The campaign to lower radiation dose across imaging procedures is not
limited to radiology, but is gaining momentum in the field of nuclear
Over the last decade, the medical imaging community has
been driving home the message to lower radiation dose levels as low as
reasonably achievable (ALARA), but much of the focus has primarily been
on patient radiation dose reduction in x-ray procedures, predominantly
in computed tomography (CT). This is despite the fact that a number of
nuclear medicine examinations deliver patient effective dose in the
range of 5 to 20 mSv per study, which is similar to that in CT.1 Additionally,
the adoption of hybrid imaging modalities, in particular positron
emission tomography with CT (PET/CT) and single photon emission computed
tomography with CT (SPECT/CT), has proliferated, further increasing the
levels of dose exposure to patients.
The focus on radiation dose
management in nuclear imaging came to a head in the last two years when
the Society of Nuclear Medicine (SNM) and the Society for Pediatric
Radiology’s Board of Directors were compelled to establish new North
American Guidelines for Radiopharmaceutical Doses for Children. These
societies expanded their pediatric radiation protection initiative by
standardizing doses (based on body weight) for [more than 11] nuclear
medicine procedures commonly performed in children.2
“It is important that we standardize dose to help ensure that all
pediatric nuclear medicine professionals consistently optimize medical
images while only using the minimum amount of radiation necessary to
obtain these images,”2 said S. Ted Treves, MD, Strategy
Leader of the Image Gently™ Nuclear Medicine Initiative, Chief of
Nuclear Medicine and Molecular Imaging at Children’s Hospital Boston and
Professor of Radiology at Harvard Medical School, Boston, MA.
the catalyst for change emerged out of a need to limit radiation
exposure to children, it applies to patients across the board, and
nuclear medicine has become equally committed as radiology to the cause
of lowering radiation dose in imaging procedures.
More CT Enhances NM Exams
to CT, lowering dose in nuclear medicine may compromise image quality,
which can adversely affect diagnostic accuracy. However, with
developments in dose reducing solutions, doctors may no longer have to
trade-off image quality for lower patient dose.
advancement in nuclear medicine has come with the development of hybrid
systems. When performing a SPECT/CT exam, coregistration is much more
straightforward when SPECT and CT are combined in a single scanner
because the patient remains in the same position during the acquisition
of both sets of images.3 This is especially valuable during oncology studies and in bone imaging.
conducting lesion localization with the low-diagnostic quality
attenuation correction CT images from most hybrids is very challenging.
As Johnsey Leef, MD, Chief of Radiology, CT and Nuclear Medicine,
Charleston Area Medical Center (CMAC), Charleston, WV, indicated, “CT
was originally just for mapping, and it was a really low-dose CT with
poor quality CT. This is something we would like to improve.”
While the nuclear medicine community calls for improved quality in CT imaging, it wants to avoid raising radiation levels.
of the advantages with PET/CT is that it begins to carve into the
volume of CT that has to be done in oncology. So, for example, the
initial staging in lymphoma certainly needs diagnostic CT, and we almost
always do a PET/CT. Many of these patients can be followed and scored
on criteria with the combination of PET and the low-dose CT to look at
changes in response to therapy. Therefore, some of the radiation dose to
an individual patient can be decreased. That’s a benefit,” said James
K. O’Donnell, MD, Associate Professor of Radiology, Case Western Reserve
University (CWRU), Director, Division of Nuclear Medicine and PET. The
challenge, however, has been acquiring quality images with low-dose CT.
dilemma has prompted the development of solutions like the Ingenuity TF
PET/CT system by Philips Healthcare. Like CT, the Ingenuity TF PET/CT
system is equipped with the Ingenuity CT platform and iDose4.
In addition, PET sensitivity and accuracy is enabled with Astonish TF
time-of-flight technology providing greater accuracy of SUV values via
list-mode reconstruction without compromising performance.
image quality is possible thanks to Astonish TF, the latest generation
in Time-of-Flight (TOF) technology, which leads to enhanced contrast by
up to 30% compared to non-TOF images. With up to 5 times higher
sensitivity than non-TOF scanners, you may be able to reduce
radiopharmaceutical dosing in some or all of your studies (Figure 1).
Also, Astonish TF technology allows Philips to select the best-in-class
list-mode reconstruction algorithm without compromising reconstruction
Similar to PET, the focus for SPECT is heavily weighted on
correlating and fusing images from CT with the SPECT images. This can
be achieved with the co-planar design of the BrightView XCT that allows
the CT and SPECT images to be acquired with little or no table motion
between the two studies. The flat-panel cone-beam CT (CBCT) component
uses low dose scans for localization purposes. And since both the SPECT
and CT acquisitions are planned from the nuclear medicine p-scope, there
is no additional dose for scout or topogram imaging that conventional
CT scanners use.
