The very low magnetic resonance imaging apparent diffusion coefficient (ADC) measure of abscess is likely due to pus’s specific T2 relaxation time

It is well known that abscess fluid (i.e., pus) tends to demonstrate a very low magnetic resonance imaging (MRI)-derived apparent diffusion coefficient (ADC) regardless of the location of the abscess (1-8). At least in the brain, it may appear counterintuitive that abscess pus, being fluid or semi-fluid, has an ADC measure lower than those of white/grey matters (Figure 1).

Figure 1 MRI of a 37-year-old woman with pyogenic abscess in the left frontal lobe (A: T2-weighted image; B: diffusion-weighted image; 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 diffusion-weighted image (asterisk in B) and low value on ADC map (C, lower than the brain tissues). 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). MRI, magnetic resonance imaging; ADC, apparent diffusion coefficient.

Recently we noted that, regardless of whether b=0 data are included for the ADC calculation, the T2 relaxation time (T2 time) of a tissue or an in vivo substance is strongly associated with ADC measure in many scenarios (9-13) (Figure S1). While ADC measure is affected by many factors, a T2 time of around 70 ms (e.g., from 60 to 80 ms) at 3 T or (or its equivalent values at other magnetic fields) may be associated with the lowest ADC measure (Figure 2). It may be further inferred that, if a tissue (or an in vivo substance) has a T2 time of slightly less than half that of body water (such as gallbladder water: 172 ms at 3 T), then this tissue (or in vivo substance) will have a low ADC measure.

Figure 2 Relationship between T2 time and ADC at 3 T. This graph is modified from Wáng and Ma (12). For data sources also see Wáng and Ma (12). The vertical dotted line at X-axis 172 ms denotes that 172 ms is the T2 time of in vivo body water at 3 T [gallbladder water as an example (14,15)]. The vertical dotted line at X-axis 70 ms denotes that a T2 time around 70 ms at 3 T is associated with very low ADC measure. ADC, apparent diffusion coefficient; ms, millisecond.

Following the discussion above, we looked at whether abscess pus has a T2 time of slightly less than half that of body water. We were able to identify four studies that reported the T2 times of abscess pus and a body fluid. We took the ratios of ‘abscess pus T2 time/body water T2 time’, and the results are shown in Figure 3 (16-21). It is shown that abscess pus has a T2 time of about half that of body water. Wall et al. reported a T2 time for abscess of 81 ms, and T2 time for muscle, liver and urine of 29, 45 and 166 ms, respectively. The T2 times of the later three are consistent with the other reports at 1.5 or 3.0 T (12,22,23). There is a notion that T2 time doesn’t change much over the range of field strengths used for routine clinical MRI (0.2 to 3.0 T) (23). Additionally, Subhawong et al. (24) described a case series of soft tissue masses, which included one abscess case and three cases of ganglion cyst. T2 signal ratio was measured as ‘lesion T2 weighting signal intensity/muscle T2 weighting signal intensity’. The abscess had a T2 signal ratio of 1.48, while the three ganglion cysts had a mean T2 signal ratio of 3.42, with the abscess’ value being 0.43 of those of the ganglion cysts (abscess ADC: 0.63 mm²/s, ganglion cyst ADC mean: 2.49 mm²/s). The specific T2 relaxation time of pyogenic abscess fluid, according to our viewpoint, contributes to very low ADC measured by MRI. We argue that abscess pus may not have truly restricted diffusion compared with many other in vivo solid tissues. Of course, in real practice, abscess pus composition may vary, and so does its ADC measure (1,4). The discussion in this letter refers to the common scenarios.

Figure 3 Ratios of ‘abscess pus T2 time/body water T2 time’ from four studies. 3-T ref is the ratio of 70 ms (the T2 time associated with the lowest ADC according to Figure 2)/172 ms (in vivo body water T2 time). Wall et al. (16) and Brown et al. (17): various abscess fluid T2 time/urine T2 time; Cieszanowski et al. (18): liver abscess fluid/liver cyst fluid. Yilmaz et al. (19): odontogenic jaw abscess fluid/odontogenic jaw cyst fluid. T2 time measurement is affected by magnetic field strength and homogeneity, pulse sequence, and data acquisition parameters, etc., therefore the ratio of ‘abscess pus T2 time/body water T2 time’ in the same study is presented instead of absolute T2 time value. Note that the composition of a body fluid may affect its T2 time (20,21). ADC, apparent diffusion coefficient; ref, reference; ms, millisecond.

Now we look at Figure 1 again. Brain tissues are generally noted to have a short T2 time with grey/dark signal on T2 weighted image. For 3T data, Wansapura et al. (25) described that the average T2 values for occipital and frontal gray matter are 41.6 and 51.8 ms, respectively, and average T2 values for occipital and frontal white matter are 48.4 and 44.7 ms, respectively. According to Figure 3, abscess pus is roughly estimated to have a T2 time of around 80 ms at 3T. Therefore, according to Figure 2, an increase of T2 time from grey/white matter values toward 80 ms would be associated with a lower ADC measure. The abscess high signal shown on Figure 1B (a high b-value diffusion-weighted image) likely does not reflect restricted fluid diffusivity, instead reflects the specific T2 time of the abscess fluid.

Acknowledgments

Funding: None.

Footnote

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-23-1363/coif). YXJW serves as the Editor-in-Chief of Quantitative Imaging in Medicine and Surgery. The author has no other conflicts of interest to declare.

Ethical Statement: The authors is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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