Emily Louise Baadsvik1, Markus Weiger1, Romain Froidevaux1, Manuela Barbara Rösler1, David Otto Brunner1, Lena Öhrström2, Patrick Eppenberger2, Frank J. Rühli2, and Klaas Paul Pruessmann1
1Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland, 2Institute of Evolutionary Medicine, University of Zurich, Zurich, Switzerland
Synopsis
Evolutionary medicine aims to study disease
development over long timescales, and through the study of mummified human
remains, tissue information dating back thousands of years becomes accessible. Due
to their status as ancient relics, nonintrusive techniques are preferable, and
to date CT imaging is the most common modality. However, CT images lack
soft-tissue contrast, making complementary MRI data desirable. Due to the
extensively dehydrated nature and short T2 times of mummified tissues, acquiring
such data is challenging. This research explored the use of the zero echo-time
sequences and a high-performance gradient in mummy MRI, yielding yet
unparalleled image quality.
Introduction
The
field of evolutionary medicine aims to broaden our understanding of disease
development by considering pathology from a long-term perspective. Through the study
of mummified human tissue timeframes of several thousand years are unlocked,
but due to their scarcity and historical value, great care must be taken when
examining such samples. Medical imaging modalities are therefore natural
contenders, and currently computed tomography (CT) is the most widespread
technique. However, CT does not provide sufficient soft-tissue contrast, and so
complementary MRI data is desired1-3. Acquiring such data is
challenging, both due to the dehydrated nature of artificially mummified
samples and the short T2 times associated with the remaining tissues (≤ a
few hundred microseconds)4. Thus mummified tissues are practically
invisible to standard MRI techniques, and specialised short-T2 approaches must
be used.
It has previously been shown that zero echo-time (ZTE)-based sequences5
together with a high-performance gradient insert6 enable high-resolution
MR imaging of samples with short T2 values7. In this work, the
ability of this system was investigated for improved depiction of mummified
human tissues. Methods
Three ancient Egyptian mummy samples (1500-1100 BC)
were imaged, one head, one foot and one hand1,2. For the
musculoskeletal samples a custom RF solenoid coil was constructed, with a PMMA
tube length of 300 mm, a diameter of 120 mm and 7 copper windings spread over 250
mm of the tube, similar to1. A single-loop surface coil of 80 mm
diameter was used for initial imaging of the hand sample. The head was imaged
with a 1H-free 8-channel array coil8. All images were acquired with
a 3T Philips Achieva scanner (Philips Healthcare, Best, the Netherlands), but using
a custom RF system9 including symmetrically biased T/R switches10.
Furthermore, a high-performance gradient insert providing a strength (G) up to
200 mT/m and a slew rate up to 1200 mT/m/ms at 100 % duty cycle was used6.
ZTE imaging sequences use a radial centre-out
pure frequency-encoding architecture, and their defining characteristic is that
the readout gradient is prepared before spin excitation in order to achieve
immediate full-speed k-space encoding. However, because the RF system has a
finite dead-time Δ (includes half the RF pulse duration, T/R switching and the filter
group delay) before signal acquisition can begin, there is data missing from
central k-space5. From the various ways of dealing with this data gap,
the hybrid filling (HYFI) technique11 was used. HYFI fills the very centre
of k-space with single-point Cartesian acquisition and then divides the remainder
of the gap into frequency-encoded radial shells, i.e. it is a hybrid between the
PETRA12 and WASPI13 techniques. In this way, it increases
SNR efficiency w.r.t. PETRA while largely maintaining image quality. See Fig. 1
for sequence details.
While
immediate k-space encoding is vital for short-T2 imaging, to avoid resolution
loss it is also necessary to complete each acquisition before significant
signal decay14. In a frequency-encoding scheme, we therefore rely on
a strong gradient to cover k-space swiftly. To investigate this effect on
images of mummified tissue, the gradient insert was used with increasing
gradient strengths from the conventional 31 mT/m up to the maximum of 200 mT/m.Results
Fig.
2 shows HYFI images of the hand sample for different values of system dead-time
(equivalent to TE) and gradient strength, using both the surface and solenoid
coils. A CT image is also included for comparison. This sample was previously
imaged with other short-T2 sequences1 at a resolution (Δr)
of (0.9 mm)3, but these images exhibit severe blurring and image
degradation. We see that while an increase in G up to around 100 mT/m
significantly improves the image quality, as expected from7,
increasing it further yields diminishing returns. Increasing the dead-time
reduces the blurry image background, which is explained by attributing the
background signal to T2 components too short to be encoded even with this
protocol: The longer we delay acquisition, the more these components have
decayed. The solenoid coil offers uniform sensitivity throughout the sample but
requires longer scan times than the surface coil.
Fig.
