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Perfusion-Weighted Magnetic Resonance Imaging in The » Rev Bras Neurol, 46 (2): 29-36, 2010 Perfusion-weighted magnetic resonance imaging in the evaluation of focal neoplastic and infectious brain lesions Perfusão por ressonância magnética na avaliação das lesões focais neoplásicas e infecciosas do encéfalo Valdeci Hélio Floriano1, José Roberto Lopes Ferraz-Filho2, Antonio Ronaldo Spotti 3, Waldir Antônio Tognola4 Resumo A ressonância magnética (RM) é o método de diagnóstico por imagem de escolha na avaliação encefálica, entretanto as técnicas convencionais de RM podem apresentar limitações por fornecerem somente parâmetros qualitativos ou anatômicos. Nas últimas décadas, têm surgido novas técnicas complementares de RM que fornecem parâmetros quantitativos proporcionando informações funcionais ou metabólico-bioquímicas. A perfusão é atualmente uma destas técnicas que vem se apresentando como uma importante ferramenta na neurorradiologia. O objetivo deste artigo é apresentar uma revisão sobre o papel da seqüência de perfusão por RM na avaliação das lesões focais neoplásicas e infecciosas, únicas ou múltiplas, do encéfalo. O estudo da perfusão encefálica pode ser realizado como método complementar às técnicas convencionais de RM, permitindo o acesso aos parâmetros hemodinâmicos de uma maneira não invasiva e demonstrando o grau de angiogênese das lesões sendo, portanto, útil na diferenciação entre lesões neoplásicas e infecciosas, tumor primário e metástase única e no seguimento pós-tratamento para a diferenciação entre recidiva tumoral e radionecrose, através da demonstração da presença ou ausência de hiperperfusão. Palavras-chave: perfusão, ressonância magnética, encéfalo, neoplasias, infecção. Abstract Magnetic resonance imaging (MRI) is the gold standard method for brain assessment however the conventional MRI techniques may present limitations by only providing qualitative or anatomic parameters. Over recent decades, new complementary MRI techniques have been developed that supply quantitative parameters providing functional or metabolic-biochemical data. Perfusion-weighted imaging, one of these techniques, has become a powerful tool in neuroradiology. The goal of this article is to present a review about the role of perfusion-weighted MRI in the evaluation of solitary or multiple, neoplastic and infectious, focal brain lesions. Brain perfusion studies can be achieved as a complementary method to conventional MRI techniques to provide hemodynamic parameters using a non-invasive technique. This method demonstrates the degree of angiogenesis of lesions and is thus useful in the differentiation between neoplastic and infectious lesions, primary tumors and solitary metastases and in the post-treatment follow up to differentiate between tumoral recurrence and radionecrosis by identifying the presence or absence of hyperperfusion. Keywords: perfusion, magnetic resonance imaging, brain, neoplasms, infection. 1 MD. Radiologist, Department of Medical Imaging, and master2 s candidate Health Sciences Program of the Faculdade de Medicina de São José do Rio Preto (FAMERP), São José do Rio Preto, São Paulo, Brazil. 2 MD, MSc. Assistant professor of Neuroradiology, Department of Medical Imaging, Faculdade de Medicina de São José do Rio Preto (FAMERP), São José do Rio Preto, São Paulo, Brazil. 3 MD, PhD. Adjunct professor of Neurosurgery, Department of Neurological Sciences, Faculdade de Medicina de São José do Rio Preto (FAMERP), São José do Rio Preto, São Paulo, Brazil. 4 MD, PhD. Adjunct professor of Neurology, Department of Neurological Sciences, Faculdade de Medicina de São José do Rio Preto (FAMERP), São José do Rio Preto, São Paulo, Brazil. Faculdade de Medicina de São José do Rio Preto - FAMERP. Departamento de Imagem Av. Brigadeiro Faria Lima, 5544 São José do Rio Preto, São Paulo, Brasil CEP 15090-000 Tel. (+ 55 17) 3201-5743 Revista Brasileira de Neurologia » Volume 46 » No 2 » abr - mai - jun, 2010 » 29 Floriano VH, Ferraz-Filho JRL, Spotti AR, Tognola WA Introduction The DSC MRI technique, first described by Rosen 45 Magnetic resonance imaging (MRI) is the gold et al. in 1991, uses rapid image sequence acquisition standard method in brain assessment. However, the that can be T1 or T2-weighted, each one of which conventional MRI techniques may present limitations by presents with specific advantages and disadvantages. The 1,16,18,44 only providing qualitative or anatomic parameters . T2-weighted DSC technique uses spin-echo (SE) or The main limitations include the difficulty to identify gradient-echo (GRE) sequences, with the latter being the exact limits of the lesion, the degree of malignancy denominated T2*. Spin-echo sequences have the and the differentiation between tumoral recurrence and advantage of diminishing artifacts at brain-bone