Quantitative Sodium (23Na) MRI in Pediatric Gliomas: Initial Experience.

BACKGROUND: 23Na MRI correlates with tumor proliferation, and studies in pediatric patients are lacking. THE PURPOSE OF THE STUDY: (1) to compare total sodium concentration (TSC) between pediatric glioma and non-neoplastic brain tissue using 23Na MRI; (2) compare tissue conspicuity of bound sodium concentration (BSC) using 23Na MRI dual echo relative to TSC imaging.
METHODS: TSC was measured in: (1) non-neoplastic brain tissues and (2) three types of manually segmented gliomas (diffuse intrinsic brainstem glioma (DIPG), recurrent supratentorial low-grade glioma (LGG), and high-grade glioma (HGG)). In a subset of patients, serial changes in both TSC and BSC (dual echo 23Na MRI) were assessed.
RESULTS: Twenty-six pediatric patients with gliomas (median age of 12.0 years, range 4.9-23.3 years) were scanned with 23Na MRI. DIPG treated with RT demonstrated higher TSC values than the uninvolved infratentorial tissues (p < 0.001). Recurrent supratentorial LGG and HGG exhibited higher TSC values than the uninvolved white matter (WM) and gray matter (GM) (p < 0.002 for LGG, and p < 0.02 for HGG). The dual echo 23Na MRI suppressed the sodium signal within both CSF and necrotic foci.
CONCLUSION: Quantitative 23Na MRI of pediatric gliomas demonstrates a range of values that are higher than non-neoplastic tissues. Dual echo 23Na MRI of BCS improves tissue conspicuity relative to TSC imaging.

1. Introduction

Pediatric brain tumors are the most common cause of cancer death in infants and children [1,2]. Conventional (1H) MRI is used in the clinical setting and is currently the standard of imaging of pediatric brain tumors and includes multiple sequences, such as T2-weighting, fluid attenuation by inversion recovery (FLAIR), and post gadolinium-based contrast agent (GBCA) T1 sequences, each offering a slightly different tissue sensitivity, but relatively little physiological specificity. Although advanced 1H MRI approaches (including diffusion, perfusion and permeability) have been proposed for tumor characterization, precise morphometric measurement can still be inaccurate. There are many tumors that do not demonstrate enhancement after GBCA administration, often negating the benefits of perfusion and permeability MRI, as well as more conventional post-GBCA T1 imaging and even if there is enhancement, it is not necessarily reflective of tumor progression. Just as in true progression, pseudoprogression is characterized by an apparent initial increase in the size of the tumor and contrast-enhancement; however, different from true progression, pseudoprogression is characterized by subsequent tumor shrinkage (regionally or globally). Thus, there are currently no imaging biomarkers that can reliably predict brain tumor progression or response to treatment. More accurate, and physiologically specific, imaging biomarkers are required both for diagnosis and effective monitoring of pediatric gliomas.

More advanced MRI techniques can provide added value in determining tumor response to treatment, but have similar limitations of inconsistency due to complex tumor microenvironments. Sodium concentrations, non-invasively measurable by sodium (23Na) MRI, have been shown to be markers of tumor proliferation in animal glioma models and may be useful in monitoring posttreatment responses [3,4]. Increased sodium accumulation has also been seen secondary to other biological processes, including neuroinflammation, compromised mitochondrial metabolism and Na/K ATPase dysfunction [5]. Sodium MRI has been beneficial in evaluation of acute ischemic strokes, multiple sclerosis, amyotrophic lateral sclerosis, migraines, and multiple tumor types, including outside of the central nervous system. Some of these disease processes have demonstrated increased sodium accumulation secondary to neuroinflammation, compromised mitochondrial metabolism and Na/K ATPase dysfunctions. In adult CNS tumors, 23Na MRI has provided additional information related to cellular metabolism, complementary to standard proton (1H) MRI [6,7].

To our knowledge, 23Na MRI has not previously been evaluated in pediatric brain tumors. The primary aim of this study was to evaluate the feasibility of sodium MRI in measuring total sodium concentration (TSC) in both uninvolved brain tissues and three types of pediatric gliomas (DIPG, LGG, HGG). Our secondary aim was to determine the added value of imaging bound sodium concentration (BSC) as measured with the dual echo 23Na MRI, with respect to tissue conspicuity. The dual-TE imaging of BSC not only provides improved imaging conspicuity of lesions by suppressing the high CSF signal seen with single TE acquisition, but also is more reflective of intracellular sodium concentration. We quantitated serial changes in TSC and BSC, by utilizing voxel based, parametric mapping techniques.