3 contains a more detailed look at the main images of the hand sample presented
in Fig. 2. The top row focuses on different dead-time images, while the bottom row
emphasises the different features that can be resolved. Fig. 4 has the same
general layout as Fig. 3, but for the foot sample. Indicative sample
constituents can be identified from the different dead-time images.
Fig.
5 illustrates the resolvable features in the head sample. CT data is included
for comparison. While both CT and HYFI capture the same general tissues (bones,
tendons, skin), the mummification substances in the skin (Fig. 2, brightest
signal) are isointense w.r.t. surrounding tissues in CT, while in MRI they
exhibit varying contrast.Discussion and Conclusion
Our
results further demonstrate the capabilities of the HYFI sequence and
high-performance gradient in short-T2 MRI, and the acquired images have higher
resolution and greater image quality than any existing work in the field. These
images can be used to identify details of the mummification process and aid researchers
with intricate knowledge of anatomy, tissue properties and pathology in studying the differences between these ancient
samples and modern humans. Acknowledgements
No acknowledgement found.References
- Özen AC, Ludwig U, Öhrström LM, Rühli FJ, Bock M.
Comparison of ultrashort echo time sequences for MRI of an ancient mummified
human hand. Magnetic Resonance in Medicine. 2016;75(2):701-708
- Öhrström LM, von Waldburg H, Speier P, Bock M, Suri
RE, Rühli FJ. Scenes from the past: MR imaging versus CT of ancient Peruvian
and Egyptian mummified tissues. RadioGraphics. 2013;33(1):291-296
- Giovannetti G, Guerrini A, Carnieri E, Salvadori PA. Magnetic resonance imaging for
the study of mummies. Magnetic Resonance Imaging. 2016;34(6):785-794
- Posh JC. Technical limitations on the use of
traditional magnetic resonance imaging in the evaluation of mummified remains:
A view from a hands-on radiologic technologist’s perspective. Anatomical
Record. 2015;298(6):1116-1124.
- Weiger M, Pruessmann KP. MRI with zero echo time.
Encyclopedia of Magnetic Resonance (John Wiley and Sons, Ltd, 2012)
- Weiger M, Overweg J, Rösler MB, Froidevaux R, Hennel
F, Wilm BJ, Penn A, Sturzenegger U, Schuth W, Mathlener M, Borgo M, Börnert P,
Leussler C, Luechinger R, Dietrich BE, Reber J, Brunner DO, Schmid T, Vionnet
L, Pruessmann KP. A high-performance gradient insert for rapid and short-T2
imaging at full duty cycle. Magnetic Resonance in Medicine.
2018;79(6):3256-3266
- Froidevaux R, Weiger M, Rösler MB, Brunner DO, Wilm B,
Dietrich B, Reber J, Pruessmann KP. Pushing the limits of short-T2 MRI: 200
mT/m gradient strength and 2 MHz bandwidth. Proceedings of the 26th
Annual Scientific Meeting of ISMRM (Paris, France, 2018)
- Rösler MB, Leussler C, Brunner DO, Schmid T, Weiger M,
Hennel F, Luechinger RC, Pruessmann KP. A head transmit-receive array for a
high performance gradient insert. Proceedings of the 27th Annual
Meeting of ISMRM (Montreal, Canada, 2019)
- Dietrich BE, Brunner DO, Wilm BJ, Barmet C, Gross S,
Kasper L, Haeberlin M, Schmid T, Vannesjo SJ, Pruessmann KP. A field camera for
MR sequence monitoring and system analysis. Magnetic Resonance in Medicine.
2016;75:1831-1840
- Brunner DO, Furrer L, Weiger M, Baumberger W, Schmid
T, Reber J, Dietrich BE, Wilm BJ, Froidevaux R, Pruessmann KP. Symmetrically
biased T/R switches for NMR and MRI with microsecond dead time. Journal of
Magnetic Resonance. 2016;263:147-155
- Froidevaux R, Weiger M, Rösler MB, Brunner DO, Pruessmann
KP. HYFI: Hybrid filling of the dead-time gap for faster zero echo time
imaging. Proceedings of the 27th Annual Meeting of ISMRM (Montreal,
Canada, 2019)
- Grodzki DM, Jakob PM, Heismann B. Ultrashort echo time
imaging using pointwise encoding time reduction with radial acquisition
(PETRA). Magnetic Resonance in Medicine. 2012;67(2):510-518
- Wu Y, Dai G, Ackerman JL, Hrovat MI, Glimcher MJ,
Snyder BD, Nazarian A, Chesler D. Water- and fat-suppressed proton projection
MRI (WASPI) of rat femur bone. Magnetic Resonance in Medicine. 2007;57(3):554-567
- Weiger M, Pruessmann KP. Short-T2 MRI: Principles and
recent advances. Progress in Nuclear Magnetic Resonance Spectroscopy. 2019