Diffusion weighted magnetic resonance imaging features of intracranial lesions

Transcription

1 INTERNATIONAL JOURNAL OF HEALTH RESEARCH IN MODERN INTEGRATED MEDICAL SCIENCES, ISSN (P), ISSN (O), Vol-2, Issue-4, Oct-Dec Original Article Diffusion weighted magnetic resonance imaging features of intracranial lesions Dr Bhogavalli Venkatarao 1, Dr Shirishpaulganta 2, Dr Kiranraju N 3, Dr Nasreensyed 4 Abstract Introduction: Diffusion weighted imaging (DWI) is a specialized magnetic resonance imaging technique that depends on the random movement of water molecules within and between the intracellular and extracellular spaces. Regions with restricted mobility of water molecules and in apparent diffusion coefficient (ADC) maps yield a greater DW-MRI signal appear bright. AIM: The study was performed to describe the features of intracranial lesions on diffusion weighted imaging and to correlate these features with ADC and T2 FLAIR images. Material and Methods: Study type: Descriptive observational study. Study period : September 2013 and March Study setting : Department of Radiology, ASRAM MEDICAL COLLEGE, ELURU. Study population : 115 patients detected to have intracranial lesions on DW MRI of the brain. Sample size : All the patients who are having the intracranial lesions and are willing to participate in the study within the study period. Results: A total of 115 patients with intracranial lesions were studied. The MRI was done on the advice of the referring doctor. The mean age of study population was 44 ± 2 years with a range of 3 days to 78 years.lesions of different kinds were observed and studied. Acute and chronic infarcts account for majority followed by Meningioma sans tuberculomas. DWI was noted to be superior to T2WI in detection of acute infarct. In evolution of acute stroke The ADC continues to decrease and is most reduced at 8 32 hours and markedly reduced for 3 5 days. This decreased diffusion is markedly hyperintense on DW images and hypointense on ADC images. DWI can differentiate between tumor and infection and can provide information about the cellularity of tumors thereby helping in characterization and grading of tumors. Conclusion: DWI along with ADC and T2 FLAIR images plays an important role in identifying and diagnosing the intracranial lesions. Keywords : ADC - Apparent Diffusion Coefficient, DWI - Diffusion Weighted Image, EPI - Echo Planar Image, FLAIR - Fluid Attenuation Inversion Recovery Introduction Diffusion weighted imaging (DWI) is a specialized magnetic resonance imaging technique that assesses local environment at the cellular level to determine changes in the random movement of water protons. Restricted diffusion appears as an area of increased signal on DWI and reduced signal on ADC maps which are calculated from a matrix of tensor vectors obtained in three planes without and with application of diffusion gradients (1). The amount of diffusion weighting of a DW image depends on the magnitude of the applied gradients, how long they are switched on, and the time between the two lobes. In 1 Dr Bhogavalli Venkatarao, Associate Professor of Radiology, ASRAM Medical College, Eluru 2 (Corresponding author) Dr Shirishpaul ganta, Assistant Professor of Radiology, ASRAM Medical College, Eluru. I.D. : shirishpaul24@gmail.com 3 Dr N Kiranraju, Assistant Professor of Radiology, MIMS, Vijayanagaram 4 Dr Nasreensyed, Postgraduate of Radiology, ASRAM Medical College, Eluru apparent diffusion coefficient (ADC) maps, regions that contain high water mobility appear bright (2). The physics of diffusion weighted imaging: Diffusion weighted imaging is a relatively newer technique that enables visualization of the inherent directionality in the brain and provides an additional contrast based on the movement of water molecules. It is a technique that can characterize water diffusion properties at each picture element of an image (3). Physics of water diffusion Molecular diffusion, or Brownian motion, was first formally described by Einstein in (4) Diffusion is the random motion of tissue water molecules. Tissue water molecules can diffuse randomly, however various intra and extracellular as well as micro and macro barriers influence diffusion (5, 6). When molecules are agitated by thermal energy alone (ie, when molecular displacement takes place through the process of diffusion), the displacement distribution is centered. This means that the average or net displacement of the molecular population is zero. This can be represented by a histogram. Displacement r is plotted on X axis and probability n/

