tissue.
    • Most non-glial tumors have little or no NAA.
    • Elevated alanine at 1.48 ppm is a signature of meningiomas.
    • They also have no NAA, very low creatine, and elevated glutamates.
  • Cerebral Ischemia and Infarction
    • When the brain becomes ischemic, it switches to anaerobic glycolysis and lactate accumulates.
    • Markedly elevated lactate is the key spectroscopic feature of cerebral hypoxia and ischemia.
    • Choline is elevated, and NAA and creatine are reduced.? If cerebral infarction ensues, lipids increase.
  • Trauma
    • MR spectroscopy is not routinely used in the acute setting of head injuries.
    • CT and MR imaging demonstrate the fractures and intracranial hemorrhage that require emergent surgical intervention.
    • On the other hand, when the patient has stabilized, MRS is helpful to assess the degree of neuronal injury and predict patient outcomes.
    • Especially in the case of diffuse axonal injury, imaging often underestimates the degree of brain damage.
    • Clinical outcome correlates inversely with the NAA/Cr ratio.
    • The presence of any lactate or lipid indicates a worse prognosis.
  • Infectious Diseases
    • As in the case of non-glial tumors, brain abscesses destroy or displace brain tissue, so NAA is not present.
    • Lactate, cytosolic acid, alanine, and acetate are characteristic metabolites in bacterial abscesses.
    • Toxoplasmosis and tuberculomas show prominent peaks from lactate and lipids.
    • Clinical investigators of HIV infection and AIDS have been very interested in the potential of MRS for measuring the effects of HIV infection on the brain and neuro-cognitive function.
    • Unfortunately, MRS has not proven very sensitive for detecting HIV encephalitis in the early stages of infection.
    • On the other hand when patients start developing neurocognitive deficits and AIDS dementia complex, the MR spectra become positive, namely with elevated choline and reduced NAA.
    • Choline is the best marker for the white matter abnormalities, and the extent of NAA depletion correlates directly with the degree of dementia.
    • MRS is also very helpful in following patients and assessing the effects of anti-viral therapies.
    • There is also considerable interest in using MRS to distinguish the common focal brain lesions in AIDS patients.
    • The most helpful marker is choline, which is elevated in lymphoma, but low or absent in toxoplasmosis, tuberuloma, and cryptococcoma.
    • Toxoplasmosis is characterized by markedly increased lactate and lipids and depletion of normal brain metabolites.
    • Tuberculoma and cryptococcoma are similar but with relatively little lactate.
    • The spectrum for PML may be similar to lymphoma, but the imaging features are distinctly different and PML may have elevated myo-inositol.
  • Pediatric Metabolic Disorders
    • MRS has a very important role in diagnosing and monitoring patients with metabolic disorders.
    • This group includes a long list of diseases that affect the gray and white matter to varying degrees.
    • The names and terminologies of these disorders are confusing because they were derived from the pathologic literature before their metabolic defects were discovered.
    • As the specific biochemical and enzyme defects are being elucidated, these diseases are being classified more appropriately.
    • The list of disorders is long and beyond the scope of this syllabus.
    • Some of the more important diseases are listed below, along with their specific metabolic markers on MR spectra.
    • Since most metabolic disorders present in infancy, it is important to understand the normal pediatric MR spectrum.
    • Compared to the adult, newborns have much less NAA, and increased choline and myo-inositol.
    • Progression to the adult pattern follows myelination.
  • Hepatic Encephalopathy
    • The spectrum of hepatic encephalopathy is characterized by markedly reduced myo-inositol.
    • Choline is also reduced, and glutamine is increased.
    • Liver failure results in excess ammonia in the blood. Ammonia is a neurotoxin and causes increased conversion of glutamate to glutamine.
    • Similar metabolic changes are seen in Reye's syndrome, an acute form of liver failure in infants.
    • The metabolic changes of hepatic encephalopathy increase after a TIPS shunt procedure, and they revert back to normal after successful liver transplantation.
  • Alzheimer's Disease
    • Although MR spectroscopy is not highly sensitive for detecting early Alzheimer's disease, as the disease progresses, the spectrum becomes abnormal.
    • Specifically, with advancing disease the NAA is reduced and myo-inositol becomes elevated.
    • Since MRS is totally non-invasive and easily obtained, myo-inositol may become an important marker for assessing new therapies for this devastating disorder.
    • Myo-inositol is also increased in Down's syndrome, a dementia that presents in childhood and is pathogenetically similar to Alzheimer's disease.
    • On the other hand, myo-inositol is not elevated in other adult dementia, so it is a helpful marker to distinguish Alzheimer's disease from the other causes of dementia.
 
                   
                      
                       
             
                       
                  
                    
                     
                      
                      
                       
                       
                            
                            
                              
                              
                             
                           
                           
                           
                                                                                                                                                                     
                        
                     
                      
                        
                       
                     
 
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