Diffusion-weighted magnetic resonance imaging (DWI) is a specialized technique that measures the degree of diffusion of water molecules over short distances.  The DWI signal is high (bright) when the apparent diffusion coefficient (ADC) is reduced, as occurs in cytotoxic damage from ischemia, inflammation, trauma, or tumor.  DWI has been reported to be helpful in detecting early cerebral ischemia.  The scope of this document with regard to DWI is limited to the use of DWI for cerebral disorders.

Perfusion-weighted magnetic resonance imaging (PWI) is often used in conjunction with DWI and is a technique used to assess cerebral blood flow and blood volume in various regions of the brain.  PWI is usually performed by injecting a contrast agent and then obtaining a series of rapid MRI images.  The images track the passage of contrast agent through the brain.

For information regarding cerebral perfusion studies using Computed Tomography (CT), please refer to RAD.00045 Cerebral Perfusion Imaging Using Computed Tomography.

Medically Necessary:

Cerebral perfusion studies using diffusion, or perfusion-weighted MRI, are considered medically necessary for the evaluation of acute cerebral ischemia.

Investigational and Not Medically Necessary:

Cerebral perfusion studies using diffusion, or perfusion-weighted MRI, are considered investigational and not medically necessary for all other indications.

Standard MRI sequences are relatively insensitive to the changes of acute ischemia within the first few hours after onset of stroke.  Several studies have provided data suggesting that DWI allows for the early detection of ischemic lesion size, site, and age.  It can detect relatively small cortical or subcortical lesions that are often poorly visualized with standard CT scans.  A recently published analysis by the Stroke Council of the American Heart Association concluded that DWI has a high sensitivity (88% to 100%) and specificity (95% to 100%) for detecting acute ischemia.  Also, the lesion volumes correlate well with stroke severity, as rated by clinical scales and outcomes (Adams, 2007, Adams, 2005; Adams, 2003).  Additionally, the AHA Council on Cardiovascular Radiology concluded that DWI and PWI are techniques that have been proven capable of demonstrating severely ischemic tissue in the acute stroke patient (Latchaw, 2003).  At the present time, there is no compelling evidence to suggest the ability of DWI and PWI to provide accurate information on the status of vascular reserves in patients with chronic ischemia or in patients with vasospasm.

While there is general agreement that DWI and PWI can be helpful as diagnostic tools, there is limited peer-reviewed, published data on how these modalities affect treatment decisions.  Nonetheless, physicians in primary and tertiary care facilities are currently utilizing diffusion and perfusion MRI in the evaluation of acute cerebral ischemia.  The strong evidence that the technology is highly sensitive for detecting acute ischemia, coupled with its adoption in clinical practice, supports the position that diffusion and perfusion MRI may be medically necessary for the evaluation of acute cerebral ischemia.

DWI and PWI are techniques utilizing magnetic resonance that were developed in the 1980s and have been studied as tools to aid in the diagnosis of cerebral ischemia.  The need for the development of these new imaging techniques has arisen out of the persistent diagnostic challenge, posed by both acute and chronic cerebral ischemia.  Non-contrast computed tomography (NCCT), for example, is highly sensitive in the detection of acute intracranial hemorrhage, but is much less sensitive in the identification of ischemic brain lesions within the first few hours of stroke onset.  Chronic, progressive occlusive disease and ischemia from vasospasm are also conditions that have been difficult to diagnose with current imaging techniques. 

It has been shown that the use of DWI and PWI imaging together can provide information about the location and extent of brain infarction within minutes of onset.  When performed in series, this imaging can also provide information about the pattern of evolution of the ischemic lesion, which may be helpful in determining treatment management options (Sen, 2005). 

Acute cerebral ischemia:  a sudden neurological affliction (injury) resulting from compromise of the cerebral blood flow/supply to brain tissue; this condition will result in reversible/transitory or irreversible neurological deficits, based on the etiology and duration of circulatory compromise

Cerebral (Ischemic) Infarction: an area of coagulation necrosis in brain tissue, (i.e., tissue death), due to local anemia resulting from obstruction of the circulation to the area

Diffusion-weighted MRI imaging (DWI):  an MRI imaging technique in which the images reflect microscopic random motion of water molecules; water molecule protons, known as the apparent diffusion coefficient (ADC) are measured and captured by this type of imaging;  studies have shown that ADC's in ischemic areas of the brain are lower by 50% or more than those of normal brain tissue, and they appear as bright areas, (i.e., hyperintensities) on DWI imaging;  studies have demonstrated changes in the ADC value occurring as early as ten minutes following onset of brain ischemia

Perfusion-weighted MRI imaging (PWI): an MRI imaging technique in which hemodynamically weighted MR sequences are based on the passage of contrast material through brain tissue; the signal intensity declines as the contrast material passes through the infarcted area and returns to normal, as it exits this area; a curve is derived from this tracing data, which represents and estimates the cerebral blood volume 

Stroke: a generic term used to represent any one or all of a group of disorders, including cerebral infarction, intracerebral hemorrhage, or subarachnoid hemorrhage; stroke is characterized by a non-convulsive, focal, neurologic deficit that lasts greater than 24 hours in duration

The following codes for treatments and procedures applicable to this document are included below for informational purposes.  Inclusion or exclusion of a procedure, diagnosis or device code(s) does not constitute or imply member coverage or provider reimbursement policy.  Please refer to the member's contract benefits in effect at the time of service to determine coverage or non-coverage of these services as it applies to an individual member. 

