New PET-MR scanner halves radiation and improves resolution
31 August 2012
Physicists at the University of Oslo have created new medical
imaging technology that combines PET and MR in a more efficient way and
reduces the radiation compared to current technology. The technology was
developed from particle physics research at CERN.
Positron emission tomography (PET) provides a spatial image of
where the cancer cells are located in the body. PET scans are harder
to interpret if medical staff cannot locate cancer cells in relation
to the skeleton and soft tissue. This can be done by comparing PET
images with an anatomical picture such as CT (computerised
tomography) or MR (magnetic resonance) scans.
CT scans provide a three-dimensional X-ray image of the body. MR
scans photograph the body using radio waves and a powerful magnetic
field. MR provides far better images of soft tissue than CT does.
The drawback of MR scans is that the examination is more expensive
and takes much longer, while the advantage is that it does not emit
ionising radiation. Currently, most hospitals combine PET and CT,
but this combination has a significant weakness.
"The radiation from such an examination is ten times higher than
the average background radiation over the course of a year. Many
cancer patients must be examined multiple times to test whether the
treatment is working. The total radiation during treatment can
therefore be very high," says Erlend Bolle, a researcher in particle
physics in the Department of Physics at University of Oslo (UiO),
Animal scanner for research
Currently there are two types of PET technologies, each adapted
to a particular use: one is adapted for clinical examinations of
patients. The other is optimised to let researchers find new and
better cancer treatments by testing new medicines on animals.
Siemens and Philips have recently launched combined PET-MR scanners
for humans. Particle physicists at UiO are the first in the world to
develop a specially adapted PET/MR solution for scans of animals.
"The high resolution in our PET scanner provides better images,
and the high sensitivity makes it possible to use only half as much
radioactivity in the examinations without it affecting the image
quality. This opens new possibilities in research, and may also
contribute to reducing radiation in clinical scanners, especially
within mammography and brain scans. We therefore hope that Philips
and Siemens find our technology interesting", says Bolle to the
research magazine Apollon.
Together with three colleagues, he has constructed a PET machine
that is so small that it can be placed inside an MR machine. Both
images can therefore be taken at the same time, and medical
personnel do not have to correct the errors that occur when two
images are combined after they have been taken.
Capturing all the photons
In a standard PET examination, radioactive isotopes are attached
to sugar molecules and injected into the body. The PET image is
taken one hour later, when the sugar has been distributed to the
entire body. Cancer cells burn sugar quicker than healthy cells.
Radioactive gamma particles therefore accumulate in cancer cells.
The gamma particles send out two sets of photons in opposite
directions. This is called parallel photons.
In order to trace the radioactive source, the PET scanner must
find which parallel photons are linked. This is one of the great
challenges for current PET scanners.
As long as the photons hit the detectors at a right angle, all is
well. When they are captured, it is possible to calculate which two
photons are linked. The problem arises when the photons hit the
detector at an angle. This leads to a great risk of imprecise
measurements of the collision points. This diminishes the image
Only half of the photons deposit all their energy on first
impact. On subsequent impacts, only some of the energy is deposited
before the photons change direction and deposit the rest of the
energy elsewhere. Current detectors have no depth information and
therefore cannot reconstruct the positions of these photons.
"In order to capture all the photons, we measure the position in
three dimensions in a five-layer detector," said Bolle.
In current machines, in order to have the photons hit the
detectors at as straight an angle as possible, it is important that
the entire patient is as centrally positioned in the machine as
possible. It is therefore important that there is great distance
between the patient and the detector. This solution has a major
"When there are large openings on both sides of the scanner, too
many photons go astray. This diminishes the image quality. The
closer the patient is to the detector, the higher the sensitivity of
the image. We have managed to double the sensitivity. In practice,
we can take the pictures twice as fast, or only use half of the
radioactive dose in order to get the same image quality as
previously," Bolle added.
In the new UiO PET scanner, good image quality can be achieved
even if the test animal is lying right next to the detectors.
The new detectors are made from entirely new crystals and light
guides. In each of the five layers of the detectors, crystal pins
are placed on top of a transverse layer of light guide fibres. This
is a completely new way of measuring gamma particles. The detectors
are placed so that the space within the new scanner is square.
Current scanners form a circle, which means that there is a gap
between each detector block and photons disappear through the gaps.
Now, there is full coverage of crystals on all sides and the scanner
can capture several million particles a second. However, this does
not happen at regular intervals, so particles are measure each
nanosecond to avoid errors.'
All the parts of the PET scanner are put together like lego
bricks. The system digitalises the data at an earlier stage than the
current PET solutions. The data can be sent to any number of
computers and the image processing takes place in parallel with the
"Though we are making a scanner for animals, it can easily be
rebuilt for hospital use," Bolle notes. He got the idea from the
large Big Bang experiment in CERN, in which enormous detectors in
the world's largest physics experiment trace the smallest particles.
The research is funded by the Research Council of Norway and the
Swiss National Science Foundation.
Source: University of Oslo.