Introduction
Postmortem imaging now plays a significant role in the paediatric and perinatal minimally invasive autopsy, largely due to a decline in parental acceptance of traditional autopsy techniques fuelling a drive to develop a novel less invasive postmortem approach [
1,
2]. Several different imaging modalities are being employed in this regard, including post-mortem (PM) skeletal radiographs, typically used to diagnose skeletal dysplasias and assess bone gestational changes [
3], and cross-sectional imaging techniques including PM computerized tomography (CT) and PM magnetic resonance imaging (PMMR), which have been reviewed extensively elsewhere [
4]. Postmortem CT is rapidly available, and readily gives vascular and bone detail, it has not been shown to be diagnostically accurate for soft tissue abnormalities in fetuses and children. PMMR has been shown to have high diagnostic accuracy for perinatal abnormalities, in both small preliminary (e.g., Breeze et al. [
5]) and large blinded studies [
6], and PMMR is widely believed to likely become the mainstay of a less invasive perinatal imaging service [
4].
However, there remain areas of potential diagnostic improvement, with true ischaemia and necrosis difficult to evaluate with conventional magnetic resonance (MR) techniques [
7]. Diffusion-weighted imaging (DWI) is a structural MR imaging technique which measures the diffusivity of water molecules. It can be measured quantitatively and expressed as an apparent diffusion coefficient (ADC) value, such that a lower ADC value represents a tissue with more restricted water diffusivity. ADC measures of body organs have been used to evaluate tumour characteristics in paediatric tumours, such as Wilms tumours and neuroblastoma [
8‐
10].
Following death, there are several changes in tissues due to tissue breakdown (autolysis) and decomposition. In the initial hours following death, tissue ischaemia will occur, followed by a period of cell lysis, membrane breakdown, fluid redistribution and gas formation/putrefaction [
11]. We hypothesise that PMMR DWI changes in individual organs will correspond to these changes, with an initial decrease in ADC values secondary to ischaemia (such as in stroke), followed by a possible increase in ADC values following cell breakdown and autolysis. Human PM specimens have recently been used to confirm DWI tractography [
12], and several animal studies have suggested an effect of PM interval on DWI changes in the brain [
13‐
17].
This study was therefore designed to establish whether (a) DWI changes are detectable by PMMR in fetuses and stillbirths, (b) PMMR DWI changes increase following death, possibly due to cellular barriers breaking down and increasing water movement, and (c) PMMR DWI changes correlate with PM interval, and could be possible surrogate markers for autolysis.
Discussion
The findings of this study demonstrate that DWI characteristics of thoraco-abdominal organs change following death, and can be evaluated using PMMR. Liver and renal cortex ADC values were lower in PM cases than normal controls. Lung ADC values were higher, and there was a significant correlation between lung ADC and PM interval (but not with ADC of any abdominal organ). Lung ADC changes could therefore be useful in estimating PM interval in future cases, although further research is needed to understand the fall in organ ADC which occurs during the PM period.
Our first observation was that live infant ADC values in our study were significantly lower than values quoted in the literature, for all of the abdominal solid organs (liver, spleen and renal cortex). This is likely to reflect both variation between acquisition protocols between different machines and vendors, as well as a possible correlation between ADC and age. Gawande et al. [
10] present a summary table of common values in the literature, but most of the studies quoted were in adults. It is recognised that paediatric solid organ ADC values are typically lower than those in adults [
20], perhaps due to changes in anatomy in growing tissues, although we were unable to demonstrate this amongst the live children in our study. The age-dependence correlation in live infants may be much shallower in gradient (over years, rather than weeks or months) than we could detect in our population under 6 months.
Our second observation was that there were differences between live and PM ADC values in different solid organs, particularly the liver and renal cortex. This may be related to inherent water content of different organs and their rate of decomposition. Although the results did not reach statistical significance for the spleen, the same trend as seen for the liver and kidney was observed, and so the same process is likely to occur. Muscle was used as a control and no trend was observed for live versus PM differences. As the normal ratio of values between organs was retained, we surmise that the same changes are therefore being seen in most organs, in keeping with generalised rather than abdominal organ specific changes. What these changes are on a cellular basis remains to be determined.
Our third observation from this dataset was that lung ADC changes correlate with postmortem interval in this study. As lung ADC values were higher, and body organ ADC values were lower than normal controls, we hypothesise that two different processes account for the ADC changes observed in this study. Lung ADC changes are likely to represent fluid accumulation, since there is no signal obtained from lung parenchyma in live infants as the lung volume is predominantly composed of air. Accumulation of fluid in the lungs and pleural spaces over time is a recognised phenomenon following death, but the imaging correlates of this have not previously been identified, nor quantified to correlate with PM interval. The accuracy of PMMR in determining PM interval may be useful in cases in which the time interval since death is unknown, such as delayed presentation and forensic cases.
