Diffusion & Perfusion Weighted Imaging
Samantha J Holdsworth1

1Lucas Center for Imaging, Department of Radiology, Stanford University, Stanford, CA, United States

Synopsis

This lecture is devoted to the basic technological aspects of diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI), using neuroimaging applications as examples, and with the concepts explained with minimal use of equations.

Background

Diffusion-weighted imaging (DWI) [1-6] is one of the most important contrast mechanisms in MRI. It has revolutionized the detection of pathologic conditions physiologic mechanisms sensitive for diffusion abnormality, such as in stroke, seizure, trauma, demyelination, tumors, infection, and others. PWI is an MRI technology that studies cerebral hemodynamics and blood flow [7-8]. PWI provides information on hypoperfused areas of brain tissue, and is used in the evaluation of focal brain lesions, primary tumors, in the differentiation of tumor recurrence and radionecrosis, and others. PWI is often combined with other MR techniques like magnetic resonance angiography (MRA) to assess vessel patency, and DWI to identify areas of reversible ischemia early before it progresses to permanent infarction [9-11].

Methods

Diffusion-weighted imaging

DWI uses the diffusion of water molecules to generate contrast in MR images. Due to the presence of cell membranes, macromolecules, fibers and other obstacles, the water molecules tend to be confined and hindered in their normal free diffusion. Water molecule diffusion patterns can therefore reveal microscopic details about tissue architecture, characterize different tissue types or pathological processes. DWI allows one to take ‘snapshots’ of this tissue water motion on a time scale of a few tens of milliseconds, using strong ‘diffusion encoding’ magnetic field gradients. On a DWI image, the grayscale pixel value is dependent on the underlying diffusivity, where voxels with high diffusion appear hypointense (e.g. CSF) and voxels with low diffusion appear hyperintense (e.g. acute stroke, Fig 1).

A DWI imaging acquisition is typically achieved with the Stejskal and Tanner method, where the strength of the diffusion encoding is represented by the ‘b-value’ [1]. A typical brain DWI sequence comprises of the acquisition of one ‘b = 0’ or T2-weighted image, followed by 3 or more diffusion-weighted ‘source’ images acquired at a b-value of 1000 (Fig. 2). These individual source images are usually combined by into a single final set for diagnosis by taking the geometric mean across these images. The combined image is known by various names: diffusion-weighted images, isotropic images, or trace images. On both the b = 0 and b = 1000 images, the contrast not only depends upon the spatially distributed diffusion coefficient of the acquired tissues, but also depends on the T2 (and sometimes T1) values. Because of this, the apparent diffusion coefficient or ADC is often calculated (based off the -logarithm ratio of b1000/b0), which helps to differentiate T2 shine through (or T1) effects and other artifacts from real diffusion lesions (Fig. 1). Sometimes the exponential ADC (or eADC [12]) is calculated which is simply based on the ratio of b1000/b0 and results in an opposite contrast (i.e. an acute infarct is seen as a hypointense on ADC and as hyperintense on eADC (Fig. 4)).

Perfusion-weighted imaging

The two most common PWI techniques comprise an exogenous method called dynamic susceptibility contrast imaging by injection of MR contrast (DSC-MRI) [7-8], and an endogenous (and non-invasive) method called arterial spin labelling (ASL) [13-17]. In DSC-MRI, a Gadolinium contrast agent is injected and a time series of fast T2*-weighted (or spin-echo) images is acquired during the first pass through the cerebral circulation. As Gadolinium passes through the tissues, it produces a reduction of T2* intensity depending on the local concentration (Fig. 3). Hemodynamic maps of cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT) or time to bolus peak (TTP) can be created by mathematical analysis of the evolution of the intensity of the signal. Both PWI and DWI are rapidly have become integral parts of the diagnostic workup in the acute stroke setting (Fig. 4).

Arterial spin labeling (ASL) uses magnetically labeled arterial blood water protons as an endogenous diffusible tracer to measure tissue blood flow [13-17]. Here, flowing spins in the carotid arteries are inverted by a radiofrequency pulse. The influx of fresh, labeled protons in the tissue of interest slightly alters the magnetization and, depending on the exchange with tissue protons (therefore the T1 relaxation time of the tissue), renders this method sensitive to the local degree of microperfusion. Hence, any change in regional blood flow will be picked up by ASL via image contrast changes. The four types of ASL preparation components are: pulsed ASL (PASL), continuous ASL (CASL), pseudo-continuous ASL (pCASL) and velocity-selective ASL (VS-ASL). The primary difference among these ASL categories is the technique that magnetically tags the inflowing blood (Fig. 5). In the past, despite being non-invasive, ASL methods have seen limited adoption clinically due to their complexity, susceptibility to motion, among other problems. However due to recent technical advances, ASL is increasingly becoming of increasing clinical interest, particularly to investigate perfusion abnormalities in cerebral stroke and patients with chronic vascular diseases (Fig. 6).

Acknowledgements

No acknowledgment found.

References

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Figures

Fig. 1 - Output from a typical brain DWI sequence.

Fig. 2 - The DWI (a.k.a. isotropic DWI, or trace image) is produced by combining at least 3 source images sensitized to diffusion along various directions. The source images may be along the laboratory x-, y-, and z-axes or in three arbitrary perpendicular orientations. However, sometimes source images are acquired in 4 (‘tetrahedral encoding’) or more directions, which boosts SNR in the final DWI. With 6 or more directions, one can also produce “diffusion tensor” images (DTI) which is used to map neural tract directional information.

Fig. 3 - (top) DSC-MRI. Time course of the T2*-weighted MR images during contrast material bolus passage. Due to the high concentration of contrast material the signal intensity decreases significantly during the peak of the bolus. (bottom). Plot of the signal from a T2*-weighted sequence after injection of a contrast bolus as it travels through a region of interest in the brain. In (b), the signal is converted to concentration versus time.

Fig. 4 - DWI is used to identify severely ischemic brain regions within minutes to hours after stroke onset, while PWI provides information on the hemodynamic status of the tissue and can detect impaired perfusion in both the ischemic core and the surrounding brain regions. This acute stroke patient has a clear DWI-PWI mismatch pattern in the right MCA territory. (top row) Diffusion-weighted images (b = 1000 s/mm2) showing the T2-weighted (b = 0) image, isotropic DWI (isoDWI), ADC (isoADC), and eADC (iso-eADC). (bottom row) CBV, CBF, Tmax, and MTT maps. The area of perfusion deficit is clearly apparent in the Tmax and MTT images. Although present, the ischemic area is less apparent on the CBF and CBV maps.

Fig. 5 - The four different ASL tagging methods used to measure CBF. Green is the imaging region and grey is the labelling region. PASL inverts spins proximal to the imaging region, while CASL and pCASL labels a narrow plane of spins continuously. VS-ASL tags spins moving at a certain velocity.

Fig. 6 - 3-year old boy with acute right MCA infarction. The clot can be seen on the T2*-weighted image (2D GRE, arrow). There is a right MCA cut off on MRA and associated decreased ASL perfusion deficit (courtesy Kristen Yeom, Stanford University).

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)