Synopsis
Once the technical limitations are cleared, Cr-CEST could
replace 31P-MRS, becoming a powerful tool for assessment of treatment outcomes
and diagnosis of muscular disorders, due to its superior spatial resolution and
sensitivity. Phantom measurements show how Cr concentration and pH are
linearly correlated with CEST contrast maps. The preliminary in-vivo measurements, with a time resolution
of 13.1s per repetition, produce an enhancement of gastrocnemius muscle of 12% during
plantar flexion exercise. However, a further
increased time resolution is anticipated for dynamic studies, close to those
routinely used in dynamic 31P-MRS.
PURPOSE
Chemical
Exchange Saturation Transfer (CEST)
is a relatively newly developed contrast mechanism
which allows to measure small amounts of
metabolites with enhanced sensitivity.[1]
Creatine (Cr) is an endogenous
metabolite with Amine protons (-NH2), which resonate around 1.8ppm
downfield from the water resonance. It is one of the major metabolites involved
in muscle (skeletal and myocardial) energy metabolism, which can be altered in
different disorders.[2]
The Creatine Kinase
(CK) reaction is the most immediate mechanism to regenerate ATP (adenosine
triphosphate), illustrated by:$$ADP + PCr + H^{+}\overset{CK}{\overset{\rightarrow}{\leftrightarrow}} Cr + ATP$$During
muscle contraction phosphocreatine (PCr), which is storage of cellular energy,
is converted into free creatine (Cr) to maintain ATP, the supply of the energy
currency, on a constant level.
Phosphorous MR
spectroscopy (31P-MRS) is nowadays the gold standard for dynamic
studies on muscle energy metabolism. However once the technical limitations are
cleared, Cr-CEST could replace 31P-MRS, becoming a powerful tool for
assessment of treatment outcomes and diagnosis of muscular disorders, due to
its superior spatial resolution and sensitivity.
MATERIALS
AND METHODS
CEST
Imaging: Performed on a whole body 7T MR Magnetom system from Siemens
Healthcare, Erlangen, Germany with a 1H 28-channel knee coil
from QED, Mayfield Village,Ohio,USA. CEST images were acquired with five Gaussian
saturation pulses of 100ms each separated by a 31ms delay and amplitude B1=3.0T for phantoms and 0.8μT for in-vivo with a Turbo-FLASH readout.
Phantom Design:
Five different
concentration ratios between PCr
and Cr (100%-0%, 75%-25%, 50%-50%, 25-75% and 0%-100%, pH=7) were prepared in
PBS (Phosphate Buffered Saline)
solution and 100% inorganic phosphate (Pi) in distilled water with a
total concentration of 50mM. Six different pH solutions were also prepared
(7.2, 7.0, 6.8, 6.6, 6.4, 6.2, [Cr]=50mM). All samples were kept in plastic sealed
syringes immersed in a cylindrical bottle filled with tap water.
Phantom sequence
parameters: Slice thickness=8mm, GRE flip angle=5º,
readout TR=650ms, TE=1.5ms, field of view=180x180mm, matrix=64x64.
Dynamic
experiment: A pneumatic
MR-compatible ergometer (Trispect, Ergospect, Innsbruck, Austria) was used for
plantar flexion exercise to exercise gastrocnemius muscles. The exam consisted
of 2min of rest, 6min of exercise at a pedal pressure of 1.8bars and 4min of
recovery (total of 60 repetitions).
Volunteer calf
muscle sequence parameters: Slice thickness=8mm, readout TR=3.4ms,
TE=1.4ms, field of view=140x140mm, matrix=64x64, each spectrum was acquired
every 13.1s.
CEST
Z-spectrum Acquisition: Range covered over ±3.5ppm in steps of
0.11ppm (63 points) for phantoms and ±2.8ppm in steps of 0.47ppm (13 points) for
in-vivo measurements, relative to bulk water resonance.
