Proton Mr Spectroscopy, Fundamental Physics and Clinical Applications

Review Article

Austin Oncol. 2016; 1(1): 1002.

Proton Mr Spectroscopy, Fundamental Physics and Clinical Applications

Kemal Arda¹* and Hasan Aydin²

¹Ankara Ataturk Rearch Hospital / Radiology Department, Turkey

²Dr. Abdurrahman Yurtaslan Oncology Training and Research Hospital/ Radiology Department, Turkey

*Corresponding author: Kemal Arda, 3281 Sk. 10/47 Yasamkent/Ankara, Turkey

Received: December 09, 2015; Accepted: January 27, 2016; Published: January 29, 2016

Abstract

Proton MR spectroscopy (H-MRS) is one of the ultrahigh MR imaging technique that has recently been used to evaluate the metabolic alterations of tissues, a molecular imaging approach, mostly depend upon the evaluation of in vivo molecular cells and tissues which allows in vivo molecular studies, inappropriate spatial resolution is one of the major shortcoming of this technique in the clinical practice.

The routine metabolites which are identified with short and long TE are: N-Acetyl Aspartate (NAA), Creatine (Cr), Choline (Cho), Lactate (Lac). Using short TE, some additional metabolites are identified such as; Lipids (lip), Glutamine and glutamate (Glx), Myo-Inositol (mI).

H-MRS was mostly performed for brain tumors, sleep apnea, epilepsy,breast lesions, thyroid nodules, prostatic tumors in order to aid and assist for diagnosis and planning of treatment.

Proton MR Spectroscopy can be combined to routine MR imaging in variable conditions by detecting metabolite alterations and measuring their resonance peak levels.

Keywords: MRI; Spectroscopy; Physics; Tumor; Nodule; Carcinoma

Introduction

Proton MR spectroscopy (H-MRS) is one of the ultrahigh MR imaging technique that has recently been used to evaluate the metabolic alterations of tissues, at first, this technique is used for the assessment and interpretation of metabolic changes in the central nervous system, mainly for brain that has been used to observe metabolite changes for different intracranial pathologies such as tumors, multiple sclerosis, stroke, tuberculomas, epilepsy, metabolic and inherited brain disorders, and traumatic injuries, nowadays its clinical use and applications are widened, evolving thyroid, breast, prostate etc [1-4].

H-MRS is believed to be a molecular imaging approach, mostly depend upon the evaluation of in vivo molecular cells and tissues which allows in vivo molecular studies, inappropriate spatial resolution is one of the major short coming of this technique in the clinical practice [2-4]. H-MRS can be acquired as an additional pulse sequence to routine MRI sequences and contribute to a multimodality study of functional and metabolic information rather than morphologic tissue properties. It aids in the diagnosis, treatment, follow up and therapy response of patients especially against brain tumors, helps to achieve the best outcome of patients [2,3,5,6].

MR spectroscopy is an unlimited technique which indicates some certain metabolites in the metabolic spectrum and one has to know the normal metabolic position of a tissue before the interpretation of metabolic alterations inside it [5-6].

Fundamental Physical Principles of H-MRS

The physical principles of H-MRS are similar to conventional MRI, the magnetic properties of atomic nucleus are the fundamentals for both methods. Various matters which have different electrical charges will have different velocities in a certain magnetic field which may provide the measurement of various metabolites [7-8].

The strength of the MR signal is directly proportional to the number of protons of that frequency in spectroscopy, spectroscopy can be described in the time domain, whereas MRS data is usually displayed in the frequency domain, area under a specific peak in the frequency domain is proportional to the number of protons, resonating at that certain frequency [8-9].

The frequency axis is proportional to the magnetic field strength so the peak locations on the axis will depend on the B0, due to the lack of natural tissue that shows zero frequency, substances are mixed to be measured with a reference but these reference materials are toxic and cannot be used in in-vivo spectroscopy. By this method, scientists express the frequency difference between the substance (that will be measured) and the reference as a non-dimension quality, quality value is represented by dcs (in parts per million) [9].Then this equation is used as follows;

dcs = f s / (f transmitter x 10-6 ) + offset

In this equation, f s represents the frequency of the sample, f transmitter shows the frequency of the transmitter and offset is the constant that references the ppm scale (in in-vivo standards). This standard is usually N-acetyl aspartate (NAA) (that has a chemical shift value of 2,01 ppm) peak for H-MRS [2-4,7-8]. MR spectrum (Figure 1) is obtained by using water supression and spectroscopic sequences as Point-Resolved Spectroscopy (PRESS), Stimulated Echo Acquisition Mode (STEAM) etc. In order to get optimized water saturation with consistent water resonance and water suppression pulses, automatic shimming of the linear x, y, z gradients have to be used for optimization of Field Of View (FOV) homogeneity, time domain data is multiplied with a Gaussian function of 90 (Centre 0, half width 256 ms), 2D Fourier transformed phase and with corrected base-line, quantified by means of frequency domain curve fitting with the assumption of a Gaussian line shape’’ by using system manufacturers [1,3,7,8]. Due to the magnetic field in homogeneities, shimming is required for H-MRS spectral data and the acquisition in which B0 field has to be performed as homogeneous as possible, use of high-channel shim coils and shimming of linear gradient coils aid in improved outcomes, particularly important at higher magnetic field strengths due to increase of induced susceptibility shifts with increased Tesla gradients [2,3,5,8-10].