|Year : 2006 | Volume
| Issue : 4 | Page : 705-710
|Clinical applications of diffusion weighted MR imaging: A review
R Rajeshkannan, S Moorthy, KP Sreekumar, R Rupa, NK Prabhu
Department of Radiology, Amrita Institute of Medical Sciences, Amrita Lane, Elamakkara (PO), Ernakulam. Kerala, India
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|Date of Submission||15-Nov-2006|
|Date of Acceptance||20-Nov-2006|
| Abstract|| |
Diffusion MR imaging is now a routine component of the brain MR imaging examination and is critical in the evaluation of stroke patients. However, high signal intensity on diffusion MR and hypo intensity on apparent diffusion co-efficient (ADC) images, which are features of acute cerebral infarction, have been reported in such diverse conditions as hemorrhage, abscess, tumor and even in Wernicke encephalopathy. Differentiating between there conditions is critical for determination of appropriate treatment. We present a systematic review of hyperintense lesions on diffusion MR images and their potential clinical applications.
Keywords: Diffusion weighted MRI
|How to cite this article:|
Rajeshkannan R, Moorthy S, Sreekumar K P, Rupa R, Prabhu N K. Clinical applications of diffusion weighted MR imaging: A review. Indian J Radiol Imaging 2006;16:705-10
Basic concepts of DW MR imaging
|How to cite this URL:|
Rajeshkannan R, Moorthy S, Sreekumar K P, Rupa R, Prabhu N K. Clinical applications of diffusion weighted MR imaging: A review. Indian J Radiol Imaging [serial online] 2006 [cited 2019 Dec 13];16:705-10. Available from: http://www.ijri.org/text.asp?2006/16/4/705/32328
Diffusion-weighted imaging is an MR imaging technique in which contrast within the image is based on microscopic motion of water. It was first described in 1965 by two physical chemists, Stejskal and Tanner. Diffusion-weighted images are obtained by adding a series of two sequential gradient pulses to a 90- 180-degree spin-echo sequence. The first gradient pulse is applied between the 90 and the 180-degree pulse. Motion after this pulse causes molecules to acquire phase shifts of their transverse magnetization. Both the 180-degree and the second gradient pulse rephase stationary spins. Phase shifts acquired in mobile molecules lead to failure of such molecules to rephase completely, resulting in substantial signal loss.
Stejskal and Tanner precisely defined this relationship as: the degree of signal drop is proportional to the exponent of the diffusion coefficient (D) and the duration and strength of the encoding gradient (b). This equation means that for spins that are less freely diffusible (i.e., those with a low diffusion coefficient) application of the additional gradient pulses results in less signal loss than for spins that are more freely diffusible.
The signal intensity (SI) of a voxel of tissue is calculated as
SI= SIo exp (-b D) where SIo is signal intensity on T2WI (or b=0), diffusion factor b=?²Gd² (?- d /3) and D= diffusion coefficient, ? is gyromagnetic ratio, G is the magnitude of, d is the width of and ? is the time between the two balanced DW gradient pulses.
According to Fick's law, true diffusion is the net movement of molecules due to concentration gradient, which cannot be differentiated from that due to pressure or thermal gradients or ionic interactions. Hence only the apparent diffusion coefficient can be calculated and signal intensity is expressed as SI= Sio exp (-b ADC).
In the brain, apparent diffusion coefficient is anisotropic particularly in white matter The ADC of white matter is less when measured perpendicular to fiber direction and more when measured parallel to the fiber direction. This can be appreciated by comparing images obtained in three orthogonal directions. Hence apparent diffusion is sampled in at least three directions and an average ADC is obtained which is termed as trace image.
Interpretation of DW images
At low diffusion weighting (small b values), there is minimal sensitivity to diffusional motions and images will show predominant T2 contrast. At high b values, the contrast is largely produced by the diffusion properties. But even at higher b values, a T2 weighted component is still present in all DW images producing "T2 shine through" effect. So all DW images should be correlated with ADC maps. Lesions with diffusion restriction appear bright on DW images and dark on ADC maps. Structures with increased diffusion like CSF will appear dark on DW images and bright on ADC maps.