2 36 INTERNATIONAL JOURNAL OF HEALTH RESEARCH IN MODERN INTEGRATED MEDICAL SCIENCES, ISSN (P), ISSN (O), Vol-2, Issue-4, Oct-Dec 2015 N is platted on Y axis. Probability refers to the proportion of molecules that are likely to undergo displacement by a distance r. Acquiring diffusion weighted image: The basic principle of diffusion imaging is that the small random motion of the molecules results in a Gaussian distribution of phases. (7) The effect of these variations is enhanced by using a T2-weighted SE, gradient-echo, or echo-planar technique and applying strong gradients. The ability of these special pulse sequences to depict diffusion depends on the strength and duration of the diffusion gradients and on the direction in which they are applied. Diffusion is independent of the relaxation times and thus adds another factor to contrast (5). A typical Stejskal Tanner sequence uses two strong gradient pulses that allow controlled diffusion weighting, according to the following equation: S = S0 e bd Where S = measured signal S0 = signal without diffusion gradients b = b factor D = diffusion coefficient Apparent diffusion coefficient (ADC) According to Fick s law, true diffusion is the net movement of molecules due to a concentration gradient. However MR imaging, cannot differentiate molecular motion due to concentration gradients from molecular motion due to pressure gradients, thermal gradients, or ionic interactions. Also, with MR imaging we do not correct for the volume fraction available or the increases in distance travelled due to tortuous pathways. Therefore, when measuring molecular motion with DW imaging, only the apparent diffusion coefficient (ADC) can be calculated. Substituting ADC for D, the signal intensity of a Voxel on a DW image is thus expressed as SI = SI0 e -bd With the original spin-echo T2-weighted sequence, even minor bulk patient motion was enough to obscure the much smaller molecular motion of diffusion. With the development of high-performance gradients, DW imaging can be performed with an echo-planar spin echo T2- weighted sequence. The use of echo-planar spin-echo T2-weighted sequence for DWI results in 1. Markedly decreased imaging time, 2. Significant reduction in motion artefacts and 3. Increased sensitivity to signal changes due to molecular motion (diffusion). As a result, it is clinically feasible to perform DW imaging. AIM : The study was performed to describe the features of intracranial lesions on diffusion weighted imaging and to correlate these features with ADC and T2 FLAIR images Materials and Methods: Study type: Descriptive study. Study period: September 2013 and March Study Setting: Department of Radiology, ASRAM MEDICAL COLLEGE, ELURU. Study population: This consists of a study of 115 patients with intracranial lesions detected on imaging. The MRI was done on the advice of the referring doctor and no patient was made to undergo MRI for the sole purpose of this study. This study evaluates the diffusion weighted imaging characteristics of intracranial lesions that were detected in these patients Inclusion criteria: The criteria for inclusion of the patients in the study included those patients who were clinically referred for diffusion weighted MRI of the brain and were detected to have any of the following 1.Infarction and hypoxic ischemic injury, 2. Infective conditions, 3.Tumors -extra axial and intraaxial, 4. Demyelination, 5. Metabolic or toxic insults to the brain, 6. Degenerative disorders. Exclusion criteria: Patients who are detected to have intracranial bleed were excluded from the study. Data Acquisition: Patients referred for diffusion weighted MRI of the brain, underwent the examination after contraindications for MRI were excluded and consent was taken. All the MRI scans in this study were performed using 1.5 T MRI scanners (SignaHDxt; GE Medical systems). Results The age of the patients with intra cranial lesions studied ranged from 3 days to 78 years with a mean of / The patients involved in the study were divided into 7 age groups viz years, years, years, years, years, years and years. There were nine patients (7.8%) in 1-10 year age group, ten (8.6%) in year age group, ten (8.6%) in year age group, sixteen (13.9%) in year age group, sixteen (13.9%) in year age group, twenty three (20%) in year age group, twenty five (21.7%) in year age group, six (5.2%) in year age group.

3 INTERNATIONAL JOURNAL OF HEALTH RESEARCH IN MODERN INTEGRATED MEDICAL SCIENCES, ISSN (P), ISSN (O), Vol-2, Issue-4, Oct-Dec TABLE 1: DISTRIBUTION OF STUDY SUBJECTS ACCORDING TO LESIONS AND AGE INTRACRANIAL LESION NUMBER ABSCESS ADEM 1 1 ACUTE INFARCT ADRENOLEUCO DYSTROPHY 1 1 ANAPLASTIC ASTROMA ARACHNOID CYST CRONIC INFARCT DEMYELINATION TOXIC 1 1 EPIDERMOID CYST 1 1 EXTRADURAL EMPYEME GBM HEMANGIOBLASTOMA 1 1 HSG ENCEPHALITIS 1 1 LOW GRADE GLIOMA LYMPHOMA 2 MEDULLOBLASTOMA 4 4 MENINGIOMA MULTIPLE SCLEROSIS NCC GRANULOMA PVL 1 1 PILOCYTICASTROMA 1 1 PRESS PRETERM HII 3 3 PROFOUND TERM HII 1 1 SCHWANNOMA SUB ACUTE INFARCT TB GRANULOMA TOTAL