When services may be Medically Necessary when criteria are met: 

When services are Investigational and Not Medically Necessary:
When criteria are not met, for all other diagnoses, or when the code describes a procedure indicated in the Position Statement section as investigational and not medically necessary.

Peer Reviewed Publications:

1. Barboriak DP. Imaging of brain tumors with diffusion-weighted and diffusion tensor MR imaging. Magn Reson Imaging Clin N Am. 2003; 11(3):379-401. 
2. Beauchamp NJ, Barker PB, Phil D, et al. Imaging of acute cerebral ischemia. Morgan H. Russell Dept. of Radiology, Division of Neuroradiology, Johns Hopkins Hospital, Baltimore, MD.  Radiology. 1999; 212:307-324.
3. Eastwood JD, Lev MH, Wintermark M, et al. Correlation of early dynamic CT perfusion imaging with whole-brain MR diffusion and perfusion imaging in acute hemispheric stroke.  Am J Neuroradiol. 2003; 24:1869-1875.
4. Fiehler J, Fiebach J, et al. Diffusion-weighted imaging in acute stroke- A tool of uncertain value? Cerebrovascular Diseases. 2002; 14:187-196. 
5. Higashida RT, Furlan AJ, Roberts H.  Technology Assessment Committee of the American Society of Interventional and Therapeutic Neuroradiology, Technology Assessment Committee of the Society of Interventional Radiology, et al.  Trial design and reporting standards for intra-arterial cerebral thrombolysis for acute ischemic stroke.  Stroke. 2003; 34(8):e109-137.  Erratum in:  Stroke. 2003; 34(11):2774.
6. Huisman TA.   Diffusion-weighted imaging: basic concepts and application in cerebral stroke and head trauma. Eur Radiol. 2003; 13(10):2283-2297.
7. Mukherji SK, Chenevert TL, et al. Diffusion-weighted magnetic resonance imaging. J Neuro-Ophthal. 2002; 22(2):118-122.
8. Parsons MW, Barber PA, Chalk J, et al. Diffusion- and perfusion-weighted MRI response to thrombolysis in stroke.  Ann Neurol. 2002; 51(1):11-13.
9. Schellinger PD, Jansen O, Fiebach JB, et al. Monitoring intravenous recombinant tissue plasminogen activator thrombolysis for acute ischemic stroke with diffusion and perfusion MRI.  Stroke. 2000; 31(6):1318-1328.
10. Schramm P, Schellinger PD, Klotz E, et al.  Comparison of perfusion computed tomography and computed tomography angiography source images with perfusion-weighted imaging and diffusion weighted imaging in patients with acute stroke of less than 6 hours duration.  Stroke. 2004; 35(7):1652-1658.

Government Agency, Medical Society, and Other Authoritative Publications:

1. Adams HP, del Zoppo G, Alberts MJ, et al. Guidelines Update: Guidelines for the Early Management of Adults With Ischemic Stroke: A Guideline From the American Heart Association/ American Stroke Association Stroke Council, Clinical Cardiology Council, Cardiovascular Radiology and Intervention Council, and the Atherosclerotic Peripheral Vascular Disease and Quality of Care Outcomes in Research Interdisciplinary Working Groups: Stroke, 2007; 38: 1655 – 1711.  Available at: http://stroke.ahajournals.org/cgi/reprint/38/5/1655  Accessed on July 31, 2009.
2. Centers for Medicare and Medicaid Services. National Coverage Determination for Magnetic Resonance Imaging (MRI). NCD #220.2. Effective March 22, 1994. Available at: http://www.cms.hhs.gov. Accessed on: July 31, 2009.
3. American College of Radiology.  Cerebrovascular Disease, ACR Appropriateness Criteria^™ 2000. Published: Masaryk T, Drayer BP, Anderson RE, et al. Cerebrovascular disease.  American College of Radiology.  ACR Appropriateness Criteria.  Radiology. 2000; 215 Suppl: 415-435. Available at: http://www.acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria/pdf/ExpertPanelonNeurologicImaging/
   CerebrovascularDiseaseDoc2.aspx  Accessed on July 31, 2009.

1. Sen, S.  Magnetic resonance imaging in acute stroke.  Last updated March 13, 2007. Available at: http://www.emedicine.com/neuro/topic431.htm  Accessed on: July 31, 2009.

Cerebral Perfusion Studies using Diffusion/Perfusion Magnetic Resonance Imaging
MRI, Cerebral Studies