Many neurometabolic disorders give rise to non-specific structural changes in the central nervous system that can be characterised with MRI, but it can be difficult to make a definitive diagnosis on the basis of imaging alone. Exceptionally, neurometabolic disorders may also have adult onset. Most patients with neurometabolic disorders show developmental delay or neurological signs and symptoms in the neonatal period or early childhood. The incidence of each individual condition is very low, while the combined incidence is estimated to be 1 per 800–2 500 births (3, 4). Neurometabolic disorders are a large and heterogeneous group of congenital conditions. In this article, we describe findings from MR spectroscopy for these diseases based on our own experience and selected literature. Olavs hospital, MR spectroscopy is used routinely for the diagnosis and follow-up of patients with such conditions. The method has greatest clinical utility in cases of suspected neurometabolic disorders or brain tumours (2). For a few conditions, findings from MR spectroscopy may have direct consequences for the follow-up and treatment of the patient. The technique is now used as a supplement to MRI, primarily in diseases of the central nervous system. The reality fell somewhat short of these initial expectations. When MRI came into use for clinical diagnosis in the mid 1980s, it was expected that also MR spectroscopy would become a key diagnostic tool, especially in oncology. MR spectroscopy can reveal metabolic changes that precede pathological structural changes in brain tissue (1). Spectra are scaled relative to the creatine signal. Water provides a strong signal at 4.7 ppm (parts per million) that is suppressed and outside the range shown. In Canavan disease (b), the signal from N-acetylaspartate is far more dominant than in the spectrum from the healthy control (c). The presence of lactate can be seen in Leigh syndrome (a). Choline (Cho), creatine (Cr), and N-acetylaspartate (NAA) appear in all spectra, and changes in these signals can often be linked to pathology. All spectra are recorded with long echo time (135 ms). Similarly, lipids are bound in fixed, immobile structures and therefore cannot usually be detected with this technique.įigure 1 MR spectra from a) a patient with Leigh syndrome, b) a patient with Canavan disease and c) a healthy person, and d) T2-weighted MRI of a patient with Canavan disease. In normal brain tissue, the amount of lactate is usually too low to be detected by MR spectroscopy. Depending on the test conditions, metabolites that are present at lower concentrations in normal brain tissue, such as myo-inositol and glutamine/glutamate, can also be detected. In addition to the water signal, the dominant peaks in MR spectra of the brain are from creatine, choline and N-acetylaspartate. Some diseases have characteristic MR spectra, which contain molecules that are not normally seen in healthy tissue, or where the relative concentrations of metabolites differ from those seen in equivalent healthy tissue (1, 2). Molecules can be differentiated on the basis of frequency differences along the x-axis, while the area of the peaks corresponds to the concentration of the molecule (Figure 1). An MR spectrum will thus reveal the molecules within tissues, provided that the molecules are mobile and present in measurable quantities (> 1 mmol/l) (1). Protons in different molecules have slightly different magnetic properties, and this difference enables small molecules in the body to be detected by MR spectroscopy. All protons in a water molecule have the same magnetic properties and are the main source of the signal in MRI.
Protons (hydrogen nuclei) display magnetic properties when in a strong magnetic field, and are the source of the signal in both methods. The technological and physical principles behind MR spectroscopy are to a large degree the same as those behind MR imaging (MRI). This article describes MR spectroscopy findings for those disorders for which the technique has greatest clinical relevance. Magnetic resonance spectroscopy (MR spectroscopy) provides information on various tissue metabolites and is a supplement to standard diagnostic MR imaging.
0 kommentar(er)