and 11,16 radionecrosis . Over recent decades, new brain-air interfaces and is more sensitive to signal complementary MRI techniques have been developed changes of the magnetic contrast agent passing through that supply quantitative parameters providing functional small vessels including capillaries. Its main disadvantage 1,35,58 or metabolic-biochemical data . Perfusion-weighted is the necessity of a larger dose of magnetic contrast imaging (PWI), one of these techniques, has become a agent to determine similar signal alterations when powerful tool in neuroradiology. compared to GRE sequences. The T1-weighted DSC Perfusion-weighted imaging may be performed as technique has the advantage of requiring a smaller dose a complementary examination to conventional MRI of magnetic contrast agent and provides a better techniques, giving non-invasive access to brain temporal resolution however its main disadvantage is hemodynamic parameters and demonstrating regional that the extravasation of contrast due to blood-brain 15,48 barrier (BBB) disruption may lead to errors in variations of microcirculation by mapping . Cerebral 16,30,37,42,60 perfusion is defined as the volume of blood that passes measurements of hemodynamic parameters . through a determined tissue mass over a unit of The disruption of the BBB, frequently observed in 11,30 time . In PWI, however, the term “perfusion” includes malignant tumors, with consequent extravasation and several hemodynamic measurements such as the cerebral recirculation of the contrast, is a limitation of the blood volume (CBV), cerebral blood flow (CBF) and perfusion method due to the T1 shortening effect that 11,39,42,58 16 the mean transit time (MTT) . causes an underestimation of the CBV . There is thus, in these cases, the necessity to correct this effect so as The clinical applications of PWI in the evaluation 6 of focal brain lesions include the differentiation between not to underestimate the values of CBV . One manner neoplastic and infectious lesions, primary tumors and to reduce it is to utilize a gamma-variate function on solitary metastasis and, in the post-treatment follow up, the measured signal intensity versus time curve which the differentiation of tumoral recurrence and provides the different parameters related to relative 9,15,26,29,42,59 radionecrosis . hemodynamics. However, this analysis does not give an The goal of this article is to present a review about absolute value of CBV, but provides a relative measurement; the relative CBV (rCBV) is proportional the role of perfusion-weighted MRI in the evaluation 9,11,16,30,34,35 of solitary or multiple, neoplastic and infectious, focal to the area under the corrected curve . brain lesions. The main contraindication of techniques that employ paramagnetic gadolinium-based contrast agents, is their Technical considerations association with the development of nephrogenic Basically there are two MRI techniques for the systemic fibrosis in patients with advanced renal failure evaluation of cerebral hemodynamics: dynamic (glomerular filtration rates of less than 30 mL/min/ 2 7,47 susceptibility contrast-enhanced (DSC), achieved with 1.73m ), in particular dialysis patients . a bolus injection of magnetic contrast agent (para- or Although DSC MRI is the technique most commonly superparamagnetic) that, on passing through the capillary employed in the clinical practice, the ASL technique has network, determines transitory alterations in the magnetic demonstrated promising results in clinical diagnosis. The field of the cerebral tissue measured by means of production of perfusion images with ASL, contrary to variations in the signal intensity, and the arterial spin labeling the techniques that use exogen contrast agents, is totally (ASL) technique which uses manipulation of the non-invasive; its contrast agent is diffusible and has magnetization of the water molecules present in the blood greater temporal resolution, reproducibility and 31,55 to obtain an endogen marker of the blood flow . measuring capacity. The limitations originate, mainly, 30 » Revista Brasileira de Neurologia » Volume 46 » No 2 » abr - mai - jun, 2010 Perfusion-weighted magnetic resonance imaging in the evaluation of focal neoplastic and infectious brain lesions from its low signal-to-noise ratio, accentuated sensitivity Although several authors have demonstrated to movement artifacts and the long time needed for significant correlations between the rCBV and the degree 12,18,29,53 the acquisition of images, which have been the impetus of malignancy of glial tumors , the increase in for research and development of different resources in the vascularization does not necessarily signify 11 an attempt to improve and, consequently, widen the malignancy . The best example is the oligodendro-
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