The marriage of anatomic data from computed tomography (CT) and magnetic resonance (MR) imaging to the metabolic data from positron emission tomography (PET) and single-photon emission computed tomography (SPECT) is an ex-ample of the value of the whole being greater than the sum of its parts.
The marriage of anatomic data from computed tomography (CT) and
magnetic resonance (MR) imaging to the metabolic data from positron
emission tomography (PET) and single-photon emission computed
tomography (SPECT) is an ex-ample of the value of the whole being
greater than the sum of its parts. Such fused images provide
radiologists, nuclear medicine specialists, and other physicians
with more meaningful data that provided by either image
individually. Although the practice of merging images has been
around for some time, at least in a rudimentary form, recent
advances in both hardware and, particularly, software technology
have taken this process to a new, more practical and efficient
Methods of image fusion
The first and most common method of image fusion is "mental
fusion," in which two images are placed side by side and the
radiologist attempts to mentally overlay the images in his or her
mind. This is the most basic and inexpensive form of image fusion.
"The only expense involved in this method is the time of the
individual performing it," said Thomas J. Hook, senior vice
president, CTI Molecular Imaging, Inc., Knoxville, TN. "The true
costs of this method, however, are the errors that are made in
trying to reach accurate conclusions by looking at the images
without overlaying them. Obviously, the cost can be quite severe if
there is a missed diagnosis or a misreading of the exam."
The next method of fusing images involves overlaying the images
either manually or through the use of simple software programs that
scale and align the images. The difficulty with this method is the
ability to accurately align two images acquired at different times.
"A human is not a static object," noted Hook. "Organs within the
body move, the person moves. So software that does simple rigid
registration typically allows you to do a better job than the
mental fusion, but it's still a relatively simplistic approach and
you still end up with mismatches in the two images."
The third level of fusion is one in which a combined image is
obtained on a hybrid imaging device, such as a PET/CT scanner. The
benefits of such a system include the ability to acquire both
images in the same plane at the same time. The drawbacks are the
considerable capital costs associated with purchasing a hybrid
imaging system and the fact that the images are still fused based
on a rigid software algorithm.
There are times, however, when such hardware-based fusion
imaging is the method of choice. "There are certain clinical
applications in which it is beneficial to have all the information
gathered at the same time, for example, in cardiac applications
where it is difficult to synchronize the images if you don't
acquire the information at the same time." said Christian P.
Behrenbruch, PhD, CEO of Mirada Solutions, Ltd., Oxford, UK.
Hardware fusion can also, at times, increase clinical
throughput. "Having the fusion on hardware can mean that the
clinical throughput is faster and the image quality may be better,"
said Behrenbruch. "Scheduling a greater number of patients for
imaging in a shorter period of time is also beneficial."
For patients, as well, hardware fusion can be beneficial.
"Hardware-based fusion brings benefit to the patient," said Clare
Jones, vice president of marketing at Mirada, "because they only
have to undergo one scan rather than two." Image distortion and
decreased resolution are still a possibility with simultaneous
image acquisition. This can cause misalignment between the center
and peripheral areas. The use of sophisticated fusion software can
address this problem.
A new level of fusion
"Those are the three initial levels of fusion," said Hook. "The
Mirada Reveal MVS workstation (Mirada Solutions and CTI Molecular
Imaging) adds a whole different level to fusion imaging because
Mirada brings what's called 'non-rigid' or 'non-linear' fusion
capability to the table."
With a nonrigid or deformable technique, one image is
manipulated to match the other. "Rather than trying to match up
entire datasets--for example, the entire PET dataset with the CT
dataset--you try to line up various regions or more localized
densities or areas of information that match up and then deform the
shape of the PET image to make it fit the shape of the CT image,"
said Todd Deterding, senior product manager for CTI. " The image
quality of the PET image remains the same, but it is placed in the
context of the CT, where it makes the most sense."
There are a variety of benefits associated with the use of this
nonrigid, software-based fusion system. First, it can be used with
images acquired at any time and on all types of modalities.
