Morphological and functional MRI, MRS, perfusion and diffusion changes after radiosurgery of brain metastasis

Abstract

Radiosurgery is a noninvasive procedure where spatially accurate and highly conformal doses of radiation are targeted at brain lesions with an ablative intent. Recently, radiosurgery has been established as an effective technique for local treatment of brain metastasis. After radiosurgery, magnetic resonance (MR) imaging plays an important role in the assessment of the therapeutic response and of any complications. The therapeutic approach depends on the imaging findings obtained after radiosurgery, which have a role in the decision making to perform additional invasive modalities (repeat resection, biopsy) to obtain a definite diagnosis and to improve the survival of patients. Conventional MR imaging findings are mainly based on morphological alterations of tumors. However, there are variable imaging findings of radiation-induced changes including radiation necrosis in the brain. Radiologists are sometimes confused by radiation-induced injuries, including radiation necrosis, that are seen on conventional MR imaging. The pattern of abnormal enhancement on follow-up conventional MR imaging closely mimics that of a recurrent brain metastasis. So, classifying newly developed abnormal enhancing lesions in follow-up of treated brain metastasis with correct diagnosis is one of the key goals in neuro-oncologic imaging. To overcome limitations of the use of morphology-based conventional MR imaging, several physiological-based functional MR imaging methods have been used, namely diffusion-weighted imaging, perfusion MR imaging, and proton MR spectroscopy, for the detection of hemodynamic, metabolic, and cellular alterations. These imaging modalities provide additional information to allow clinicians to make proper decisions regarding patient treatment.

Introduction

Radiosurgery has been widely used to treat meningiomas, schwannomas, pituitary adenomas, metastatic brain tumors, malignant gliomas and other neurological diseases as an alternative method in place of surgical resection [1], [2], [3], [4], [5], [6], [7]. Follow-up imaging studies are needed to assess the results of radiosurgery and to detect any complications. The introduction of magnetic resonance (MR) imaging has revolutionized the clinical effectiveness of radiosurgery by improving the identification and characterization of target tissues by precisely characterizing treatment response. The role of MR imaging in assessing the response to radiosurgery in post-treatment has been studied for variable diseases [8], [9], [10], [11], [12], [13]. Conventional MR imaging is an anatomy-based procedure and morphologic alterations are the key to resolve abnormal findings on follow-up imaging studies after radiosurgery. However, the histological response of brain normal tissue and tumors to radiosurgery has confounded correct follow-up evaluations. Recurrent tumors and radiation-induced changes often appear at the same location within or near the region of the irradiated area, and both occurrences resemble contrast-enhancing, expansile brain lesions surrounded by edema. It is difficult with the use of conventional MR imaging to assess abnormal imaging findings after radiosurgery as lesions that are related to a residual or recurrent brain tumor, or as lesions that are related to non-tumorous, radiation-induced changes [14]. Thus, an accurate diagnosis of radiation-induced changes on MR imaging has been challenging. If a lesion deviates from usual conventional MR imaging findings on follow-up, additional information is required to make a correct diagnosis using functional MR imaging. Currently, diffusion-weighted imaging (DWI), perfusion-weighted imaging, and proton MR spectroscopy are used as physiological-based functional MR imaging methods. Diffusion-weighted imaging (DWI) is based on the detection of changes in the random motion of protons in water, and its use enables the characterization of tissues and pathological processes at a microscopic level [15], [16]. Perfusion-weighted imaging is an important tool to measure the degree of brain tumor angiogenesis and capillary permeability, both of which are major biological markers of malignancy, grading, and prognosis, especially for gliomas [17], [18], [19], [20]. MR spectroscopy can provide a noninvasive window to analyze neurometabolism. Pattern analysis of MR spectroscopy data can be successfully applied to detect metabolic alterations of brain tumors and other non-neoplastic lesions [21], [22].

This review provides a comprehensive overview of the radiation biology to understand MR imaging changes. In addition, this review evaluates conventional and functional MR imaging after radiosurgery of metastatic brain tumors in order to obtain more accurate follow-up results and ultimately to improve patient clinical outcome.