2. Materials and Methods

2.1. Participants

Study participants were recruited from patients who were participating in glial-associated antigen peptide vaccine trials through the Pediatric Neuro-Oncology Clinic (RJ) at the Children’s Hospital of Pittsburgh (Supplemental Table S1).

2.2. Sodium MR Imaging Acquisition

This study was approved by the institutional ethics committee and informed consent was obtained in all cases.

2.3. Sodium MR Imaging Acquisition

23Na MRI images were acquired on a 3T Siemens Scanner (TIM Trio, Siemens AG, Erlangen, Germany) with a dual-tuned (1H-23Na) head volume coil (Advanced Imaging Research, Cleveland, OH, USA). A custom-developed pulse sequence, twisted projection imaging (TPI) [8], was used to acquire total sodium imaging data in all subjects with the following optimized single echo technique: FOV = 220 mm, matrix size = 64 × 64 × 64, voxel size = 3.44 mm (3D isotropic), TE = 0.5 ms, TR = 100 ms, averages = 4, and total acquisition time (TA) = 10 min 38 sec. For a subset of patients, both total sodium and bound sodium imaging was calculated using a two-TE technique which was developed and implemented later in the study [9]: FOV = 220 mm, matrix size = 64 × 64 × 64, voxel size = 3.44 mm (3D isotropic), TE1/TE2 = 0.5/5 ms, TR = 100 ms, averages = 4, and TA = 10 min 38 sec for each TE.

2.4. Post-Processing and Quantitative Sodium MR Imaging

Baseline analysis: intensity of the TE1 and short-T2 images were linearly calibrated using the CSF region (TSC = 145 mM) and the noise-only background (TSC = 0 mM). Please refer to Appendix A for details about the calculation of sodium signal and additional serial analysis approach. The region of interest (ROI) quantification for normal-appearing grey matter (GM) and white matter (WM) for both TE1 and short-T2 images was performed. Follow-up sodium images were registered to the anatomical T2 or FLAIR proton images using 6 degrees of freedom rigid body transformation in medical imaging processing, analysis, and visualization (MIPAV). Intensity of all sodium images was normalized relative to that of vitreous fluid. Sodium concentration was then measured in the CSF, vitreous, uninvolved GM, WM, brainstem, and background noise, using uniform ROIs placed in these regions across the participants to ensure sodium measurements were derived from a consistent volume. The primary site of tumors on sodium images were identified and manually segmented with the aid of the regular proton images.

2.5. Statistical Analysis

For the primary analysis, analysis of variance (ANOVA) was used to test differences in the total sodium concentration change among the tumor types at the primary lesion ROIs for the three groups. Additional ANOVA was performed among these groups for each of the uninvolved tissue regions (vitreous, CSF, grey matter, and white matter) (Table 1). Linear regression was used to analyze the relationship between total sodium concentration in these regions and the age of the participant. An exploratory univariate regression analysis was performed to correlate serial changes in TSC (mM) with change in tumor volume (mm3) as measured on concurrent conventional MRIs. An exploratory agreement analysis between the serial change in TSC (mM) and conventional MRI response was calculated by performing a Cohen’s kappa. Possible sex-based difference in sodium concentration was also examined using a t-test on the mean TSC from the aforementioned ROIs. The false discovery rate controlling procedure method was used to correct for family-wise error in multiple comparisons.

Table 1

ANOVA of 23Na of Uninvolved Structures and Pediatric Glioma.

	N-Used	R2	Coeff	DoF	F-Value	p-Value *
CSF	26	0.0448	14.8171	2/25	0.54	0.5901
Vitreous Humor	26	0.0351	14.2010	2/25	0.42	0.6634
GM	26	0.2611	11.6462	2/25	4.06	0.0308 *
WM	26	0.1803	12.5907	2/25	2.53	0.1016
Tumor	26	0.4926	24.1698	2/25	11.17	0.0004 *

* Corrected for multiple comparison with FDR procedure.

3. Results

3.1. Clinical Characteristics

A total of 26 participants with gliomas were included in the cohort with a median age of 12.0 years; range 4.9–23.3 years; 14 males (a 20+ year old patient was initially diagnosed with a pediatric glioma as a pediatric patient and continued monitoring into adulthood, thus was recruited into this study): DIPG (n = 11, median age of 8.8 years), recurrent supratentorial LGG (n = 6, median age = 16.5 years) and recurrent supratentorial HGG (n = 9, median age = 12.7 years) (Flow chart of participant recruitment displayed in Supplemental Figure S1).