4 38 INTERNATIONAL JOURNAL OF HEALTH RESEARCH IN MODERN INTEGRATED MEDICAL SCIENCES, ISSN (P), ISSN (O), Vol-2, Issue-4, Oct-Dec 2015 The anisotropic nature of diffusion in the brain can be appreciated by comparing images obtained with DW gradients applied in three orthogonal directions as shown in the (Figure 1). Gx Gy Gz Figure 1: Axial DW MR Images with the diffusion gradients applied along the x (Gx, left), y (Gy, middle), and z (Gz, right) axes demonstrate anisotropy. The signal intensity decreases when the white matter tracts run in the same direction as the DW gradient because water protons move preferentially in this direction. Note that the corpus callosum (arrow on left image) is hypointense when the gradient is applied in the x (right-to-left) direction, the frontal and posterior white matter (arrowheads) are hypointense when the gradient is applied in the y (anterior-to-posterior) direction, and the corticospinal tracts (arrow on right image) are hypointense when the gradient is applied in the z (superior-to-inferior) direction. In each of the images, the signal intensity is equal to the signal intensity on echo-planar T2 weighted images decreased by an amount related to the rate of diffusion. Large discontinuities is the bulk magnetic susceptibility, such as those that occur at tissue- air interfaces produces local magnetic field gradients which notorious for their contribution to image distortion, particularly in EPI. In addition to the image distortion, susceptibility variations within the brain adversely affect DWIs because the additional local gradients act like diffusion gradients causing the b-matrix to be spatially varying. Fortunately, this affect appears to be limited to the volume of the brain adjacent to the sinuses. (Fig 2: Susceptibility artefacts) (FIGURE 2: SERIAL MRI SHOWING SUSCEPTIBILITY ARTEFACTS) T2 shine through effect: The relative contributions of abnormal diffusion and the intrinsic T2 properties are difficult to determine on diffusion-weighted alone. A T2 hyperintense lesion can, in the absence of restricted diffusion, still appear hyperintense on DW images, a phenomenon called T2 shine-through. The resultant hyperintense signal on diffusion-weighted images, which is attributable to the intrinsic T2 signal characteristics of the tissue, has been termed the T2 shine-through effect. T2 Washout: Refers to presence of isointense lesions on DWI i.e. normal DWI with increase in the apparent diffusion coefficient. This phenomenon resulting from a balance of T2 effects and increased water diffusibility has been term as T2 Washout phenomenon. It is result of intravoxeldephasing related to the increased water diffusion in the vasogenicedema that washes out the inherent increased T2 signal intensity in the lesions. A malignant mass lesion also causes significant mass effect and associated vasogenicedema which causes T2 Washout phenomenon as seen in the figure 3. FIGURE 3: WASH OUT PHENOMENA. Diffusion/T2 blackout: The term diffusion blackout refers to hypointensity on T2, DWI and ADC map. This pitfall of susceptibility induced signal losses complicating ADC measurements in acute hematomas. T2 blackout effect can also be seen in Tumor calcification induced susceptibility effect. Thus T2 blackout effect is corollary of the T2 shine-through effect. (Figure 4) Figure 4 : T 2 BLOCK OUT PHENOMENA

5 INTERNATIONAL JOURNAL OF HEALTH RESEARCH IN MODERN INTEGRATED MEDICAL SCIENCES, ISSN (P), ISSN (O), Vol-2, Issue-4, Oct-Dec Infarcts:In this study restricted diffusion was noted in all the 100% (30) acute infarcts patients in DWI. In T2WI no change was noted in 16.6% (5) of acute infarcts. Thus DWI was noted to be superior to T2WI in detection of acute infarct. Figure 5: COMPARING DWI AND FLAIR IMAGES IN ACUTE INFARCT. In chronic infarcts the signal on DWI and ADC images is variable and depends on a combination of T2 signal and increased ADC values. Time course of lesion evolution in acute stroke: The ADC continues to decrease and is most reduced at 8 32 hours. The ADC remains markedly reduced for 3 5 days. This decreased diffusion is markedly hyperintense on DW images (which are generated with a combination of T2- weighted and DW imaging) and hypointense on ADC images. The ADC returns to baseline at 1 4 weeks. (Figure 6: DW imaging reliability in acute stroke) Hypoxic ischemic injury: Diffusion-weighted imaging showed abnormal high signal intensity in the brain in patients in whom the conventional MR sequences were initially normal. Infections: In the present study 6 cases of the tubercular granulomas were observed. Diffusion restriction was noted, probably denoting presence of necrosis. Granulomas could not be detected on DWI alone and needed ADC and T2W images for lesion detection probably due to the poor spatial resolution of diffusion weighted imaging. Lai et al (6) have showed that abscess cavity shows high signal intensity on DWI and a low signal on ADC image. This is not seen in the necrotic component of brain tumors. They concluded that DWI may enable one to distinguish brain tumors from necrotic tumors. Also it helps in the evaluation of partially treated abscesses and to look for their recurrence Tumors: Intra axial tumors: DWI can differentiate between tumor and infection and can provide information about the cellularity of tumors thereby helping in characterization and grading of tumors. Extra axial tumors: Only few showed restricted diffusion depending on their cellularity. We observed that study 33% of meningiomas showing true diffusion restriction.diffusion weighted MR plays a key role in differentiating arachnoid from epidermoid cysts. In this study all 5 cases of arachnoid cysts had signal similar to CSF on DWI and ADC images. The single case of epidermoid cyst noted in this study had restricted diffusion. Demyelination The four cases of demyelination seen in this study did not show restricted diffusion and had increased signal on T2 FLAIR images. Pyogenic infections: Abscess cavities and empyema are homogeneously hyperintense on DW MR images. Although intracranial abscesses and intracranial neoplasms may appear similar on images obtained with conventional MR sequences, the signal intensity of the abscess cavity is markedly higher and the ADC ratios are lower than those of necrotic tumors on DW MR images.

