The Effect of Paramagnetic Contrast in Choline Peak in Patients with Glioblastoma Multiforme Might Not Be Significant

Discussion

MR spectroscopy has become a useful tool in the characterization and evaluation of brain tumors. MR spectra of brain tumors usually show elevated Cho levels and decreased levels of N-acetylaspartate, reflecting increased cellular membrane turnover and neuronal cell dysfunction/death, respectively.^{1⇓⇓⇓⇓–6,8,13⇓–15} Nelson ⁸ showed that the relative increase in Cho and decrease in NAA is critical for defining the spatial extent of the metabolic abnormality corresponding to an active tumor. Nelson developed an index for clinical applications, termed Cho-to-NAA index that, based on its value, assigns a probability value of tumor presence. Fountas et al¹⁶ showed that the higher the grade of the astrocytomas, the higher the Cho peak and the lower the Cr peak. They were able to differentiate between astrocytoma grades II, III and IV by using the Cho/Cr ratio, and proposed considering this ratio as a malignant index for the histologic grading of these tumors.

The use of the paramagnetic contrast before the acquisition of the spectroscopy may aid voxel positioning in the most representative and solid portion of the tumor, but some experimental studies have suggested that Gd-contrast could lead to loss of the Cho signal, leading to errors in tumor evaluation.^10,11 Therefore, it is important to characterize the effect of the paramagnetic contrast on the metabolite values, particularly Cho.

Previous studies have shown that when using T2-weighted spectroscopy (ie, using a long TE), the Cho peak area and line shape are influenced by paramagnetic contrast agents.^10,11 These 2 studies reported a mean decrease of Cho peak area of 12% and 15% when using chemical shift imaging point-resolved spectroscopy (TR/TE: 1500/135 ms).

Although we used the same acquisition technique in our study, we did not find statistically significant differences before and after contrast in the Cho peak area in the tumor, nor in the contralateral hemisphere, calculating mean values for the enhancing and nonenhancing tumoral voxels and also in the contralateral hemisphere.

The paramagnetic contrast can enhance both longitudinal and transversal metabolite relaxation, leading to effective shorter T1 and T2 relaxation times. A T1 shortening would result in a metabolite intensity gain, especially if the spectrum is acquired with a short TR. On the other hand, a T2 shortening will result in a loss of metabolite intensity, becoming more pronounced the longer the TE of the acquisition. Based on an in vitro study, Murphy et al¹⁷ showed that, for the acquisition parameters TE/TR = 135/1500 ms, the actual contrast concentration in the voxel selected for the ¹H-MR spectroscopy study will determine which of both effects will dominate, that is, whether an increase or decrease of metabolite intensity will be observed. A smaller Gd concentration would predominantly cause an increase in the T1 relaxation of the Cho, leading to an increase in its signal. At higher concentrations of contrast, transverse relaxation enhancement becomes more significant and metabolite intensity begins to drop.¹⁷ Although the changes in the mean Cho peak area found in our study were not statistically significant, it is interesting to note that, individually, we found a large variation before and after contrast administration, ranging from negative to positive values, probably depending on the contrast concentration in each voxel as described by Murphy et al.¹⁷ After contrast administration we observed a mean increase in the Cho peak area in the nonenhancing tumoral voxels, where an increase in the T1 relaxation predominates leading to an increase in the Cho signal and a mean decrease in the Cho peak area in the enhancing tumoral voxels, where T2 relaxation becomes more pronounced leading to a decrease in the Cho signal.

We found it interesting to pick the voxel of the highest Cho concentration in each patient and to study the effect of the Gd in this particular voxel, as some authors suggested that this voxel could represent the highest metabolic activity area of the tumor and could be used to classify the degree of the tumoral malignance, and to target stereotatic biopsy and radiation therapy.⁸ When considering the mean values for the maximum Cho peak area value in each patient, a statistically significant decrease was noted in the Cho peak area mean value after contrast injection in the enhancing tumoral voxels where the Gd concentration is higher, and a tendency of increase in the nonenhancing tumoral voxels Cho peak area mean value, though not statistically significant (P = .077), where the Gd concentration is lower.

