MRI slow diffusion coefficient (SDC) shows liver pyogenic abscess has faster diffusion than adjacent liver parenchyma

It is well known that pyogenic abscess fluid (i.e., pus) tends to demonstrate a very low magnetic resonance imaging (MRI) apparent diffusion coefficient (ADC) regardless of the location of the abscess (1-7). The same as pyogenic abscesses in other places, abscess pyogenic fluid in the liver is known to be associated with very low ADC (8-12). For example, Figure 1 and Figure 2 show abscesses in the brain and in the liver respectively, both demonstrating very low ADC values (1,8). It has been suggested that the low ADC of abscess fluid might be related to the high viscosity of the liquid, however, myxoma is known to have a very high ADC value (13,14).

Figure 1.

MRI of a 37-year-old woman with a pyogenic abscess in the left frontal lobe (A) T2 weighted image; (B) DWI; (C) ADC map; (D) contrast enhanced T1 weighted image. The central component of the lesion shows high signal on T2 weighted image, high signal on DWI (asterisk in B) and low value on ADC map (C, lower than the brain tissues, arrow). Arrow in (D) denotes a ring enhancement after contrast agent administration. It appears unreasonable that the abscess content has a so low ADC measure. Adapted from Feraco et al. (1). ADC, apparent diffusion coefficient; DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging.

Figure 2.

DWI (A) and ADC (B) map of a liver pyogenic abscess (arrows). The abscess liquid has very high DWI signal and very low ADC. It appears unreasonable that the abscess content has a so low ADC measure. Adapted from Castillo and Manterola (8). ADC, apparent diffusion coefficient; DWI, diffusion-weighted imaging.

We argue that there exists a ‘T2-ADC curve’, with ADC measure strongly affected by T2 relaxation time (15,16). T2 can be divided into short T2 band (<60 ms), intermediate T2 band (60–80 ms), and long T2 band (>80 ms). For the short T2 time band, there is a negative correlation between T2 and ADC. For the long T2 time band, there is a positive correlation between T2 and ADC. A tissue likely measures a low ADC if its T2 is close to 70 ms. ADC value is high when: (I) the signal decay between b=0 image and high b value image is fast such as the cases for tissues with short T2 (such as the cases for cartilage and muscle), or (II) tissue has a very long T2 and appear high signal on b=0 image so that there is large amounts of signal to decay between b=0 image and high b value image (such as the cases for intervertebral disc, myxoma, and chondrosarcoma) (16,17). ADC measures a low value when tissue T2 is in the intermediate range of 60–80 ms (15-17). Note that, an application of the diffusion gradients will lead to a slight decrease in observed T2 for tissues, which can be interpreted as an application of diffusion gradients is associated with a longer time of echo (TE) for data acquisition (18). We also found that the abscess pus is likely to have a T2 of about 70–80 ms, and the specific T2 of pyogenic abscess fluid likely contributes to its very low ADC measured by MRI (19). We argue that abscess pus may not have truly restricted diffusion compared with many other in vivo solid tissues (19). Of course, in real practice, abscess pus composition may vary which is partially dependent on the development stage of the abscess, and so does its ADC measure.

Recently, we proposed a metric termed ‘slow diffusion coefficient (SDC)’ (20). In its basic form, SDC is derived from a high b value diffusion-weighted imaging (DWI) image and a higher b value DWI image:

$$SDC=\left[S\left(b_1\right)-S\left(b_2\right)\right]/\left(b_2-b_1\right)$$ [1]

where b₁ and b₂ refer to a high b value (e.g., 500 mm²/s for the case of liver DWI) and a higher b value, respectively (e.g., 700 mm²/s for the case of liver DWI); S(b₁) and S(b₂) denote the image signal-intensity acquired at the high b value and the higher b value, respectively. SDC is expressed in [arbitrary unit (au)/s].

With the conventional ADC measure, the spleen has a much lower ADC than liver, hepatocellular carcinomas (HCCs) have a lower ADC than liver parenchyma, and simple liver cysts have a higher ADC than liver hemangiomas. On the other hand, with SDC analysis, we showed that the spleen has a faster diffusion than the liver, HCCs have a faster diffusion than liver parenchyma, and liver hemangiomas have a faster diffusion than simple liver cysts (20). The liver and spleen have a similar amount of blood perfusion, the spleen is waterier than the liver, and the spleen tissue has a higher contrast-enhanced computed tomography (CT) extracellular volume fraction than the liver (21,22). HCCs are mostly associated with increased blood supply and increased proportion of arterial blood supply and with edema. It is more reasonable with SDC results that the spleen and HCC have a faster diffusion than liver parenchyma. Due to the ‘flushing’ of blood flow inside the hemangioma, it is also more reasonable with SDC results that the diffusion of liver hemangioma liquid is faster than the more ’static’ liquid of the liver cysts (20).

We have earlier collected liver intravoxel incoherent motion (IVIM) series images for a patient with a liver abscess. The data acquisition was conducted with institutional ethical approval and with informed consent obtained from the patient. Imaging was conducted with a Siemens Avanto 1.5T MR scanner. Parameters for DWI included: repetition time (TR) =6,900 ms, TE =81 ms, slice thickness =6 mm, inter-slice gap =1.8 mm, field of view =285×380 mm, matrix =115×192, number-of-excitations =2. SDC pixelwise map was reconstructed by b=500 and b=700 s/mm² DWI images, and the results are shown in Figure 3. Figure 3 shows an abscess that has internal content with overall SDC signal higher than that of the adjacent liver parenchyma.

Figure 3.

A 37-year-old diabetes mellitus man with a liver pyogenic abscess. (A) is post-gadolinium T1-weighted (T1W + C) image, a liver abscess is seen with peripheral rim (arrows). (B) is DWI of b value =700 s/mm², with the abscess showing high signal (arrows). (C) is ADC map (with b values of 0 and 1,000 s/mm²), the abscess (arrows) had internal content with mixed signals and some areas showing slightly lower value (asterisks) than that of the adjacent liver parenchyma. The peripheral rim of the abscess has higher ADC, which is a sign for abscess as described by Park et al. (11). (D) is SDC map (with b values of 500 and 700 s/mm²), the abscess (arrows) had internal content with mixed higher signals than that of the adjacent liver parenchyma. Note that, as shown in (C), ADC of the spleen is notably lower than ADC of the liver parenchyma. On (D), SDC of the spleen is faintly higher than SDC of the liver parenchyma, and the overall SDC of the abscess is higher than SDC of the spleen. Overall, SDC map of this case suffers from low signal-to-noise ratio. ADC, apparent diffusion coefficient; DWI, diffusion-weighted imaging; SDC, slow diffusion coefficient.

In conclusion, the data described in this letter support our earlier argument that the abscess pus does not have truly restricted diffusion compared with many other in vivo solid tissues, despite showing very low ADC value (19).

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