3.2. Quantitative Total Sodium Concentration of Normal Brain and Brain Tumors

Total sodium concentrations (mean ± SD) were as follows: uninvolved (non-neoplastic) tissue: CSF = 126.6 ± 11.3 mM, vitreous fluid = 99.8 ± 14.9 mM, grey matter = 58.9 ± 5.7 mM, white matter = 52.5 ± 4.7 mM, and normal brainstem = 24.90 ± 4.5 mM (Figure 1 left). An inverse relationship between TSC and age was observed in the GM (R = 0.53207; p-value = 0.0035), and WM (R = 0.50744; p-value = 0.0061) (Supplemental Figure S2) (Supplemental Table S2). Compared to uninvolved brainstem (measured from portions of the uninvolved pons in participants with supratentorial LGG and HGG), DIPG s/p RT had significantly higher total sodium concentrations (p < 0.0001) (Figure 1, right; examples in Figure 2). Among participants with recurrent LGG and HGG, the total sodium concentration in the tumors were compared to their own uninvolved WM and GM measurements. For participants with recurrent supratentorial LGG and HGG, both groups exhibited higher total sodium concentrations compared to their uninvolved WM and GM (p = 0.0005 WM and p = 0.0011 GM for LGG) and (p = 0.0042 for WM and p = 0.01781 for GM for those with HGG). (Figure 1, right).

Figure 1

Total sodium concentration (TSC) of the uninvolved cerebral tissue, vitreous humor, and cerebrospinal fluid (CSF), against tumor type. T-test comparing tumor grade ** = p < 0.05; *** p < 0.001. GM = grey matter, WM = white matter, DIPG = diffuse infiltrating pontine glioma, LGG = low-grade glioma, HGG = high-grade glioma.

Figure 2

Sodium MRI images demonstrating relative total sodium concentration (TSC) between three different types of pediatric gliomas. There is reduced relative TSC in the pediatric diffuse intrinsic pontine glioma (DIPG) compared to the supratentorial low-grade and high-grade gliomas. There is no difference in relatively high TSC between the low-grade and high-grade gliomas (arrows point to the tumors).

3.3. Serial Quantitative Intra-Tumoral Total Sodium Concentration of Pediatric Glioma

Eight participants underwent serial 23Na MRI (total of 20 exams) to measure intratumoral TSC (Figure 3, Supplemental Figures S3 and S4) (Table 2). The eight patients included one with recurrent LGG, four with recurrent supratentorial HGG, and three with DIPG following the completion of RT. Six of these participants underwent only one follow-up sodium scan, whereas two patients with HGG underwent two to four follow-up scans. There was no change in TSC (within ± 1 SD mM) in 7/12 (58.3%) serial exams, increased in 2/12 (16.7%) serial exams, and decreased in 3/12 (25.0%) serial exams. As an exploratory analysis, we observed that although serial % change in TSC did not correlate with % change in concurrent tumor size, serial change in TSC did moderately agree with qualitative multi-modal conventional MRI treatment response assessment in an exploratory analysis (Table 2).

Figure 3

Initial (top row) and follow-up (bottom row) sodium MRI scans of supratentorial pediatric gliomas with (A) increased total sodium concentration (TSC) corresponding to tumor progression in a supratentorial high-grade glioma (arrows) (HGG), and (B) no change in TSC corresponding to a stable supratentorial low-grade glioma (arrows) (LGG).

Table 2

Serial follow-up sodium (23Na) MRI Scans in Evaluation of Pediatric Glioma Immunotherapy.