"Ultrasound, X-ray, MRI, PET, CT--you can bring in all these
modalities," noted Behrenbruch. "For example, you don't find an MR
system bolted onto a PET system. The physics of creating such a
system are challenging. Yet there are all kinds of applications,
such as in neurology, where a radiologist would love to compare the
results of MR and PET images."
"The Reveal MVS workstation can give you a 'pseudo device' even
though those things don't exist," agreed Hook. "Obviously the more
images you can 'morph' and overlay, the more power it gives the
radiologist or nuclear medicine physician reading the exam to
actually make a more definitive diagnosis."
Another advantage of software solutions is the ability to
perform longitudinal comparisons. The user can compare images over
time to assess treatment response or any other variable. "There are
all kinds of possible applications," Behrenbruch explained. "When
you have an oncology patient being worked up and they are about to
be sent to surgery, by the time they are sitting on the operating
table, hundreds of thousands of dollars have been spent on that
person, probably including a PET scan, an MR, and a CT scan. The
more information you can integrate to build a complete picture of
what's happening inside the body, the better the benefit for the
Other applications of deformable software-based fusion include
use in comparing the results of contrast-enhanced breast MR and the
metabolic activity from a PET image, particularly when assessing
response to therapy over time. "If you are looking at liver
lesions, a helpful modality for that is ultrasound," noted
Behrenbruch. "So if you could do ultrasound/CT fusion, that would
have a lot of practical applications, particularly when you want to
start doing ablation."
Another benefit to the software-based solution is that it sits
in your information domain and, therefore, provides flexibility
when upgrading hardware components.
"If your PACS evolves, for example," said Behrenbruch, "you add
a new plug-in and you get better DICOM connectivity, or if you
switch to Web-based PACS, information-type applications can move
with it. In the case of hardware-based fusion, the obvious
disadvantage is that if you want to upgrade your hardware
system-for example, if you want to use a different detector
material or you want to upgrade to a 132-slice CT scanner when it
becomes available-you would then have to have someone upgrade part
of your system or send it to the factory for a couple of weeks. So
software fusion has intrinsic value."
Economies of time and money
There are also economic benefits to this system in terms of both
time and money. "The economic reality is that, sometimes just for
dollars and cents reasons and sometimes for space considerations, a
PET/CT is not an option for a provider," said Hook. "The only
option may be to use a workstation with very sophisticated software
to get a fused image."
"There are a lot of people doing functional imaging who don't
have the economic clout to get a PET/CT scanner," said Behrenbruch.
"There are also some very good operational examples as well in
which PET/CT scanners are not practical, such as mobile machines."
The cost of outfitting a coach for mobile PET/CT and the cost of
driving an extra 10 tons of weight can be prohibitive.
The workstation-based system is also time efficient. "Fusion
software used to be incredibly slow and labor-intensive," noted
Behrenbruch. "You had to click lots of points down, or if it was a
more sophisticated type of fusion, you had to wait-sometimes for a
couple hours-for it to register the two images together. Today,
it's instantaneous. With our platform, the average time to complete
image fusion is about 20 seconds."
"In many cases it's not actually the radiologist performing
these functions," added Jones. "With our fusion software it very
well may be the technologist who performs the fusion and passes the
case to the radiologist for review."
"That has been one of the challenges of fusion software,"
continued Behrenbruch. "Unless you can make the fusion really
robust, you can't leave it in the hands of a technologist."
Combining software and hardware
Hardware- and software-based fusion are not mutually exclusive.
"It's also equally powerful that that same MVS Reveal workstation
can be used in a PET/CT environment," said Hook. "You can collect
the same dataset on a PET/CT and still use the REVEAL MVS to remove
even more errors from the fused images. It's really a tool to be
used across the industry."
"Software is totally complementary with a hardware solution,"
concluded Behrenbruch. "There are very different reasons why you do
each one. In the future, what we will see is software-based
technologies augmenting the capabilities of both systems in terms
of accuracy, longitudinal comparison, and in really integrating
other modalities. It's like glue in a multiple imaging