CEST Asymmetry ratio: Spectra were normalized with a non-saturated
signal (S0) using the equation: $$$CEST\small{asym}=\frac{\normalsize{S}\tiny{sat}\small{(-\triangle\omega})-\normalsize{S}\tiny{sat}\small{(+\triangle\omega})}{\normalsize{S}\tiny{0}}$$$ where the chemical shifts -∆ω and +∆ω correspond to the integration
ranges ±[1.4ppm-2.3ppm] and ±[1.5ppm-2.1ppm]. B0 inhomogeneities
were normalized as described in [3].
RESULTS
Phantom spectra and CEST asymmetry ratios in Figure1 show how CEST peaks
behave for different ratios between PCr and Cr, as well as for tap water and Pi
when pH is constant.
In-vivo spectra and CEST asymmetry ratios in Figure4 show a CEST effect
of ≈12% along the gastrocnemius muscle and ≈7% in soleus at 1.9ppm during
plantar flexion exercise. CEST ratios of ≈6% and ≈4.5% were measured in gastrocnemius
and soleus at resting state.
The CEST-maps presented in Figure3 show how the contrast evolves for
different metabolites concentration and pH emulating different instants within
exercise (increasing Cr concentration and decreasing pH). Figure4 illustrates the
correlation of (a) metabolites concentration and (b) pH with CEST contrast.
The preliminary in-vivo CEST map presented in Figure5 shows the CEST
enhancement during exercise compared with the resting state.
DISCUSSION
CEST contrast increases in-vivo from 6% to 10% due to exercise (Figure1)
around the Cr resonance frequency (1.8ppm) in the principle muscles involved in
plantar flexion exercise, soleus and in particular the gastrocnemius muscle.
In phantom studies free Cr achieved 4-times higher CEST contrast
compared with PCr (Figures.2,3&4) for a fixed pH=7.
The CEST contrast ratio is linearly correlated with Cr
concentration (around 0.6% per mM), the other metabolites present had no
considerable contribution due to their much slower exchange rate.[4] There
is also a linear relation with pH in the physiological ranged with a slope
of 6.2% per pH unit at 7T. Phillip Zhe Sun et al. also found a Cr-CEST dependence of 2% per pH unit
at 3T and 8% per pH unit for 9.4T.[5]
Motion artifacts are a limitation that will have
to overcome for reliable quantification of dynamic exams.
CONCLUSION
This study shows the feasibility to map Cr-CEST
contrast
in-vivo with exercise. However, a further increased time resolution is
anticipated for dynamic studies, close to those routinely used in dynamic
31P-MRS.
Acknowledgements
My gratitude to Oesterreichische
Nationalbank (OeNB) for the
grant “Jubläumsfond” #16133.References
1. van Zijl, P.C. and N.N. Yadav, Chemical exchange saturation transfer
(CEST): what is in a name and what isn't? Magnetic Resonance in Medicine,
2011. 65(4): p. 927-948.
2. Kogan, F.,
H. Hariharan, and R. Reddy, Chemical
Exchange Saturation Transfer (CEST) Imaging: Description of Technique and
Potential Clinical Applications. Curr Radiol Rep, 2013. 1(2): p. 102-114.
3. Poblador_Rodriguez,
E., Creatine CEST In Human Muscle, in
Department of Signals and Systems.
Division of Signal Processing and Biomedical Engineering. 2015, Chalmers
University of Technology, Sweden. Master Thesis in Biomedical Engineering,
EX068/2015
4. Haris, M.,
et al., Exchange rates of creatine kinase
metabolites: feasibility of imaging creatine by chemical exchange saturation
transfer MRI. NMR Biomed, 2012. 25(11):
p. 1305-1309.
5. Sun, P.Z.,
et al., Investigation of optimizing and
translating pH-sensitive pulsed-chemical exchange saturation transfer (CEST)
imaging to a 3T clinical scanner. Magn Reson Med, 2008. 60(4): p. 834-841.