ID withScan#	Tumor Total Sodium ConcentrationTSC, mM			Change in TSC			Conventional Radiographic Response to Treatment at Time of Follow-Up Imaging	Volume (mm3)	Volume Change (Absolute in mm3/% Change)	Concordant/Discordant (between Changes on Na MRI and Conventional MRI)
	Type	Mean	Std.Dev.	Change	% Change	Direction *
4002-1	DIPG	54.5052	13.373	6.6723	12	No change	Stable, No tumor progression.	20.8
4002-2		61.1775	12.1866					25.7	4.9/19%	Concordant
4006-1	DIPG	51.1066	11.9815	2.0928	4	No change	Stable, No tumor progression	41.3
4006-2		53.1994	12.2965					46.4	5.1/11%	Concordant
4020-1	DIPG	33.7777	5.1005	22.9931	68	Increase	Stable, No tumor progression, increase in necrosis within tumor	24.7
4020-2		56.7708	14.3001					58.9	34.2/58%	Concordant
4017-1	LGG	101.9751	15.2094	−2.7367	−3	No change	Stable, No tumor progression	85.7
4017-2		99.2384	15.4893					84.6	−1.1/−1.3%	Concordant
4010-1	HGG	92.5898	12.3779				First serial scan showed decrease in size of the tumor. The second serial scan showed a decrease in size of the tumor The third serial scan showed necrosis with increase in size of the tumor as well as new enhancing lesionThe fourth serial scan showed no change in size of the tumor	163.6
4010-2		73.9051	15.4974	−18.6847	−20	Decrease		120.7	−42.9/−36%	Concordant
4010-3		60.0018	14.2077	−13.9033	−19	Decrease		45.3	−75.4/−166%	Concordant
4010-4		67.2067	20.0919	7.2049	12	No change		43.5	−1.8/−4.1%	Discordant
4010-5		62.0964	13.6286	−5.1103	−8	No change in TSC; decrease in BSC including recurrent lesion between last two dual echo scans		33.8	−9.7/−28.7%	Concordant
4023-1	HGG	49.477	3.7975	7.7313	16	Increase	Interval increase in non-enhancing edema. Infiltrating abnormal signal intensity representing tumor progression	19.2
4023-2		57.2083	4.3588					39.4	20.2/51.3%	Concordant
4025-1	HGG	47.0821	8.8937	−11.7503	−25	Decrease in TSC and decrease in BSC	Interval increase in size and enhancement of tumor representing recurrence	26.8
4025-2		35.3318	5.7061					20.4	−6.4/−31.4%	Concordant
4027-1	HGG	77.1108	13.6791			No change in TSC on both serial scans; increase in BSC in recurrent lesion between last two dual echo scans	Progressive increase in size of tumor representing tumor progression	1
4027-2		81.9984	17.2532	4.8876	6			12.9	11.9/92.2%	Discordant
4027-3		78.9372	18.4989	−3.0612	−4			48.6	35.7/73.5%	Discordant

* determined by a change in +/−1 SD TSC; BSC: volume-fraction weighted bound sodium concentration.

3.4. Quantitative Intra-Tumoral Bound Sodium Concentration of Pediatric Gliomas Including Serial Imaging

Bound sodium concentration (vBSC) was measured in five patients who underwent the two-TE imaging. We computed vBSC for uninvolved GM (13.8 ± 8.67 mM) and uninvolved WM (14.1 ± 4.49 mM) falls within the accepted range for intracellular sodium in the brain (12–15 mM) (Supplemental Table S3). Particularly for tumors near the ventricular system, the high sodium concentrations within CSF caused significant interference, which was overcome by performing subsequent evaluations using the dual-TE sodium MRI to measure BSC. The dual-TE sodium MRI suppresses the sodium signal within both CSF and necrotic foci, resulting in improved conspicuity of both non-neoplastic and neoplastic tissue located near peripheral cortex and ventricular CSF (Figure 4 and Supplemental Figure S5). Among the group of eight patients that had serial TSC measurements, three patients had serial BSC measurements (Table 2). In one case (Figure 5A), a supratentorial HGG demonstrated no significant change in TSC at one month and four months after baseline sodium imaging, however, BSC did increase and correlated with the conventional imaging of tumor progression. The patient depicted in Figure 5B demonstrated a decrease in TSC and BSC, two months after the baseline sodium MRI, which correlated with a decrease in tumor volume on concurrent conventional MRI. The same patient in Figure 5B demonstrated increased BSC in the region of evolving necrosis (Supplemental Figure S5). Another patient with a HGG (Figure 5C) had a concomitant decrease in TSC and BSC that correlated with a decrease in tumor volume (as measured on conventional MRI) nine months after sodium MRI baseline (Table 2).

Figure 4

Serial total sodium MRI images at two different axial levels from a participant with supratentorial high-grade glioma treated with immunotherapy. This time series demonstrates a decrease in TSC relative to uninvolved tissue, preceding the eventual lesion size reduction in the tumor (white arrow) as noted by fluid-attenuated inversion recovery (FLAIR) imaging (Supplemental Figure S3). Note, a separate necrotic recurrent lesion (green arrow) also depicted in the sodium vBSC images (Supplemental Figure S5).

