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Year : 2002  |  Volume : 12  |  Issue : 1  |  Page : 51-58
Magnetization transfer MR imaging in central nervous system infections


Department of Radio-Diagnosis, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India

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Keywords: Magnetisation Transfer(MT), Magnetisation transfer ratio (MTR), Central Nervous System Infections

How to cite this article:
Gupta R. Magnetization transfer MR imaging in central nervous system infections. Indian J Radiol Imaging 2002;12:51-8

How to cite this URL:
Gupta R. Magnetization transfer MR imaging in central nervous system infections. Indian J Radiol Imaging [serial online] 2002 [cited 2019 Jul 24];12:51-8. Available from: http://www.ijri.org/text.asp?2002/12/1/51/28417

   Introduction Top


Magnetization transfer (MT) is a unique contrast mechanism in magnetic resonance (MR) imaging that has been known for the past decade. Magnetization transfer contrast (MTC) is most useful in two basic areas, improving image contrast and tissue characterization. MTC has also proven to be extremely useful in the reduction of background signal in MR angiography and improves the appreciation of tissue enhancement by intravenous contrast agents. Today MT is accepted as an additional way to generate unique contrast in MRI that can be used to our advantage in a variety of clinical applications [1],[2]. Various researchers have studied the utility of MT in central nervous system pathology and appear to be establishing as a useful diagnostic tool in characterization of a variety of central nervous system (CNS) infections. The role of MT has been studied in the detection and diagnosis of meningitis, encephalitis, CNS tuberculosis, neurocysticercosis and brain abscess. In this article we review the clinical applications of MTC in the evaluation of the CNS infections.

Physical Basis of Magnetization Transfer

The signal obtained in clinical MR imaging comes from mobile protons. These are largely present in free water with a smaller contribution from lipid. There are, in addition, a large number of protons in tissues contained in macromolecules or in water bound to these macromolecules. No signal is normally detected from these protons by standard clinical imaging techniques because they have a very short T2 value (of the order of 1 ms or less) and any transverse magnetization is rapidly dephased before data collection is possible. There is, however, a constant exchange of magnetization between the protons in these two pools, the free pool and the bound pool, which occurs by through-space dipole-dipole interactions or probably less importantly direct chemical exchange. By means of this interchange, the bound pool influences the signal obtained from the free pool even though the bound pool cannot be directly visualized. In MT imaging, the normal equilibrium between free and bound pools is perturbed and the resulting contrast is referred to as magnetization transfer contrast and indicates the exchange processes with the bound pool in that particular tissue. Wolff and Balaban[3] first produced in vivo images with MTC and described two-pool concept of magnetization transfer. They coined the terms free (Hf) and restricted (Hr) proton pools to describe the exchange compartments. These authors perturbed the normal equilibrium between free and bound pools by selectively saturating the bound pool [Figure - 1].

Selective saturation: It is possible to saturate the macromolecular spins (i.e. to reduce their magnetization to zero) preferentially using radio frequency pulse. The restricted motion of protons results in a very short spin-spin relaxation rate (T2). This results in a very broad absorption line shape (20-40 kHz) than the mobile spins (narrow water resonance peak of 15Hz), making them as much as 106 times more sensitive to an appropriately placed radio frequency pulse. The saturated macromolecular spins, with zero magnetization will exchange for water spins having magnetization of one. Thus, when the magnetization of the water spins is subsequently measured, it will be found to be less than one. The absolute reduction in magnetization will clearly depend, on the rate of exchange between the two spin populations, and hence can be detected with MR imaging.

MT Contrast: The reduction of magnetization in the water spins will be manifest as less signal, or less brightness on MR image, when compared to the "control" image obtained without the MT preparation. The decrease will be larger in regions where exchange of magnetization is more efficient, determined by the relative proportion of the hydrogen atoms of the two pools, their intrinsic relaxation times and the exchange rate. Although saturation is never perfectly selective in vivo, contrast between areas exhibiting varying degrees of MT effect is developed and superimposed upon the intrinsic contrast of the baseline image, be it proton-density weighted (W) image, T1-W image, or some combination.

Saturation transfer techniques: Three techniques for saturating the bound water have been studied: (a) off-resonance (radio frequency pulses applied at a frequency that is offset from the "free" water resonance) continuous wave excitation [3], (b) off-resonance shaped pulses, and (c) on-resonance binomial pulses[4],[5]. The off-resonance shaped pulses are now the commonest saturation transfer techniques for clinical imaging[6].

Magnetization transfer ratio: It is a quantitative measure of the MT effect on tissues. It is the degree of signal suppression of a given tissue compared with the conventional PD or T1-W image. The MT ratio (MTR) may be simply obtained by collecting a pair of identical images (PD or T1-W), one with and one without MT saturation.. For each region of interest (ROI), MTR is calculated from the two images using the formula:

MTR = {1 - Ms }x 100%

Mo

Where Mo, Ms represent the signal intensity with the saturation pulse off and on, respectively.