Our final observation was that there was also no correlation between PM interval and ADC changes in other body organs in our population. Both the timing and nature of these changes are interesting. Our results imply a rapid change in ADC within hours after death which stabilises within 24–72 hours, which we did not detect given our PM interval range of days rather than hours after death. Several previous studies have described an effect of PM interval on DWI or DTI characteristics of animal brains in the first few hours after death [
13,
14], although different immersion/fixation techniques may have affected these results [
13‐
16]. The present study avoids the interaction of immersion and fixation techniques by imaging whole-body PM cases. Other studies have shown that ADC values in the PM adult brain are significantly lower than normal controls (49–72 % lower), and may correlate with PM interval, although only three time intervals were compared (<24 hours, 24–48 hours, and >48 hours) [
17]. Interestingly, our study found a correlation with lung ADC which continued beyond 14 days. However, the degree of inter-individual variability (between cases) for organ ADC values may be greater than the correlation with PM interval, thus limiting the accuracy of results. Serial measurements on the same case may provide further data regarding this mechanism.
We acknowledge that unlike lung changes, other organ ADC changes are unlikely to represent simple fluid accumulation. DWI measures the diffusivity, or freedom of movement of water molecules, with the magnitude of signal loss between dephasing and rephasing gradients proportional to the diffusivity of the tissue [
21]. Several factors interact to change the ADC values, including water content, tissue cellularity and integrity of intracellular membranes [
22]. Whilst there was a decrease in ADC values likely secondary to ischaemia, histological analysis is required to shed light on which of these factors is likely to contribute greatest to the imaging changes identified. Autolysis-associated histological changes are well recognised in PM samples, with variation between tissue types, but quantitation of such changes in relation to factors including cause of death and PM interval is difficult. The liver and kidney are both metabolically and enzymatically active tissues and it is generally accepted that autolytic activity causes cellular breakdown, but the precise mechanism relating changes in ADC values over time to histological features would require serial sampling from the early and immediate PM period, perhaps in an animal study.
Despite this being the first study which addresses changes in foetal and stillbirth organs using PMMR, the limitations include the relatively small sample size, and that we were unable to further elucidate specific mechanisms of body organ changes across different organ types. The data is also based on a comparison of foetal PM cases and live infants, with a small (gestational) age difference. An alternative dataset for comparison with foetal PM ADC values would be antenatal foetal body organ and lung MRI, but these were not available at the time of this study. However, results from the foetal MRI literature largely support our findings. For example, Savelli et al. [
23] found that normal foetal renal ADC values ranged between 1.06 and 1.33 x10
-3 mm
2/s, which supports our findings as these values are comparable to our infant group but significantly higher than the PM group. Other groups have assessed foetal lung ADC changes with debate as to whether there is true a correlation with gestational age. Manganaro et al. [
24] found that foetal lung values increased from 1.2 x10
-3 mm
2/s at 18 weeks to 3.9 x10
-3 mm
2/s at 36 weeks, with a mean lung ADC of 2.35 ± 0.6 x10
-3 mm
2/s, and correlation suggesting that foetal lung ADC can be used as a surrogate marker of lung maturity. However, two other papers document foetal lung ADCs between 1.63 and 2.13 x10
-3 mm
2/s which were highly reproducible and largely dependent upon gravitational changes on the lung irrespective of gestational age [
25,
26]. Our PM lung ADC values in a similar group of late foetal deaths are much lower than the values quoted for normal foetuses, and we did not find a correlation with gestation.
We also used a temperature correction based on unrestricted diffusion, whereas different properties in different tissues could display different sensitivity to temperature. Whilst this could bias the between-group comparison (in favour of not finding a difference between groups), it does not affect the observed correlation between PM interval and lung ADCs. In addition, this method has been used to measure temperature changes in tissue with reasonable accuracy [
19]. Finally, we excluded cases with lung abnormalities or organ disease, in which the presence of additional pathology could affect these results.
In summary, PM lung ADC values probably reflect fluid accumulation in the lungs after death, and correlate with PM interval. This could be a useful imaging marker of PM interval in future cases. However, further research is needed to understand the timing and nature of the different changes in individual organ ADC values which occurs soon after death.
Acknowledgements
The scientific guarantor of this publication is Owen J. Arthurs. The authors of this manuscript declare no relationships with any companies, whose products or services may be related to the subject matter of the article.
OA is funded by an NIHR Clinician Scientist Fellowship, AMT and NJS are funded by NIHR Senior Investigator awards, Great Ormond Street Children’s Charity and the Great Ormond Street Hospital Biomedical Research Centre. None of the funding bodies had any role in analysis of data, results or conclusions of the study.
This article presents independent research funded by the National Institute for Health Research (NIHR) and supported by the Great Ormond Street Hospital Biomedical Research Centre. The views expressed are those of the author(s) and not necessarily those of the National Health Service (NHS), the NIHR or the Department of Health.
One of the authors has significant statistical expertise. Written informed consent was obtained from all subjects (patients) in this study. None of the study has been previously reported. Methodology: prospective recruitment with retrospective comparison, case-control study.