Magnetic resonance analyses of cooling dynamics of brown adipose tissue in healthy adults

The main function of brown adipose tissue (BAT) is to convert chemical energy
stored in lipids into thermal energy (heat). Therefore, BAT is considered as a
promising target to combat cardiometabolic diseases. Cold exposure is the main
physiological stimulus for BAT activation. Cold exposure stimulates cation
channels in the skin, which stimulate afferent nerves that transfer the cold
perception to the hypothalamus. Once the signal is received by the
hypothalamus, it triggers two thermogenic responses in the body: non-shivering
and shivering thermogenesis. Non-shivering thermogenesis is mainly produced by
the activation of BAT, where the hypothalamus activates BAT via the sympathetic
nervous system, and heat is generated by the combustion of free fatty acids
within the mitochondrial machinery of brown adipocytes. The combustion of free
fatty acids decreases the lipid content in BAT, and perfusion is increased upon
activation. In shivering thermogenesis, skeletal muscle are rapidly
contracting, and thereby generate heat. However, this classical distinction
between non-shivering and shivering thermogenesis is under debate, as it was
recently shown that also several skeletal muscles are active during
non-shivering thermogenesis. It is still unclear how the physiological
responses in BAT and skeletal muscle are triggered in response to cold, and how
they are coordinated by the hypothalamus.

The most common method used to evaluate BAT activity in humans is fluorine-18
deoxyglucose [18F]FDG PET-CT. This technique quantifies metabolic tissue
activity based on the uptake of a glucose analogue, even though it is known
that the primary substrate for BAT is fatty acids. Moreover, this technique is
invasive and depends on radiation. Alternatively, magnetic resonance (MR)
imaging has been proposed as a non-invasive technique to quantify the metabolic
activity of this tissue via water-fat quantification. As such, it targets
lipids directly. MR imaging of the BAT depot in humans located in the
supraclavicular fossa has shown decreases in fat fraction (FF) of this depot
after cooling, indicating the sensitivity of the technique to detect
physiological changes.

Another important advantage of MR is that it enables the use of multiple scans
within one imaging session. This allows assessment of other parameters, such as
perfusion or temperature, but also other tissues, such as skeletal muscles or
the brain. While methods to assess FF in the supraclavicular fossa are
relatively well established, protocols for alternative parameters (i.e.,
perfusion, temperature etc.), or the use of protocols to simultaneously assess
multiple tissues within one cooling session, are still an active research
topic.

1. To adapt and optimize the MR protocol to enable multi-parametric dynamic
acquisitions during 1 hour of mild-cold exposure.

2. To investigate tissue dynamics of metabolic activity of the hypothalamus,
BAT, and skeletal muscles, measured by dynamic MR acquisitions, in combination
with blood markers during mild-cold exposure in healthy adults.

This study is an intervention study, which will be conducted at the Leiden
University Medical Center (LUMC). For objective one, we have included one study
visit and the duration of this study day is circa 120 minutes. For objective
two, we have included three visits, a screening visit and two study visits. The
duration of the screening visit is circa 30 minutes and the study visits will
each take circa 120 minutes.

Subjects will be exposed to mild-cold using a dedicated cooling device
(Blanketrol® III, Cincinnati Sub-Zero (CSZ) Products, Inc). The system consists
of a heater, a compressor, a circulating pump and blankets/pads. This device
can be utilized for hypo-and hyperthermia applications and offers a rapid or
gradual temperature management control.

One or two water-circulating blankets will be placed around the subject
positioned in the MR scanner and the blankets will be connected to the
Blanketrol® III system via the waveguide. The system displays the water
temperature, which will be initially set to circa 32°C to maintain
thermoneutral conditions, after 20 minutes at thermoneutrality we will directly
set the temperature to 18°C to initiate the cooling phase. The thermal
perception of subjects will be monitored using a numeric rating scale
(1=comfortable and 10=extreme cold) every 15 minutes. When the subject reports
shivering, we will switch off the cooling device and get the subject out of the
scanner.

For objective 2, we have also included a thermoneutral control measurement.
This measurement has the same set-up as the cooling experiment, but the
temperature of the blankets will be kept constant at 32°C for the entire length
of the measurement.

Subjects with contraindications for MR imaging will be excluded. There are no
known risks associated with the use of the MR scanner. The mild cooling
protocol, in which a special cooling device is used, is generally well
tolerated and no side effects are expected. The occurrence of hypothermia as
well as potential other disadvantageous effects are highly unlikely and also
have never occurred before in previous studies (p16-023, p16-078) that have
been performed at the LUMC. Subjects have no personal benefit from
participating in this study. With respect to the burden of the second part of
the study (objective 2), in total 67.5 mL of blood will be drawn per subject.