Diffusion-weighted magnetic resonance imaging and its application to cancer

Abstract

Diffusion-weighted magnetic resonance imaging (DW-MRI) provides image contrast through measurement of the diffusion properties of water within tissues. Application of diffusion sensitising gradients to the MR pulse sequence allows water molecular displacement over distances of 1-20 microm to be recognised. Diffusion can be predominantly unidirectional (anisotropic) or not (isotropic). Combining images obtained with different amounts of diffusion weighting provides an apparent diffusion coefficient (ADC) map. In cancer imaging DW-MRI has been used to distinguish brain tumours from peritumoural oedema. It is also increasingly exploited to differentiate benign and malignant lesions in liver, breast and prostate where increased cellularity of malignant lesions restricts water motion in a reduced extracellular space. It is proving valuable in monitoring treatment where changes due to cell swelling and apoptosis are measurable as changes in ADC at an earlier stage than subsequent conventional radiological response indicators.

Figure 1

A representation of a tumour or tissue displaying heterogeneous cellularity. The mean path length ‘ L’ travelled by protons in the extracellular fluid within a specific observation time is much greater in regions of low cellularity where random motion is not impeded by the presence of cellular membranes.

Figure 2

Signal change during application of diffusion gradients. Water that remains in the same location relative to diffusion gradient experiences firstly the dephasing effects of the first gradient followed by equal and opposite rephasing via the second diffusion gradient: it is not dephased as a result of the diffusion gradients (top row). Water that moves in the direction of the diffusion gradients between their application will experience either too much or too little phase reversal, because it moves to a position where it experiences a different field strength in the second diffusion pulse than it did in the first (lower row, green). The resulting ‘dephasing’ causes a drop in signal in the diffusion-weighted image. Note that water moving perpendicularly to the diffusion gradient experiences an equal and opposite gradient field strength and is rephased (lower row, red).

Figure 3

Primary brain tumour. Transverse DW-MRI (EPI 4184/91 ms [TR/TE]) through the brain at the mid-ventricular level at b-values of 0 (A), 500 (B) and 1000 s/mm ² (C) sensitized in the antero-posterior direction and isotropic ADC map (D). A mass lesion in the right parieto-occipital region is seen in (A) (arrow) with surrounding high signal intensity from peritumoural oedema in the white matter (open arrow). In (B) there is a reduction of signal from within the tumour which becomes more marked as diffusion weighting increases ((C), arrow). This reflects breakdown of brain tissue structure and increased diffusion. On the ADC map this region of tumour with high diffusivity is bright. In the surrounding oedematous tissue water diffuses less freely because of intact tissue architecture (open arrow). Note also the high diffusivity in the ventricular CSF and in the white matter tracts running antero-posteriorly in the direction of diffusion sensitization ((C), black arrow). The anisotropic diffusion within the white matter tracts is not represented in the ‘trace’ ADC image (D).

Figure 4

Liver metastasis from colorectal cancer. Transverse T2W (FSE 2500/80 ms [TR/TE] (A), post-contrast T1W (GRE 200/12/70 ⁰ ms [TR/TE/FA] (B), and diffusion-weighted images (single-shot EPI 1850/56 ms [TR/TE], (C) (b = 0, 150, 500 s/mm ²) through the liver showing a metastatic deposit (arrow) and a cyst (open arrow). The metastasis shows increasingly restricted diffusion at increasing b-values. Signal from the cyst on the other hand decays away as diffusivity is high.

Figure 5

Primary prostate cancer. Transverse T2W image (FSE 2096/90 ms [TR/effective TE] (A) and an isotropic ADC map (B) at the same level through the prostate apex calculated from images (b = 0, 300, 500, 800 s/mm ²) with diffusion-weighted gradients sensitised in three planes. The tumour which is poorly seen as an ill-defined low-signal intensity area on T2W (arrow) is clearly demarcated as an area of restricted diffusion (arrow) on the ADC map.

Figure 6

Recurrent ovarian cancer. Coronal T1W (FSE 600/14 ms [TR/TE] (A), STIR (IR 1833/15/165 ms [TR/TE/TI] (B) images and maximum intensity projection of a whole body diffusion-weighted image ((C), b = 0, 1000 s/mm ²) showing left pelvic side-wall lymphadenopathy. Although the nodes are well defined in (A) and (B), the differentiation between cystic (open arrow) and solid (small arrow) component of tumour is clearly apparent on the diffusion-weighted image.