6 40 INTERNATIONAL JOURNAL OF HEALTH RESEARCH IN MODERN INTEGRATED MEDICAL SCIENCES, ISSN (P), ISSN (O), Vol-2, Issue-4, Oct-Dec 2015 Herpes encephalitis: Herpes encephalitis lesions are characterized by marked hyperintensity on DW MR Images, with ADC ratios of these lesions to normal brain parenchyma ranging from 0.48 to On follow-up conventional T1-weighted and T2-weighted MR images, these areas demonstrate encephalomalacic change. DW MR imaging may aid in distinguishing herpes lesions from infiltrative temporal lobe tumors because the ADCs of herpes lesions are low while the ADCs of various tumors are elevated or in the normal range. Multiple Sclerosis: Acute plaques have significantly higher ADCs than do chronic plaques. The elevated diffusion may result from an increase in the size of the extracellular space due to oedema and demyelination acutely and to axonal loss and gliosis chronically. Others In the present study hypertensive encephalopathy showed that the edema of is of vasogenic type. None of the cases of PRES seen in this study had features of restricted diffusion. No signal change was noted in periventricular leucomalacia seen in this study, while the single case of adrenoleucodystrophy showed features of vasogenicedema. Discussion: Diffusion weighted imaging provides image contrast that is dependent on the molecular motion of water, which may be substantially altered by disease. The normal random movement of water molecules is affected by various pathological conditions which alter the signal generated on DWI. These features can be used to identify and characterize the lesions (8). Results of this study are correlated with a study done by Gonzalez et al (9) who concluded that DWI is superior to conventional MRI in the diagnosis and characterization of acute infarct. Schaefer et al (10) concluded that HII lesions not seen on routine MR images are identified on DW MR Images. When lesions are identified on conventional images, lesion conspicuity is increased and lesion extent is seen to be larger on DW MR Images. References : 1. Haaga JR, Dogra VS, Forsting M, Gilkeson RC, Ha HK, Sundaram M. CT and MRI of whole body. 5th ed. China: Elsevier; p. 54, Atlas SW, editors. Magnetic resonance imaging of the brain and spine. 4th ed. China: Lippincott Williams and Wilkins; p Susumu Mori, Barker PB. Diffusion magnetic resonance imaging: its principle and applications. The anatomical record 1999; 257: Hagmann P, Jonasson L, Maeder P, Thiran JP, Weeden VJ, Meuli R. Understanding diffusion MR techniques: from scalar diffusion weighted imaging to diffusion tensor imaging and beyond. Radiographics 2006;26: Robert B, Lufkin R. The MRI Manual. 2nd ed. New York: Hagendorn ; p Abragam A. The principles of nuclear magnetism. London: Oxford University Press; Allen PS. Some fundamental principles of nuclear magnetic resonance in medical physics, Monograph No. 21: The physics of MRI. New York: AIP press; p Schaefer PW, Grant PE, Gonzalez RG. Diffusion weighted MR imaging of the brain. Radiology 2000 november;217: Gonzalez RG. Clinical MRI of acute ischemic stroke. J MagnReson Imaging 2012;36: Schaefer PW, Hassankhani A, Putman C, et al. Characterization and evolution of diffusion MR imaging abnormalities in stroke patients undergoing intra-arterial thrombolysis. AJNR Am J Neuroradiol 2004;25:951 7 Conclusion: DWI along with ADC and T2 FLAIR images plays an important role in identifying and diagnosing the intracranial lesions.