Researchers in Norway
develop PET scanner that
produces 50% less radiation
than current PET technology
Small device will fit in MR scanner for simultaneous
By Yngve Vogt
Current cancer examinations involve high levels of radiation. Based on
the Big Bang research in CERN, particle physicists at University in Oslo
have created a brand new technology that combines the PET and MR medical
imaging technologies. This combination involves much less radiation
than current technology.
PET (Positron Emission Tomography)
provides a spatial image of where the cancer
cells are located in the body. PET scans are
harder to interpret if medical staff cannot
situate the location of 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 xray
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. The advantage is that MR
does not emit ionising radiation.
Currently, most hospitals combine PET
and CT, but this combination has a significant
“The radiation from such an examination is 10 times higher than the average
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), Norway.
Currently, there are two types of PET
technologies, each adapted to a particular
use: One is adapted for clinical examinations
of patients. The other technology is optimised to let researchers find new and
better cancer treatments by testing new
medicines on animals.
Siemens and Philips have recently
launched a new PET/MR combination for
patients. However, particle physicists at
UiO are the first in the world to develop a
specially adapted PET/MR solution for
research 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
mammography and brain scans. We therefore
hope that Philips and Siemens find
our technology interesting,” says Bolle.
Together with his 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.
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. They are
referred to as 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
photons are linked. The problem arises when
the photons hit the detector at an angle
other than a right angle. This leads to a risk
of imprecise measurements of the collision
points and diminishes the image quality.
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,” Bolle says.
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 weakness.
“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.”
In the new UiO PET scanner, good image quality can be achieved even if the test
subject is lying right next to the detectors.
“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.”
Crystal pins and light fibres
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,” says Bolle.
“The detectors are placed so that the
space within the new scanner is square.
“Today, the scanners form a circle. This
means that there is a gap between each
detector block, and photons disappear
through the gaps. Now, we have full
coverage of crystals on all sides. We can
capture several million particles a second.
However, this does not happen at regular
intervals. We measure each nanosecond.
If we do not measure fast enough, we can
Digitalising the data
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. The
image processing takes place in parallel
with the examination.
“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 are being used to trace the
world’s smallest particles.
The research is funded by the Research
Council of Norway and the Swiss National
of upload: 22nd Jan 2013