Diffusion-weighted imaging in the abdomen and pelvis: concepts and applications

Abstract

Diffusion-weighted magnetic resonance (MR) imaging allows the detection of focal solid and cystic lesions in the abdomen and pelvis and, if pitfalls are to be avoided, is most effectively used in conjunction with other imaging sequences. It is important to recognize that the strength of the diffusion sensitizing gradient (b value) can and should be adjusted to ensure optimal evaluation of the body region or organ being imaged, and that more than one b value is necessary for tissue characterization. The success of lesion detection and characterization largely depends on the extent of tissue cellularity because increased cellularity is associated with impeded diffusion, as indicated by a reduction in the apparent diffusion coefficient. It is also important to recognize that certain normal tissues such as the endometrium are highly cellular and as such demonstrate restricted diffusion, which should not be misinterpreted as disease. Impeded diffusion can also be seen in highly viscous cystic lesions such as abscesses. Diffusion-weighted imaging is an evolving technology with the potential to improve tissue characterization when findings are interpreted in conjunction with findings obtained with other conventional MR imaging sequences.

Figure 1

Schematic illustrates water molecule movement. In A, water molecules in a container alone move randomly (Brownian motion). In B, highly cellular tissue impedes the movement of water molecules. Their movement can be categorized as intravascular, intracellular, or extracellular. In C, tissue of low cellularity or with defective cells permits greater water molecule movement.

Figure 10a

Prostate cancer in a 64-year-old man. (a) On an axial T2-weighted MR image obtained with an endorectal coil, the peripheral zone of the prostate gland appears normal (ie, has high signal intensity), but a questionable focus of hypointensity is seen in the anterior left central portion of the gland (arrow). (b) Axial ADC map more clearly depicts the questionable area as a hypointense focal region of restricted diffusion (arrow).

Figure 10b

Prostate cancer in a 64-year-old man. (a) On an axial T2-weighted MR image obtained with an endorectal coil, the peripheral zone of the prostate gland appears normal (ie, has high signal intensity), but a questionable focus of hypointensity is seen in the anterior left central portion of the gland (arrow). (b) Axial ADC map more clearly depicts the questionable area as a hypointense focal region of restricted diffusion (arrow).

Figure 11a

Renal cell carcinoma in a 54-year-old woman. (a) Fast spin-echo T2-weighted MR image shows a mass (arrow) that arises from the left kidney. The mass demonstrates intermediate signal intensity, a finding that is consistent with a solid mass or hemorrhagic cyst. (b) Axial diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (c) On an axial ADC map, the mass (arrow) demonstrates low signal intensity (restricted diffusion), a finding that is consistent with a solid tumor.

Figure 11b

Renal cell carcinoma in a 54-year-old woman. (a) Fast spin-echo T2-weighted MR image shows a mass (arrow) that arises from the left kidney. The mass demonstrates intermediate signal intensity, a finding that is consistent with a solid mass or hemorrhagic cyst. (b) Axial diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (c) On an axial ADC map, the mass (arrow) demonstrates low signal intensity (restricted diffusion), a finding that is consistent with a solid tumor.

Figure 11c

Renal cell carcinoma in a 54-year-old woman. (a) Fast spin-echo T2-weighted MR image shows a mass (arrow) that arises from the left kidney. The mass demonstrates intermediate signal intensity, a finding that is consistent with a solid mass or hemorrhagic cyst. (b) Axial diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (c) On an axial ADC map, the mass (arrow) demonstrates low signal intensity (restricted diffusion), a finding that is consistent with a solid tumor.

Figure 2a

(a) Schematic illustrates the effect of a diffusion-weighted sequence on water molecules (solid circles) within highly cellular tissue or a restricted environment. The diffusion-weighted sequence is fundamentally a T2-weighted sequence with the application of a dephasing gradient (diffusion sensitizing gradient) prior to the 180° RF pulse, followed by a symmetric rephasing gradient after the 180° pulse. Water molecules within a restricted environment do not move long distances and acquire phase shifts during the application of the first gradient that are cancelled out by phase shifts acquired during the second (opposing) gradient. As a result, no net loss in signal intensity occurs (aside from normal T2 decay). (b) Schematic illustrates the effect of a diffusion-weighted sequence on water molecules (solid circles) within tissue with low cellularity or a less restricted environment. Water molecules within a less restricted environment can move long distances. Such highly mobile molecules acquire phase information from the first gradient, but because of their motion, their signal does not completely rephase with the second gradient, resulting in a net loss in signal intensity in addition to normal T2 decay. SE = spin-echo.

