Clinical Magnetic Resonance Imaging of Ancient Dry Human Mummies Without Rehydration FREE

To the Editor: Ancient human mummies are a unique source to study the evolution of disease.¹ Noninvasive imaging of such historic tissues is of increasing interest in paleoanthropological and paleopathological studies, with conventional radiograph and computed tomography (CT) scan being the standard modalities.² However, the ability to differentiate soft tissues by CT scan in mummies is limited and the ionizing radiation is of uncertain safety to the samples. Clinical magnetic resonance imaging (MRI) has been applied to ancient dry tissues after sample-altering rehydration,^3- 4 a process deemed necessary due to the lack of unbound protons. We show the ability of standard clinical MRI to visualize historic dry tissues without rehydration by use of a newly available MRI pulse sequence.

Ancient artificially embalmed Egyptian mummies (1 head, 2 hands, and 1 foot; circa 1500-1100 BCE; private collection of F.R.) and a naturally mummified Peruvian corpse (circa 1100 CE; Museum of History and Ethnography, St Gallen, Switzerland) were examined using a 3-dimensional ultra-short-echo time (UTE) sequence⁵ on a standard 1.5-T clinical MRI scanner (256² matrix; 32 768 projections, nonselective radiofrequency pulses of 60 microseconds duration; Magnetom Avanto, Siemens AG Medical Solutions, Erlangen, Germany). The 3-dimensional isotropic original data sets were cropped around the sample and volume rendered on standard Leonardo workstations (Siemens AG Medical Solutions) and by Amira version 4.1 software (ZUSE Institute, Berlin, Germany). Correlative multislice CT imaging of all samples was conducted (Orthopedic University Hospital Balgrist, Zurich, Switzerland).

We analyzed proton density–weighted images (isotropic spatial resolution, 0.8-1.1 mm) of soft tissues, bones, mummification-related wrappings, or embalming materials (Figure 1A and B). All samples showed transverse relaxation times (T2) of approximately 300 microseconds, except 1 mummy hand with a T2 of 1.5 milliseconds. Longitudinal relaxation time (T1) was approximately 5 to 10 milliseconds. The magnetic resonance images generally allowed spatial dry mummy tissue discrimination. Subchondral bone appeared bright in comparison with CT scan, and different bone qualities (cortical vs trabecular) could be assessed (Figure 2). Tissues with a high content of collagen type I such as the anuli fibrosi of the intervertebral disk were visible. Among other visualized structures, arteries and ligaments could be discriminated (Figure 3), as well as bone marrow, meninges, and teeth.

Magnetic resonance imaging using a UTE sequence and standard clinical hardware may be a suitable modality for noninvasive studies of dry soft tissues in paleoanthropological, paleopathological, or forensic research. Yet radiology of mummified samples should be interpreted cautiously because dehydration changes the imaging properties (density on radiograph and CT scan, hydrogen density and mobility on MRI) of the tissues. The relaxation times of tissue such as mummified muscles and bone are generally very short (<1 millisecond) in comparison with those in vivo (eg, T2 of human calf muscle is approximately 25 milliseconds), which has previously made magnetic resonance–based imaging impossible. However, the UTE sequence allows for imaging of dry tissues with extremely short relaxation times.

Clinical MRI allows for a sustainable approach⁶ favored for rare historic specimens. Our study suggests that morphological alterations by invasive rehydration of the sample before imaging can now be avoided for dry tissues. This study is limited by evaluation of only 2 specimens, and further research is needed to assess the usefulness of the technique on a broader range of tissues. If this is confirmed, the technique may allow anatomical variations and pathological alterations such as atherosclerotic lesions, intervertebral disk protrusion, or degenerative arthritis to be effectively examined qualitatively as well as quantitatively by spatial proton density distribution. In addition, embalming substances show a large signal variation in the MRI, allowing for improved analysis of chemically diverse materials.