Figure 5

Two-TE sodium MRI scans showing tumor progression in high-grade glioma (Panel (A) [pt. ID 4027]), and response to therapy in supratentorial astrocytoma (Panel (B) [pt. ID 4010]) and in high-grade glioma (Panel (C) [pt. ID 4025). In the tumor regions of the bound sodium images are pixels of vBSC with a value greater than 1 standard deviation (S.D.) from the average vBSC value (~22 mM) over the tumor.

4. Discussion

This study has demonstrated the feasibility of performing 23Na MRI of brain tumors within the pediatric population and was able to distinguish uninvolved brain tissue from neoplastic glial tissue in pediatric patients. The total sodium concentration (TSC) for CSF, vitreous, GM, and WM in our study were similar to what has been reported in the adult literature [7]. We detect high TSC in both recurrent low- and high-grade pediatric gliomas that have distinctive histologies, suggesting that 23Na MRI may have less utility in assessing baseline tumor grade in contrast to the serial assessment of therapeutic responses (future studies are warranted). 23Na MRI can assess metabolic changes in tissues, e.g., cell integrity and tissue viability with validated repeatability and reproducibility [10,11,12,13,14,15,16,17,18,19]. The sensitivity of sodium imaging stems from the tightly controlled sodium ion homeostasis in healthy tissues which maintains a large concentration gradient between intracellular sodium concentration (ISC) at 10–15 mM and extracellular sodium at 145 mM. Importantly, TSC is elevated in tumors due to increased intracellular sodium (reflecting dysfunction of Na+-K+ pumps on the cell membrane) and/or an increased proportion of extracellular space (changes in cell morphology) [20].

Total sodium concentration in brain tumors can have limitations because of the high sodium signal seen in CSF/necrosis that can mask the intra-tumoral sodium signal related to proliferation. We show the added value of measuring volume-fraction weighted bound sodium concentration (as a proxy of BSC) with dual-TE imaging, which can saturate TSC-high sodium related signal in CSF/necrotic areas, providing better conspicuity of lesions relative to non-neoplastic structures. Dual TE 23Na MRI helped mitigate this limitation and potentially enhances its ability to determine heterogeneous treatment responses.

23Na MRI is known to be a marker of tumor proliferation in animal glioma models and has shown utility in monitoring posttreatment necrosis and treatment responses in animals [3,4]. As such, we explored the relationship between quantitative serial TSC/BSC measurements and both tumor volume and radiographic response assessment in a small sample of patients. Interestingly, we observed that although serial % change in TSC did not correlate with % change in concurrent tumor size, serial change in TSC did moderately agree with qualitative multi-modal conventional MRI treatment response assessment in an exploratory analysis. In contrast to serial TSC imaging, voxel-based parametric mapping of serial change in BSC did appear to be more reflective of the tumor response assessment within individual patients (in an exploratory analysis). We show that two patients with serial BSC serial changes were concordant with TSC and tumor volume as determined by conventional proton MRI. Studies have investigated the ability to differentiate bound sodium (thought to be reflective of intracellular sodium) and extracellular sodium with various techniques [21,22]. These studies are exploratory and future studies with larger sample sizes are needed to further confirm these results.

The limitations of this study include the small number of heterogeneously treated tumor cases studied, which makes it difficult to compare between the tumor-type groups. For example, the DIPG cohort received radiation therapy 3–4 months prior to obtaining the sodium MRIs, which may cause a treatment related decrease in metabolic activity (lower TSC). There might be a partial volume effect due to the low resolution of the sodium MRI acquisitions, as well as the low SNR of 23Na imaging compared to proton imaging. However, with this limitation in mind, the analysis examined intensity of the tumor regions and not the volume measurements. Partial volume effect could potentially affect variation in intensity at the border voxels of the tumor regions, in the context of average quantities, the effect is likely within acceptable limits and will be assessed in future studies.

5. Conclusions

In conclusion, we demonstrate the feasibility of quantitatively evaluating 23Na by MRI in uninvolved brain tissue of pediatric glioma patients, with similar values to sodium concentrations seen in adults. Diffuse intrinsic brainstem gliomas post-RT and supratentorial gliomas demonstrated total sodium concentrations (TSC) greater than adjacent uninvolved brain tissue. We also show the additional benefit of dual-echo Na MRI (bound sodium concentration) to improve visualization of tumor by distinguishing it from the surrounding tissue and CSF. Future studies are needed to determine the value of 23Na MRI in delineating response to treatment in pediatric gliomas.