Diffusion imaging in Brain ischemia
Acute cerebral infract is characterized by hyperintensity on DW image and low (ADC) values. Many theories were proposed to explain the diffusion restriction in acute cerebral ischemia ,
- The most probable theory is that the changes are due to increase in the intracellular- extra cellular water ratio secondary to disruption of intracellular energy metabolism and loss of ionic gradients.
- With cellular swelling, there is reduction in extra cellular space and increased tortuosity of extra cellular space pathways.
- Increased intracellular viscosity due to dissociation and fragmentation of intracellular components.
An important event in the pathophysiological cascade that leads to infarction following ischemia is net movement of water from extra cellular space in to intra cellular compartment without increase in total water content in the affected zone. Hence T2 weighted image will be normal at this stage. Later when endothelial breakdown leading to vasogenic edema and total increase in water content, the T2 weighted image will show bright signal. On the other hand DW imaging is capable of identifying the infarct even before the appearance of vasogenic edema.
Evaluation of acute stroke on DWI
The DWI and ADC maps show changes in ischemic brain within minutes to few hours after symptom onset, when no abnormalities are typically seen on conventional MRI and CT ,,[Figure - 1]. The signal intensity of acute stroke on DW images increase during the first week after symptom onset and decrease thereafter, but signal remains hyper intense for a long period (up to 72 days in the study by Lausberg et al) .
The ADC values decline rapidly after the onset of ischemia and subsequently increase from dark to bright 7-10 days later ,,. This property may be used to differentiate the lesion older than 10 days from more acute ones [Figure - 2]. Chronic infarcts are characterized by elevated diffusion and appear hypo, iso or hyper intense on DW images and hyperintense on ADC maps.
Reversibility of ischemic lesions on DW images
All the lesions with diffusion restriction may not progress to complete infarction . There are few reports of normalization of initial diffusion restriction in well- controlled animal models of ischemia and in human studies ,. In one study, the mean ADC ratios of reversible regions as compared to normal side ranged from 0.8 to1.1. In the irreversible region, ADC ratio was lower ranging from 0.66 to 0.96 . In our series of 100 patients complete reversal of diffusion bright lesions was observed in two patients [Figure - 3].
False-negative DW images have been reported in patients with very small lacunar brainstem or deep gray nuclei infraction as well in regions of decreased perfusion, which latter progressed to infarction.
Hyper intensity on DW images is reported in hyperacute (intracellular oxyHb) and late sub acute (extra cellular meth Hb) stages of hemorrhge [Figure - 4]. The ADC values are reported to be decreased in hyperacute stage and increased in late sub acute stage ,. ADC values of hematomas that are dark on T2 WI cannot be accurately calculated due to the "T2 blackout effect", which is the corollary of the T2 shine-through effect 
Intracranial mass lesions
Extra axial masses
Arachnoid cyst versus Epidermoid cyst
Epidermoid tumors demonstrate ADCs similar to those of gray matter and lower than those of CSF  and appear markedly hyperintense compared with CSF and brain tissue on DW images [Figure - 5]. Conversely, arachnoid cysts demonstrate very high ADCs, and appear similar to CSF on DW and ADC images [Figure - 6].
Most benign meningiomas are isointense on DW images and ADC maps. High signal intensity on DW images and reduced ADC values (0.4-0.69 x10¯³mm²/sec) suggest malignant meningiomas . Diffusion restriction in malignant meningiomas is probably due to high tumor cellularity 
Intra axial masses
The signal intensity of gliomas on DW images is variable (hyper-iso-, or hypointense). Several reports have shown that DW imaging can be used to grade the gliomas . Glioma grade correlates inversely with minimum ADC values that can be explained on the basis of increasing tumor cellularity with grade.
Initial reports suggested that the enhancing components of lymphomas show diffusion restriction [Figure - 7]. When compared with gliomas, lymphomas were shown to have lower ADC, likely because of their higher cellularity .
Ring enhancing tumors
The necrotic component of brain tumor (GBM and metastases) show marked hypo intensity on DW images and increased ADC values due to increased free water [Figure - 8]. This finding can be used to differentiate necrotic tumors from cerebral abscess, which demonstrates marked diffusion restriction . The differential diagnosis of intra cerebral necrotic tumors and cerebral abscess is frequently difficult on conventional MR images as both can present as ring enhancing lesions. The DW images must allow differentiation between necrotic tumors and cerebral abscess
Herpes encephalitis lesions are characterized by marked hyperintensity on DW MR images with ADC ratio of these lesions to normal brain parenchyma ranging from 0.48 to 0.66 [Figure - 9]. The restricted diffusion is explained by cytotoxic edema in tissue undergoing necrosis. 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 tumors are elevated or in the normal range .