Other factors that could have contributed to this large variation include the heterogeneity of the GBM itself, the variability inherent in the method, and the type of Gd-contrast agent used. The large variation in voxel values involving the Cho peak area before and after contrast injection observed in our study might indicate that is advisable to consider several voxels within the tumor, instead of choosing just one to calculate the values for Cho when IV contrast injection is performed before data acquisition, particularly when analyzing heterogeneous contrast-enhancing tumors such as glioblastoma multiforme.

One factor that is important to consider when studying paramagnetic contrast interaction with brain metabolites is cellular compartmentalization. The cell membrane inhibits intracellular metabolites from coming into contact with extracellular contrast agents. Because of their charge and structure, they do not freely pass through the intact cell membrane.^9,18,19 This means that for the contrast agents to interact directly with the metabolites, the metabolites should be in the extracellular compartment. Sijens et al¹⁰ consider that there is a pool of extracellular Cho that is increased in brain tumors and interacts with the paramagnetic contrast, leading to a dipolar relaxation with loss of signal of this metabolite. In the dipolar relaxation there is a dipole-dipole interaction between the protons of the metabolite and the unpaired electrons of the Gd, which is responsible for the T1 and T2 shortening of the metabolites. This would explain why the effect of Gd is more appreciable on the Cho signal, and also present in the extracellular space, than on other metabolites.

Theoretically, contrast agents could also affect the measured MR signal indirectly by inducing T2* susceptibility derived relaxation of intracellular metabolite from the extracellular space.⁹ A nonhomogeneous magnetic field within the volume of interest leads to slight differences in the precession frequency of the hydrogen protons within the same molecule, increasing the metabolite line width, which could cause loss of the signal-to-noise ratio and overlap of the metabolite peaks.¹⁸ Hence, paramagnetic contrast distribution could create magnetic field perturbation, leading to metabolite peak widening. We calculated the width of the line fitted to the Cho peak before and after contrast administration across the tumoral area and the contralateral hemisphere, considering the mean value per patient and also the width of the Cho peak maximum value voxel in the enhancing and nonenhancing tumoral voxels, and in the contralateral hemisphere in each patient. Although no statistically significant differences were found, a tendency of increase in the Cho peak width mean value was observed when considering the mean values for the Cho peak width in the enhancing tumoral voxels. This suggests that Gd injection really could cause a T2* susceptibility effect on the spectrum. This can be important if the Cho signal is estimated only by the measurement of peak amplitudes, because T2* line broadening leads to lower peak heights.

A limitation of our study was that we could not control which Gd complex was to be used. The properties of the contrast agent itself are likely to influence the efficacy of paramagnetic relaxation of metabolite protons. These differences may occur due to different complex charges and/or different chelate structures, which would prevent or facilitate the Gd coming closer to the metabolite protons. The 3 more commonly used contrast agents, which were also used in our study—GD-DTPA, GD-DOTA, and GD-DTPA-BMA—have different charges and molecular structure. GD-DTPA and GD-DTPA-BMA are structurally analogous, though they possess different overall charges, −2 in GD-DTPA and 0 in GD-DTPA-BMA. The complex GD-DOTA has an overall charge of −1 but has a macrocyclic structure, which makes a direct interaction much more difficult compared with the open structures of DTPA and DTPA-BMA.¹⁷ In an in vitro study, Murphy et al¹⁷ found that these 3 Gd agents are all able to efficiently enhance the relaxation of the Cho methyl protons, which have a positive overall charge, with GD-DTPA being more efficient in enhancing the relaxation of Cho than the other 2 paramagnetic complexes, with a clear relationship between relaxivity and the ionic charge of the metabolite and of the contrast agent.