Whole body MRI: improved lesion detection and characterization with diffusion weighted techniques

Abstract

Diffusion-weighted imaging (DWI) is an established functional imaging technique that interrogates the delicate balance of water movement at the cellular level. Technological advances enable this technique to be applied to whole-body MRI. Theory, b-value selection, common artifacts and target to background for optimized viewing will be reviewed for applications in the neck, chest, abdomen, and pelvis. Whole-body imaging with DWI allows novel applications of MRI to aid in evaluation of conditions such as multiple myeloma, lymphoma, and skeletal metastases, while the quantitative nature of this technique permits evaluation of response to therapy. Persisting signal at high b-values from restricted hypercellular tissue and viscous fluid also permits applications of DWI beyond oncologic imaging. DWI, when used in conjunction with routine imaging, can assist in detecting hemorrhagic degradation products, infection/abscess, and inflammation in colitis, while aiding with discrimination of free fluid and empyema, while limiting the need for intravenous contrast. DWI in conjunction with routine anatomic images provides a platform to improve lesion detection and characterization with findings rivaling other combined anatomic and functional imaging techniques, with the added benefit of no ionizing radiation.

Keywords: MRI; whole body MRI; apparent diffusion coefficient ADC; b-value; diffusion-weighted imaging DWI; oncology imaging.

Copyright © 2013 Wiley Periodicals, Inc.

Figures

Figure 1

Application of water motion probing diffusion sensitizing gradients between polarizing RF pulses. Free unrestricted water movement in between application of the diffusion sensitizing gradients results in low signal. Increased signal is produced in tissue where water is restricted from moving between the two motion probing diffusion sensing gradient applications (bottom row).

Figure 2

b-value, or diffusion gradient field strength. It is dependent on gradient field strength amplitude, A, duration of application of the diffusion gradient, d, and the time interval between applying the two motion probing pulses, t.

Figure 3

Diffusion imaging of a cross section through the abdomen at b-values of 0 (a), 200 (b), 800 (c), and 1000(d) s/mm2. At a b-value of 0, note the dark areas representing fluid, flowing or static. At a b-value of 200, flowing fluids are no longer visible. As b-values increase, there is a resultant loss of background tissue, with minimal normal background liver seen at b-value of 1000 (d).

Figure 4

Generic body tissue diffusion curve. Log signal intensity versus b-values for most body tissues show a biexponential behavior with an inflection around b-value of 100 s/mm2. At b-values below 100 (shaded), there is a tissue-dependent and additive perfusion flow effect resulting in increased signal intensity which can skew ADC calculations. At b-values above 100, the perfusion effects in the body are limited, resulting in an essentially linear true diffusion slope.

Figure 5

Tissue diffusion IVIM curve. The apparent diffusion (ADC) is the absolute value of the slope of the curve. As higher b-values are used, the background signal decreases.

Figure 6

Tissue diffusion curve behavior for native tissue, and a restricted lesion within the tissue (a). The restricted lesion has a less steep slope and thus low ADC value. In body imaging typically the greatest separation in signal intensity between native and restricted tissue with background tissue visibility occurs near b-value of 500 s/mm2 (b). Outlines the difference in slope/ADC from near horizontal for solid tissue to the steepest for vascular lesions. The slope differences represent the premise for assessing ADC change of a solid restricted mass initially becoming edematous with treatment with resultant rising ADC values.

Figure 7

ADC misregistration due to breathing. Upper left b-500, lower left b-50 demonstrates a lesion which is conspicuous from background tissue, with the lesion signal intensity dropping on the b-500 image. The relative signal change is typical for a simple cyst, as confirmed with hyperintense features on the axial T2 (lower middle) and coronal STIR sequences (right). The ADC map (upper center) shows no apparent hyperintense signal characteristic of a cyst due to the misregistration. Note the noise of the ADC map in the liver from breathing movement, which is not as apparent in the left kidney due to less movement.

Figure 8

Coronal whole-body images. The b-value image at b-0, shown as white on black, represents a fat saturated fluid sensitive sequence (a), however, the purpose of DWI as a functional technique is to maximize lesion conspicuity by maximizing signal to noise. Typically this is performed with 3 × 3 mm by 5 mm thick slices, as compared with the 1.6 mm × 1.6 mm inplane STIR sequence which demonstrates exquisite anatomic detail (b).

Figure 9

Axial b-500 image (a) demonstrates loss of signal within the left aspect of the liver from cardiac pulsation. The anatomy is not obscured on the axial T2 (b) image. This loss of signal is less problematic at low b-values and can be practically minimized by increasing the number of averages for DWI, or cardiac gating.

Figure 10

Whole-body coronal T1, DWI (b-500 shown as black on white), and STIR images demonstrating the conspicuity of the left adnexal subacute hemorrhage in an endometrioma due to the paramagnetic effects and restricted diffusion of blood degradation products.

Figure 11

Axial b-500 (a) readily demonstrates the increased signal from localized pyelonephritis, which cannot be identified on axial T2 (b) images. The subtle loss of arterial phase enhancement from pyelonephritis is shown in (c), with near complete loss of lesion conspicuity on delayed contrast enhanced images (d).

Figure 12

Prostate imaging. DWI b-800 demonstrates marked susceptibility from gas in the rectum (a) obscuring visualization of the peripheral zone. This can be simply rectified noninvasively by prone positioning (b) of the patient, or with rectal gel.