Year : 2003 | Volume
: 13 | Issue : 2 | Page : 213--217
Role of diffusion weighted MR images in early diagnosis of cerebral infarction
K Rima, G Rohit, P Anjali, C Veena
Department of Radiodiagnosis, M.A.M.C and Lok Nayak Hospital, Jawaharlal Nehru Marg, Delhi-110002, India
198, Gagan Vihar, Delhi 110051
Objectives : The purpose of this study was to determine the role of Diffusion weighted (DW) MR Imaging in the early diagnosis and staging of evolution of cerebral infarction. Material and Methods : Forty patients of suspected cerebral infarction werestudied over an eight month period by echoplanar diffusion weighted (DW) MR images obtained on a 1.5 Tesla. MR Scanner the criteria for inclusion in the study included a clinical diagnosis of stroke, focal neurological deficits and clinical signs and symptoms persisting longer than 24 hours after onset. DW images were visually analyzed and compared with conventional spin-echo and FLAIR images for changes in signal intensity. Results Diffusion weighted images showed changes in signal intensity in all (11/11) lesions less than 1 day old, 15/16 lesions which were 1 to 7 days old, 4/7 lesions 8-14 days old and 1/12 lesions more than 14 days old. Conclusion : Abnormal signal intensity was present on all DW MR studies obtained in patients within 24 hours of symptom onset and in more than 94 percent of patients within the first two weeks after ictus. The percentage of abnormal diffusion studies decreased with time and changes in signal intensity were rare if MR scanning was done more than two weeks after the onset of clinical symptoms.
|How to cite this article:|
Rima K, Rohit G, Anjali P, Veena C. Role of diffusion weighted MR images in early diagnosis of cerebral infarction.Indian J Radiol Imaging 2003;13:213-217
|How to cite this URL:|
Rima K, Rohit G, Anjali P, Veena C. Role of diffusion weighted MR images in early diagnosis of cerebral infarction. Indian J Radiol Imaging [serial online] 2003 [cited 2018 Jun 17 ];13:213-217
Available from: http://www.ijri.org/text.asp?2003/13/2/213/28664
Stroke is a leading cause of death and is a major cause of long term disability. Clinical trials on treatment of stroke have used CT to select or exclude patients for thrombolysis. In some patients CT can be used to detect vasogenic edema in infarcted brain tissue within hours of the onset of symptoms . However, in many patients positive CT findings do not appear within the time window in which thrombolysis can be useful.
Conventional spin-echo MRI is more sensitive to the accumulation of tissue water than CT and this increased tissue water content due to edema provides the signal contrast in ischemic parenchyma on MR imaging. However, the MR image is often normal in the first 8 hours and gradually becomes more hyperintense during the first 24 hours .
Diffusion Weighted imaging provides a technique for mapping proton contrast that reflects the microvascular water environment. This MR imaging technique is sensitive to early ischemic insult. Diffusion Weighted imaging is performed with a pulse sequence capable of measuring water translation over short distances (diffusion). This water diffusion is much slower in regions of ischemia compared with normal brain. Diffusion Weighted imaging is highly sensitive to water proton translations that slow very early in the ischemic episode (within minutes).
We report our results with DW MR imaging technique to evaluate patients with stroke.
Materials and Methods
Over an 8-month period, forty patients ranging in age from thirty to seventy years underwent DW MR imaging. These patients were referred with complaints of focal neurological deficit, hemiparesis/hemiplegia and other clinical findings consistent with stroke. All patients were studied on a 1.5 Tesla MR scanner (Sonata, Siemens AG Erlangen, Germany) using conventional sequences as well as echoplanar DW sequence.
The conventional scanning protocol of the MR examination included sagittal and axial 5mm thick T1 weighted (TR/TE- 550/15) spin-echo sequence, an axial 5mm thick TSE PD/T2 weighted sequence (3000/100/20) and a coronal 4mm thick FLAIR sequence (TR/ effectiveTE- 800/112, TI- 2700msec). After the conventional portion of the study, a single shot spin-echo echoplanar DW imaging sequence was obtained. Acquisition parameters included 10,000/98 TR/TE, single excitation, 30 cm FOV, 128X128 matrix and a 5mm section thickness with a 2.5mm interslice gap. Gradient strength corresponding to b values (diffusion sensitivities) of 0, 500 and 1000 second/mm2 were used. Fifteen axial images covering the entire brain were obtained. The conventional MR sequences and DW images were reviewed in conjunction with relevant clinical history. Signal intensity abnormalities noted on T2 W images, FLAIR and DW images were compared. Apparent diffusion coefficient (ADC) maps were also obtained.