Creutz feldt-Jacob disease
DW MR images in patients with Crentzfeldt- Jakob (CJD) disease have demonstrated hyperintense lesions in the cortex and basal ganglia . DW imaging is more sensitive than T2 and FLAIR in detecting cortical abnormalities [Figure - 10]. The restricted diffusion observed in CJD is consistent with spongiform change. Areas of diffusion restriction is useful in differentiating CJD from progressive multi focal leukoencephalopathy and SSPE 
The cerebral abscesses have been reported to show central hyperintensity on DW images and strongly reduced ADC values in the range of 0.27-0.64 x10¯³mm²/sec [Figure - 11]. Restricted diffusion in abscesses is believed to result from the relatively high viscocity and cellularity of pus .
The increased signal intensity on DW image and a low ADC value are usual in inflammatory granulomas 
Most of the multiple scerosis lesions show increased ADC values. Occasionally homogenously enhancing lesions may show reduced ADC values . We observed a case of central pontine myelinolysis showing diffusion restriction [Figure - 12].
Diffuse axonal injury may be characterized by restricted diffusion [Figure - 13]. In addition, most lesions are more conspicuous on DW images than on routine T2 weighted images . Thus DW MR imaging may be important for the prospective determination of extent of traumatic injury, the degree of irreversible injury (number of lesion with low ADCs), and the long-term prognosis.
Wernicke Encephalopathy (WE)
Konchu et al  reported two cases of WE characterized by diffusion restriction in thalamus, mamillary bodies and periaqueductal gray matter [Figure - 14]. They concluded that the MRI abnormalities in WE might be owing to the "reversible cytotoxic edema" caused by vitamin B1 deficiency.
Sustained seizure activity:
There are few case reports showing reversible restricted water diffusibility on DW images early after onset of sustained seizure activity in the absence of related T2 or FLAIR imaging abnormalities 
Recently Koch et al  reported hyper intense lesion of DW images (suggestive of cytotoxic edema) within predominantly hyper intense areas on T2 WI and hypo intense areas on DW images (suggestive of vasogenic edema) in a case of eclampsia.
Inborn error of metabolism
Cavellri et al have demonstrated diffusion restriction during the active phase of Maple syrup urine disease with ADC values falling to 20% to 30% of their normal values . We observed a case of Maple syrup urine disease showing diffusion restriction in deep cerebellar, medial temporal and centrum semiovale [Figure - 15].
Diffusion tensor imaging (DTI)
In contrast to DW imaging which is a one dimensional technique, DTI is inherently three dimentional; one must apply diffusion gradients in at least six directions in order to provide enough information to estimate the six independent elements of the diffusion tensor equation . DTI is an area of burgeoning research in both technical refinements and clinical applications. DTI is found useful in myelin disorders, ischemic lesions, epilepsy and neurodevelopmental disorders.
To summaries DW imaging has a major role in the following clinical situations:
- Early identification of ischemic stroke.
- Differentiation of acute from chronic stroke.
- Differentiation of acute stroke from other stroke mimics
- Differentiation of Epidermoid cyst from arachnoid cyst.
- Differentiation of abscess from necrotic tumors.
- Assessment of cortical lesions in CJD.
- Differentiation of Herpes encephalitis from diffuse temporal gliomas.
- Assessing full extent of diffuse axonal injury.
- Grading of gliomas and meningiomas-need further study.
DW MR imaging provides unique information about the physiologic state of brain tissue. It has tremendous potential for helping direct the treatment of acute ischemic stroke. It may prove to be valuable in the evaluation of a wide variety of other diseases, as described in this review. Further investigations are needed to optimize the possible contribution of diffusion imaging in other clinical applications.
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Department of Radiology, Amrita Institute of Medical Sciences, Amrita Lane, Elamakkara (PO), Ernakulam, Pin: 682026, Kerala
Source of Support: None, Conflict of Interest: None
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15]
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