Controlling parameters in saturation transfer: There are several key parameters in the off-resonance saturation transfer technique that can alter the quantitative and qualitative MTC. The MTR measured depends upon the degree of saturation of the bound water, and the degree of the direct saturation of the signal of the mobile protons and the exchange rate. The principle determinants being the base sequence used, characteristics of the saturating pulse (pulse type, power, duration, time between pulses, duty cycle i.e. number of pulses per TR, bandwidth, its effective flip angle), offset frequency of the saturating pulse, and field strength [6]. All of these characteristics alter the degree of saturation obtained and must be known before quantitative comparisons can be made between clinical studies. Numerical values may be given, or the information displayed as a difference image.

Clinical Applications

Meningitis

MR imaging has significantly increased the sensitivity over computed tomography (CT) in the detection and evaluation of complications of meningeal disease. Recently, post-contrast T1-W MT imaging has improved the sensitivity of detection of meningitis [7],[8],[9]. Runge et al [9] found that the inflamed meninges were not visible on pre-contrast T1-W images in experimental animals in whom pyogenic meningitis had been introduced. The sensitivity of detection of abnormal meningeal enhancement on conventional T1-W images was significantly increased with the use of triple dose of contrast agent compared to the routine single dose of 0.1 mmol GD-DTPA. The similar sensitivity of detection of abnormal meningeal enhancement was achieved with the single dose of contrast agent with concurrent use of MT imaging. In the study by Gupta et al[10], the abnormal meninges were distinctly visible as periparenchymal hyperintensity on pre-contrast T1-W MT images in all the cases of tuberculous meningitis. These meninges were not seen or, were barely visible on conventional spin-echo (CSE) images, and enhanced on post-contrast T1-W MT images [Figure - 2]. They even found that inflamed meninges in tuberculous meningitis show significantly different MT ratio than those of inflamed meninges in meningitis of non-tuberculous origin. The MT ratio from the thickened meninges of tuberculous meningitis was significantly lower (19.49 ± 1.22) than that from the meninges in cryptococcal (27.2 ± 1.7) and pyogenic disease (30 ± 0.17) and significantly higher in the meninges in viral meningo-encephalitis (8.2 ± 0.8). The difference in the MT ratio of meningitis stemming from different causative agents could be due to the difference in the amount of protein in the exudates.

Encephalitis

The role of MT MR imaging has been studied in herpes simplex virus (HSV encephalitis [11] and HIV encephalitis [12]. MR imaging is more specific than CT in identifying the HSV-1 encephalitis lesions, which show characteristic distribution with involvement of the temporal lobes and the orbital surfaces of the frontal lobes, which may extend to the insular cortex, cerebral convexity, and posterior occipital cortex. The basal ganglia tend to be spared. The post-contrast T1-W images obtained with MT saturation showed greater central nervous system involvement than was apparent on the CSE images [11], specifically, at delineating generalized meningeal enhancement as well as focal areas of brain involvement not seen on non-contrast T2-W images or conventional post-contrast T1-W images [Figure - 3].

MR imaging in HIV encephalitis shows diffuse brain atrophy as the most common finding. Bilateral symmetric, often periventricular lesions are the next most common finding seen as hyperintense signal on T2-W images [13]. These lesions may be mimicked by the lesions of progressive multifocal leucoencephalopathy (PML). Differentiation between the two is critical as the prognosis is much better for the HIV encephalitis. By determining the MTR it may be possible to differentiate these two entities with HIV encephalitis lesions having significantly higher MTR values (40% ± 3.8) compared to MTR of PML lesions (22% ± 2.3). MTR may also suggest the HIV encephalitis involvement of normal appearing white matter, with involved white matter showing lower MTR (44% ± 2.6) compared to normal white matter in control subjects (47% ± 2.3) [12].

Tuberculomas

CNS tuberculosis manifests as leptomeningeal and/or parenchymal involvement, which usually presents as either solitary or multiple tuberculoma formation. CT and MR imaging are the main techniques used in its localization and characterization [14],[15],[16]. The MR imaging features are not specific and overlap with other intra-cranial diseases, such as cysticercosis, metastases, and primary brain neoplasm. The MR imaging features of the individual tuberculoma depend on whether the granuloma is non-caseating, caseating with a solid center, or caseating with a liquid center [17],[18]. The non-caseating granuloma usually is hypointense relative to brain on T1-W images and hyperintense on T2-W images and, shows homogenous post-contrast enhancement. The caseating granuloma with solid caseation appears relatively hypointense or isointense on T1-W images with solid caseation appears relatively hypointense or isointense on T1-W images and isointense to hypointense on T2-W images [Figure - 4]a,b. The granulomas with central liquefaction of the caseous material appear centrally hypointense on T1-W image and hyperintense on T2-W images with a peripheral hypointense rim on T2-W images. Gd-DTA enhanced T1-W images show rim enhancement in caseating granulomas. Pre-contrast T1-W MT imaging helps to better assess the disease load in CNS tuberculosis by improving the detectability of the lesions, with more number of tuberculomas detected on pre-contrast MT images compared to routine SE images. These T2 invisible tuberculomas appear as hyperintense lesions on pre-contrast MT images [10]. Detection of CSE invisible tuberculomas on MT images is the result of the lower transfer of magnetization in tuberculomas as compared with surrounding brain parenchyma. Recently, MT imaging study of the intracranial T2 hypointense tuberculomas with histopathology correlation has been published [19]. On T1-W MT imaging these granulomas have a hypointense core and a hyperintense rim [Figure - 4]c. On histopathology, the T2 and MT hypointense central core matched with the solid caseation while the MT hyperintense rim (usually not seen on T2-W images as it merges with surrounding edema) showed variable amounts of cellular infiltrate, granulomas, Langhan's giant cells and gliosis/fibrosis. The MT ratio is also different between the inner core and outer rim (lower MTR from the outer rim than the inner core). It may also be possible to differentiate T2 hypointense tuberculoma from T2 hypointense cysticercus granuloma with the use of MTR, as cysticercus granulomas show significantly higher MT ratio compared to tuberculomas [10].

