Comparison of perfusion, diffusion, and MR spectroscopy between low-grade enhancing pilocytic astrocytomas and high-grade astrocytomas

Abstract

Background and purpose: The differentiation of pilocytic astrocytomas and high-grade astrocytomas is sometimes difficult. There are limited comparisons in the literature of the advanced MR imaging findings of pilocytic astrocytomas versus high-grade astrocytomas. The purpose of this study was to assess the MR imaging, PWI, DWI, and MR spectroscopy characteristics of pilocytic astrocytomas compared with high-grade astrocytomas.

Materials and methods: Sixteen patients with pilocytic astrocytomas and 22 patients with high-grade astrocytomas (8-66 years of age; mean, 36 ± 17 years) were evaluated by using a 1.5T MR imaging unit. MR imaging, PWI, DWI, and MR spectroscopy were used to determine the differences between pilocytic astrocytomas and high-grade astrocytomas. The sensitivity, specificity, and the area under the receiver operating characteristic curve of all analyzed parameters at respective cutoff values were determined.

Results: The relative cerebral blood volume values were significantly lower in pilocytic astrocytomas compared with the high-grade astrocytomas (1.4 ± 0.9 versus 3.3 ± 1.4; P = .0008). The ADC values were significantly higher in pilocytic astrocytomas compared with high-grade astrocytomas (1.5 × 10(-3) ± 0.4 versus 1.2 × 10(-3) ± 0.3; P = .01). The lipid-lactate in tumor/creatine in tumor ratios were significantly lower in pilocytic astrocytomas compared with high-grade astrocytomas (8.3 ± 11.2 versus 43.3 ± 59.2; P = .03). The threshold values ≥1.33 for relative cerebral blood volume provide sensitivity, specificity, positive predictive values, and negative predictive values of 100%, 67%, 87%, and 100%, respectively, for differentiating high-grade astrocytomas from pilocytic astrocytomas. The optimal threshold values were ≤1.60 for ADC, ≥7.06 for lipid-lactate in tumor/creatine in tumor, and ≥2.11 for lipid-lactate in tumor/lipid-lactate in normal contralateral tissue.

Conclusions: Lower relative cerebral blood volume and higher ADC values favor a diagnosis of pilocytic astrocytoma, while higher lipid-lactate in tumor/creatine in tumor ratios plus necrosis favor a diagnosis of high-grade astrocytomas.

Fig 1.

Comparison of rCBV between PA and HGA on PWI. A and B, Solid-cystic PA in the left cerebellar hemisphere. A, Contrast-enhanced axial T1-weighted imaging demonstrates intense homogeneous enhancement in the solid portion of the tumor and lack of enhancement of the walls of the cystic portion. B, PWI, with an rCBV color overlay map, shows low rCBV on the solid portion of PA. C and D, GBM in the left frontoparietal lobe. C, Contrast-enhanced axial T1-weighted imaging demonstrates ring and irregular peripheral enhancement with central necrosis. D, PWI with an rCBV color overlay map shows increased rCBV in the solid tumor area.

Fig 2.

DWI in PA and HGA. A and B, Solid-cystic PA in the left cerebellar hemisphere. The solid part of this tumor is hypointense on DWI (A) and hyperintense on ADC map (B) in comparison with normal brain parenchyma. C and D, The frontoparietal HGA (GBM) shows decreased water diffusion (C) in the more hyperintense solid tumor area on DWI, which is more hypointense (D) on ADC than the normal brain parenchyma and indicates greater cellularity of this GBM than is observed in the PA shown (A and B).

Fig 3.

Comparison of the Lip-Lac^tu/Cr^tu ratio on short-TE multivoxel MR spectroscopy in PA and HGA. A and C, Axial T2-weighted imaging was used for positioning of the regions of interest in the tumor. A and B, The solid homogeneous enhancing hypothalamic-chiasmatic PA has a Lip-Lac peak. The (C and D) necrotic ring-enhancing GBM located on the lateral wall of the left lateral ventricular trigone also has an elevated Lip-Lac peak (D), but the Lip-Lac^tu/Cr^tu ratio in this tumor is higher than that found in the PA (B). A and B reproduced with permission from Aragao et al.