Diffusion Weighted images showed increased signal intensity in all of the ten patients (11 lesions) evaluated within 24 hours of the onset of ictus. The bright lesions on DW images showed hypointense signal on ADC maps. Four of these eleven lesions had no signal abnormality on either T2 weighted or FLAIR images [Figure 1]a,b&c. All regions of acute stroke which demonstrated decreased ADC corresponded to the same infarct region seen at DW imaging. The signal to noise ratio allowed detection of lesions as small as a few millimeters in size.
In the thirteen patients studied between days 1 and 7, abnormal high signal intensity was seen in fifteen (94%) of the sixteen cerebral lesions noted. The majority of these lesions were hypointense on ADC maps [Figure 2]a,b,c&d. Two patients had false negative findings on T2 W images (i.e. subacute lesions were missed as compared with DW imaging) caused by i) Absence of signal abnormalities in one patient imaged 36 hours after stroke ii) Lesion diameter of only 3mm in the other patient.
As compared with T2 W imaging in which 13/16 subacute lesions were diagnosed and three were false negative, DW imaging was diagnostic in 15/16 lesions and missed recent ischemic damage (i.e. was falsely negative) in only one patient, who was examined seven days after stroke and had small subcortical lesions.
The percentage of abnormal DW images decreased as a function of time after clinical ictus [Table 1]. The oldest lesion showing high signal intensity on DW images with b values of 500 and 1000 was scanned at fifteen days. However, none of the infarcts evaluated sixteen days or more after ictus revealed hyperintensity on DW images with b values of 500 and 1000 [Figure 3]a,b&c.
The first few hours of stroke are the most critical for attempts to salvage brain tissue. Therefore morphological information pertinent to early treatment decisions such as the exact size of the lesion can be crucial. Unequivocal imaging evidence of an ischemic lesion may serve both to confirm clinical suspicion of an ischemic event and to rule out other differential diagnosis. The pattern of damage per se may be indicative of stroke origin. Finally, counselling of the patient and prognostic significance are facilitated by clear insight into the extent and location of morphological damage .
In general ischemic lesions are well seen on conventional T2 W MR images on which they appear as areas of high signal intensity. Acute lesions are diagnosed because of their characteristic shape, confinement to specific vascular territories and presence of mass effect. However in the case of small lesions these findings are not always present. The coexistence of previous focal and diffuse ischemic brain damage with a similar signal intensity to that of acute lesions on conventional T2 W images may further complicate delineation of recent ischemic damage. In contrast, DW imaging is based on the specific changes in the motion of water molecules that follow ischemic damage. Ischemic lesions appear bright on DW imaging sequences, especially with high b values only for a few days and can be distinguished from normal and chronically ischemic brain, including nonspecific white matter hyperintensities .
Several studies have shown the efficacy of DW imaging to depict morphological brain changes within the initial period of focal ischemia and the usefulness of this technique for determining the location and size of brain damage in the setting of acute stroke has been extensively documented .
Using simple visual analysis of DW images we found that areas of cerebral infarction may have persistent high signal intensity as long as 14 days after clinical ictus. Furthermore the signal intensity of the diffusion abnormality decreases with time and reverts to normal by the end of the second week. These lesions showed corresponding decreased signal on ADC maps. Creation of ADC maps negates the T2 "shinethrough" effect that can contribute to lesion signal hyperintensity on DW imaging .
Apparent diffusion slowing (decrease in ADC values) after ischemia reflects the shift of extracellular water protons, which move quickly into a restricted intracellular compartment. This may be due to changes in cellular membrane permeability to water protons and/or intracellular increases in osmolality after ischemia ,.
The contribution of DW imaging was greatest for the detection of small lesions within the first few days after stroke, and in patients with preexisting brain damage. At the end of the first week, the ADC within ischemic brain parenchyma starts to rise and goes through a period of pseudonormalisation before values become higher than those of normal tissue in areas that have undergone necrosis . At this time the sensitivity of DW imaging for detecting recent ischemic damage starts to diminish, and may even fail to pinpoint such lesions.
In the subacute period, contrast arises from both T2 "shinethrough"and decreased ADC. At a certain point after ADC pseudonormalisation, the contribution to MR signal coming from prolonged T2 values is compensated by the contrast contribution coming from increased ADC on DW imaging sequences. The advantages of DWI over T2 W MR imaging has been mentioned in previous reports on the use of DWI in stroke patients . Similar findings have been observed in our study.
In conclusion, we found abnormal signal intensity on DW images and ADC maps in all our stroke patients presenting within 24 hours of the onset of ictus and in upto 94% of patients studied within 14 days of stroke. We also found clear advantages of DW imaging over conventional T2W sequences in acute and subacute periods of stroke. Our results indicate that DW MR imaging along with ADC maps may have an important role in the early diagnosis and management of stroke patients. These findings indicate an inclusion of DW imaging in standard MR imaging protocols of stroke patients.
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