The principle of MRI is the following:
First, the main magnetic field
aligns the spins of hydrogen nuclei,
which are bound in fat or water in
patient cells in a certain direction.
Then, radio frequency pulses are
transmitted to deflect this created
magnetization to create measurable signals.
Gradient pulses are used to modify the
magnetic field locally, such that signals
from different regions can be distinguished.
Finally, the MRI system measures these signals
to create images of the human body.
Gradients are crucial for
spatial encoding of the signals.
They are important to determine from where
in the patient body the signal comes from.
So strong gradients can help visualize
very fine structures and characterize
tissue properties. In detail.
certain applications need
very large gradient pulses.
By increasing the magnitude, we can make
the pulses shorter.
Despite the time-saving,
this also increases the measured signal.
So like if you travel faster you can shorten
your journey and you get less tired.
Also, certain applications can benefit if the
gradient strength can be modified quickly.
So we need both high gradient
amplitude and short ramp times.
So like a sports car,
it should drive fast,
but it should also accelerate quickly.
For years, developers look into how to
best increase the gradient strength.
However, with standard technology,
a further increase of the gradient
strength would lead to reduced speed by
which the system could reach its power.
So with the new technology,
we aim at both, large gradient amplitudes,
so large gradient strength,
and short ramp times for high slew rates.
We start with the concept design that
has all the ingredients to generate
the ideal gradient field and this
design then turns into a design
which accounts for all aspects of
the real world of manufacturing.
The idea is not just to increase it slightly,
but to make a significant step in
the available gradient strength.
We achieve this step by using a new
technology, a double GPA technology,
which means we drive our gradient coil
with two gradient power amplifiers.
The main challenge is to enable
the multi-GPA technology.
Therefore we have to divide the gradient
coil into two separate circuits.
Due to this requirement,
we doubled our set of interfaces and
electrical connectors, which results
in a very complex layer structure.
It's so difficult because space
is limited and the more field
we want to generate,
the more conductors we need,
and so we have to find a compromise
between available space and the
ideal aspects of an ideal gradient coil.
Another difficulty always is to
translate the physical design concept
into a manufacturable product,
cause a manufacturable product
always has far more requirements
than the physical design concept.
Usually we have only one
GPA for one gradient.
The idea now is to have two
individual GPAs supporting and
controlling two separate parts
of the same gradient coil.
This would allow us to increase
the speed at a given power,
so that we would have a maximum
speed at maximum power.
The main challenge was to
truly synchronize both GPAs.
This required a huge team
effort.
Imagine you are riding
two horses at the same time.
This requires you to make them run
in parallel at the exact same speed.
Many additional teams are involved to
implement such a multi GPA technology.
There's the GPA itself that needs to
extend the existing technology also to
make it usable in a multi-GPA setup.
Then we have the physics group that
needs to identify procedures that
measure and also align the individual
contributions of the GPAs and the
correlating gradient coil parts.
Then we have the system software that
introduces an abstraction layer to
the application such that the system
feels like one powerful system
instead of executing individual instructions
that need to be precisely synchronized.
Then we have the service software that
needs to be adapted to site multi-
GPA system and also to maintain it.
Next we have infrastructure that
needs to be extended to site
now more than one GPA.
There were many contributions to
build the prototype and also to
adapt the application to make use
of the new possibilities.
All in all we invested
till now more than two years to make this
happen in a really big team effort.