Neurocysticercosis

MR imaging manifestations of neurocysticercosis vary with the stage of the disease. Four stages being described [20-22]: live innocuous cyst, early degenerating stage, cysticercus granuloma and healing lesions. MT MR imaging in neuro cysticercus lesions [23] showed that T1-W MT imaging improved lesion detectability and the lesions that were "CSE invisible" were detected. These lesions appeared as central hypointense core with peripheral hyperintensity. A small number of healing lesions (T2 hypointense) were not visible on T1-W MT images and these were all located in white matter. Remaining lesions visible on CSE images were also visible on T1-W MT images. They also calculated MTR from different regions of the lesions in different stages of evolution. Maximum MTR was calculated from healing lesions and from the core of SE invisible lesions while innocuous lesions showed almost no magnetization transfer. The visibility of a lesion on T1-W MT sequence depended on its MTR and its location in the cerebral hemisphere (cortical gray matter, white matter or deep gray matter). The visibility of the lesion is decided by the difference in the MTR of the lesion and of the surrounding brain parenchyma.

T1-W MT images are also important in demonstrating perilesional gliosis in treated neuro cysticercus lesions. On MRI gliosis is observed in a small number of patients with epilepsy and appears as hyperintense on T2-W images. Gliosis is seen in a large number of patients on histopathology as opposed to what is seen on T2-W MR images. The use of T1-W MT imaging improves detection of perilesional gliosis not visible on CSE MR imaging, and is seen as hyperintensity around the lesion [Figure - 5]. Gliotic areas show low MTR compared to the gray matter and white matter. The demonstration of gliosis seems to have practical importance in the management of epilepsy in patients with solitary cysticercal cyst in brain. Its incidence is higher in those patients in whom the primary lesion has not disappeared completely after albendazole therapy and the patients with gliosis may have seizures that could be difficult to control with a single anti-epileptic drug and may have a higher incidence of seizure recurrence after withdrawing anti-epileptic drug therapy [24],[25].

Brain abscess

The neuropathologic progression of abscess formation in the brain is divided into four stages: early cerebritis (days 1-3), late cerebritis (days 4-9), early capsule stage (days 10-13) and late capsule stage (days 14 and later) [26]. Runge et al [27], evaluated the role of T1-W MT images in early brain infection (early and late cerebritis stage) using a canine model and found that use of MT along with high-contrast dose improved visualization of lesion enhancement. Quantitative MT imaging may help in differentiating cystic tumors and infarction from infection, with cystic centers of infection having a significantly higher MTR than infarct and cystic tumors[28].

MR imaging features are nonspecific with respect to the causative organism for patients with brain abscess [Figure - 6]. For management purposes it is important to differentiate pyogenic from tuberculosis brain abscesses. Gupta et al [29] studied the role of MT MR imaging and in vivo proton MR spectroscopy (MRS) in differentiating these two entities. They found that amino acids detected in the core of pyogenic brain abscesses on in vivo proton MRS were not seen in tuberculous abscesses. The wall of the tuberculous abscess contains large numbers of mycobacteria inside and outside the cells, along with chronic nonspecific inflammatory cells, while the wall of the pyogenic abscess contains inflammatory cells. The tubercle bacilli are rich in lipids and their presence in large numbers is probably responsible for the significantly lower MT ratio (19.89 ± 1.55) from tuberculous abscess wall compared to the MT ratio from the wall of the pyogenic abscess (25.56 ± 1.61). MT MR imaging may also differentiate tuberculous brain abscess from caseating tuberculomas. The hypointense peripheral rim on T2-W images for tuberculous abscess appears hyperintense on T1-W MT images, whereas for tuberculomas, a hyperintense rim on T1-W images surrounds the T2 hypointensity.


   Conclusion Top


We feel that MTC is an important technique for detection and diagnosis of various CNS infections and should form a part of routine diagnostic imaging protocol in cases suspected of infective disease of the CNS on clinical basis.


   Acknowledgement Top


Sanjeev Chawla acknowledges the financial assistance from Council of Scietific and Industrial Research, New Delhi, India.

 
   References Top

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Correspondence Address:
R Gupta
Department of Radio-Diagnosis, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow
India
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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6]

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