Figure 2b

(a) Schematic illustrates the effect of a diffusion-weighted sequence on water molecules (solid circles) within highly cellular tissue or a restricted environment. The diffusion-weighted sequence is fundamentally a T2-weighted sequence with the application of a dephasing gradient (diffusion sensitizing gradient) prior to the 180° RF pulse, followed by a symmetric rephasing gradient after the 180° pulse. Water molecules within a restricted environment do not move long distances and acquire phase shifts during the application of the first gradient that are cancelled out by phase shifts acquired during the second (opposing) gradient. As a result, no net loss in signal intensity occurs (aside from normal T2 decay). (b) Schematic illustrates the effect of a diffusion-weighted sequence on water molecules (solid circles) within tissue with low cellularity or a less restricted environment. Water molecules within a less restricted environment can move long distances. Such highly mobile molecules acquire phase information from the first gradient, but because of their motion, their signal does not completely rephase with the second gradient, resulting in a net loss in signal intensity in addition to normal T2 decay. SE = spin-echo.

Figure 3

Axial diffusion-weighted image (b = 0 sec/mm²) obtained in a 60-year-old woman shows a signal void within the inferior vena cava (arrow). Small b values will result in decreased signal of highly mobile water molecules such as occur within vessels. Such images are referred to as black-blood images due to the decreased signal of the fast-flowing blood within vessels.

Figure 4

Sagittal diffusion-weighted image (b = 800 sec/mm²) obtained in a 38-year-old woman shows the endometrium with normal high signal intensity (arrow).

Figure 5a

(a) Graph illustrates signal intensity versus b values at diffusion-weighted imaging (DWI) of tissue with normal versus restricted diffusion. (b) Graph illustrates the logarithm of signal intensity versus b values at diffusion-weighted imaging of normal liver versus liver tumor. The signal of water molecules decays exponentially with increasing b values for different tissue types. The decay in signal is reduced in tissues with restricted diffusion (eg, tumor). The ADC represents the slope (gradient) of the plotted lines. The greater the number of b values used in the analysis, the more accurate the ADC calculation.

Figure 5b

(a) Graph illustrates signal intensity versus b values at diffusion-weighted imaging (DWI) of tissue with normal versus restricted diffusion. (b) Graph illustrates the logarithm of signal intensity versus b values at diffusion-weighted imaging of normal liver versus liver tumor. The signal of water molecules decays exponentially with increasing b values for different tissue types. The decay in signal is reduced in tissues with restricted diffusion (eg, tumor). The ADC represents the slope (gradient) of the plotted lines. The greater the number of b values used in the analysis, the more accurate the ADC calculation.

Figure 6a

T2 shine-through in a 42-year-old woman with a small cyst in the left hepatic lobe. (a) T2-weighted image shows a cyst (arrow) with very high signal intensity despite its small size. (b) On a diffusion-weighted image (b = 500 sec/mm²), the cyst (arrow) reflects T2 shine-through rather than restricted diffusion.

Figure 6b

T2 shine-through in a 42-year-old woman with a small cyst in the left hepatic lobe. (a) T2-weighted image shows a cyst (arrow) with very high signal intensity despite its small size. (b) On a diffusion-weighted image (b = 500 sec/mm²), the cyst (arrow) reflects T2 shine-through rather than restricted diffusion.

Figure 7a

Hemangioma. (a) Gadolinium-enhanced gradient-echo T1-weighted MR image shows a hemangioma with peripheral nodular enhancement (arrow). (b) On a fast spin-echo T2-weighted MR image, the hemangioma (arrow) is lobulated and demonstrates very high signal intensity. (c) On a diffusion-weighted image (b = 500 sec/mm²), the hemangioma (arrow) demonstrates high signal intensity due to slow-flowing blood. (d) ADC map shows the diffusion of the hemangioma (arrow) to be similar to that of background liver (ie, unimpeded).

Figure 7b

Hemangioma. (a) Gadolinium-enhanced gradient-echo T1-weighted MR image shows a hemangioma with peripheral nodular enhancement (arrow). (b) On a fast spin-echo T2-weighted MR image, the hemangioma (arrow) is lobulated and demonstrates very high signal intensity. (c) On a diffusion-weighted image (b = 500 sec/mm²), the hemangioma (arrow) demonstrates high signal intensity due to slow-flowing blood. (d) ADC map shows the diffusion of the hemangioma (arrow) to be similar to that of background liver (ie, unimpeded).

