Is it really mitochondrial Pi?
A study into the origin of the alkaline inorganic phosphate signal as observed by 31P MRS at 7T

Assessment of mitochondrial content and capacity in skeletal muscle is
important in clinical research, for instance in diabetes, as well as in aging
research and sports medicine. Current methods for determination of
mitochondrial content are either invasive as they require a biopsy from the
muscle tissue, or they are not feasible in a clinical setting, as exercise
inside an MRI scanner is needed. Therefore, a non-invasive method to determine
mitochondrial content that does not require exercise would offer a significant
advantage for patients.
In a previous study at the LUMC, phosphorous magnetic resonance spectroscopy
(31P MRS) of skeletal muscle was used to detect an extra signal for inorganic
phosphate. This signal, which can only be detected at 7T due to the increased
signal to noise ratio and increased spectral resolution, was attributed to the
mitochondrial inorganic phosphate pool. If this signal really originates from
the mitochondria, this MR-tractable mitochondrial biomarker in resting muscle
could offer a significant advantage because it offers a non-invasive
determination of mitochondrial density.
A follow-up study showed a significant increase of this signal in endurance
trained athletes, as well as a very high correlation with the phosphocreatine
recovery time, a well accepted marker of mitochondrial capacity which requires
in-magnet exercise. Both results provide further evidence for the origination
of the signal from mitochondria because of the known increase in mitochondrial
density in endurance trained athletes.
However, it could still be possible that a significant portion of the signal is
originating from blood. Only when the blood contribution to the signal is
known, further research can be done on the use of this signal as a biomarker
for mitochondrial content. We propose to study this by reversibly increasing
the blood volume in the tissue of interest by inflating a cuff around the upper
leg resulting in venous occlusion, and acquiring 31P MRS data in the lower leg
before and during the inflation.

The objective of the study is to be able to determine the contribution of blood
inorganic phosphate to the signal observed in skeletal muscle.

The study will be performed on the 7 tesla Philips human MRI scanner in the
C.J. Gorter Center for High Field MRI in the Department of Radiology at the
Leiden University Medical Center. Subject will be placed in a supine position,
with a MR surface coil placed on their calf muscle. An inflatable cuff will be
placed around the upper leg, but the cuff will not be inflated at the start of
the scan.
First some preparation scans will be made, and this will take about 15 minutes.
Thereafter, a 31P MR spectrum will be acquired, which takes about 10 minutes.
Subsequently, the cuff will be inflated to a pressure of 60 mmHg, using a
manual blood pressure Sphygmomanometer, which is located outside the scan room.
The cuff will stay inflated for a maximum of 11 minutes, during which a 10
minute MR spectrum will be acquired. After that, the cuff will be deflated by
opening the valve of the Sphygmomanometer, resulting in a gradually lowering of
the pressure in the cuff.

There are no known risks associated with participating in an MRI study, as long
as risk groups are excluded. Subjects with intracranial or intraocular metal, a
pacemaker, and claustrophobia will be excluded because of potential
contraindications of MRI in such subjects. The partial
occlusion of the veins by using an inflatable cuff in healthy subjects for
about 11 minutes is not harmful or dangerous. In many
previous studies partial or full venous occlusion has been performed in healthy
subjects, without adverse effects.