Figure 7c

Hemangioma. (a) Gadolinium-enhanced gradient-echo T1-weighted MR image shows a hemangioma with peripheral nodular enhancement (arrow). (b) On a fast spin-echo T2-weighted MR image, the hemangioma (arrow) is lobulated and demonstrates very high signal intensity. (c) On a diffusion-weighted image (b = 500 sec/mm²), the hemangioma (arrow) demonstrates high signal intensity due to slow-flowing blood. (d) ADC map shows the diffusion of the hemangioma (arrow) to be similar to that of background liver (ie, unimpeded).

Figure 7d

Hemangioma. (a) Gadolinium-enhanced gradient-echo T1-weighted MR image shows a hemangioma with peripheral nodular enhancement (arrow). (b) On a fast spin-echo T2-weighted MR image, the hemangioma (arrow) is lobulated and demonstrates very high signal intensity. (c) On a diffusion-weighted image (b = 500 sec/mm²), the hemangioma (arrow) demonstrates high signal intensity due to slow-flowing blood. (d) ADC map shows the diffusion of the hemangioma (arrow) to be similar to that of background liver (ie, unimpeded).

Figure 8a

Hepatocellular carcinoma in an 81-year-old woman. (a) Gadolinium-enhanced T1-weighted MR image shows a hypervascular mass (arrow). (b) On a fat-suppressed fast spin-echo T2-weighted MR image, the mass is slightly hyperintense (arrow). (c) Diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (d) On an ADC map, the mass demonstrates restricted diffusion (arrow).

Figure 8b

Hepatocellular carcinoma in an 81-year-old woman. (a) Gadolinium-enhanced T1-weighted MR image shows a hypervascular mass (arrow). (b) On a fat-suppressed fast spin-echo T2-weighted MR image, the mass is slightly hyperintense (arrow). (c) Diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (d) On an ADC map, the mass demonstrates restricted diffusion (arrow).

Figure 8c

Hepatocellular carcinoma in an 81-year-old woman. (a) Gadolinium-enhanced T1-weighted MR image shows a hypervascular mass (arrow). (b) On a fat-suppressed fast spin-echo T2-weighted MR image, the mass is slightly hyperintense (arrow). (c) Diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (d) On an ADC map, the mass demonstrates restricted diffusion (arrow).

Figure 8d

Hepatocellular carcinoma in an 81-year-old woman. (a) Gadolinium-enhanced T1-weighted MR image shows a hypervascular mass (arrow). (b) On a fat-suppressed fast spin-echo T2-weighted MR image, the mass is slightly hyperintense (arrow). (c) Diffusion-weighted image (b = 500 sec/mm²) shows the mass with high signal intensity (arrow). (d) On an ADC map, the mass demonstrates restricted diffusion (arrow).

Figure 9a

Diffuse liver disease. (a) ADC map obtained in a 36-year-old woman with grade 0 steatosis and stage 0 fibrosis shows no pathologic findings. Arrow indicates the right hepatic lobe. (b, c) ADC maps obtained in a 56-year-old man with nonalcoholic fatty liver disease, grade 1 steatosis, and stage 0 fibrosis (b) and in a 50- year-old woman with nonalcoholic fatty liver disease, grade 1 steatosis, and stage 1 fibrosis (c) show restricted diffusion in the right hepatic lobe (arrow). Regions of interest placed in the right hepatic lobe may be used to measure ADC. ADC measurements of the left lateral segment are typically degraded by cardiac motion.

Figure 9b

Diffuse liver disease. (a) ADC map obtained in a 36-year-old woman with grade 0 steatosis and stage 0 fibrosis shows no pathologic findings. Arrow indicates the right hepatic lobe. (b, c) ADC maps obtained in a 56-year-old man with nonalcoholic fatty liver disease, grade 1 steatosis, and stage 0 fibrosis (b) and in a 50- year-old woman with nonalcoholic fatty liver disease, grade 1 steatosis, and stage 1 fibrosis (c) show restricted diffusion in the right hepatic lobe (arrow). Regions of interest placed in the right hepatic lobe may be used to measure ADC. ADC measurements of the left lateral segment are typically degraded by cardiac motion.

Figure 9c

Diffuse liver disease. (a) ADC map obtained in a 36-year-old woman with grade 0 steatosis and stage 0 fibrosis shows no pathologic findings. Arrow indicates the right hepatic lobe. (b, c) ADC maps obtained in a 56-year-old man with nonalcoholic fatty liver disease, grade 1 steatosis, and stage 0 fibrosis (b) and in a 50- year-old woman with nonalcoholic fatty liver disease, grade 1 steatosis, and stage 1 fibrosis (c) show restricted diffusion in the right hepatic lobe (arrow). Regions of interest placed in the right hepatic lobe may be used to measure ADC. ADC measurements of the left lateral segment are typically degraded by cardiac motion.