The BrightView XCT has full iterative technology
(FIT), which uniquely supports iterative reconstruction on both the
SPECT and CT (Figure 2). On the SPECT side, the Astonish technology,
which incorporates resolution recovery into the iterative reconstruction
process, provides improved spatial resolution. On the CT side,
iterative reconstruction is used to improve image quality through
increased uniformity and reduced noise.4
“We have the
BrightView XCT SPECT/CT, which uses the flat panel. It has the full
iterative reconstruction of CT data. That aids us a lot on just routine
SPECT, bone, MIBG studies, parathyroid studies, and all the different
kinds of studies you want to use the data for,” said George M. Ebert,
MD, PhD, Associate Professor of Radiology, Chief, Informatics and
Imaging Technology, Co-Director of Clinical MRI, University of Vermont
College of Medicine, Burlington, VT, in affiliation with Fletcher Allen
Dose management: A balancing act
management of oncological diseases requires repeated PET/CT or SPECT/CT
imaging for diagnosis, staging, and monitoring. This adds up to multiple
exposures to radiation throughout the therapy, and a need for dose
On the PET side, the advancements in the
Ingenuity TF PET/CT System are driven by Astonish TF, which enhances
lesion detectability with acquisition time that can be as fast as 30
sec/bed. Dr. Ebert noted how much faster the upgraded Astonish TF
completes image reconstruction than the previous version (Figure 3).
can, in many cases, see lesions in the lung that are 4 mm and 3
mm—something that we never expected to see in PET,” said Dr. Ebert. “We
used to have a threshold of approximately 8 mm in PET in the old days.
So we’re seeing smaller lesions.”
A recent NIH study5 found that TOF improves lung and liver lesion detection in patients. This is especially true in heavy patients.
of our problems is that patients are getting heavier. Traditionally the
only way to compensate for that has been [to] increase radiation dose
to maintain image quality,” said Dr. Ebert. “Using Astonish TF, we can
take a large patient and get spectacular image quality. The time penalty
that we used to incur to do the Time of Flight is completely gone. So
now we can use Time of Flight on every single case, for all
applications, whether they’re cardiac or oncology. That alone has
improved our stance a lot.”
“Setting appropriate levels to either
reconstruction so that you’re able to achieve diagnostic quality images
has also been a factor in SPECT/ CT,” noted Dr. O’Donnell, who works
with the BrightView XCT. This system is uniquely outfitted with both
iterative SPECT and CT reconstruction capabilities. “We have the
flat-panel detectors with the attenuation correction and the iterative
reconstruction that’s gone into SPECT,” said Dr. O’Donnell.
is important because as Dr. O’Donnell pointed out, “Dose does add up
after a while, when you’re doing repetitive studies, for example, with a
regular follow-up with Indium octreotide scans and SPECT or a regular
follow-up with FDG scans in oncology.”
List mode supports advanced imaging
techniques in PET imaging, such as improvements in standard uptake
value (SUV) accuracy, are leveraging the TOF for fast scan times.
you do time-of-flight, you basically are counting every single photon,
its direction, and how long it took to get from one side to hit the
ring,” explained Dr. Ebert.
The list mode feature helps Dr.
O’Donnell and his clinical team to perform advanced imaging studies and
the latest techniques in oncology.
“We’re doing quantitative
blood flow, and you need list mode for that because you have to capture
that initial arterial input into the blood. Because it happens so fast,
you can try framing rates, but it’s much better to have the list mode,
where the computer’s concentrating on just gathering all,” said Dr.
“For cardiac and oncology imaging, if you want to
post-process this into some sort of data study program—either
respiratory data or cardiac data—it is one of the big advantages of list
mode,” noted Dr. Ebert.
Stress-only imaging to reduce dose
technique for reducing dose that can benefit patients and clinicians is
stress-only imaging. A method used for myocardial perfusion that can be
applied in SPECT/CT imaging studies, stress-only imaging demonstrates
that perfusion and gated imaging can be performed on half-count data
with high diagnostic accuracy and acceptable image quality.
recent multi-center trial designed to test performance of the Astonish
algorithm for myocardial perfusion SPECT imaging, researchers compared
the clinical performance of Astonish on both full-count and half-count
stress-rest data, both with and without attenuation and scatter
The study showed that absorbed radiation
doses can be reduced by injecting less activity while imaging for
conventional imaging times.7 Results from stress-only
imaging, which requires the use of attenuation correction, demonstrate
that perfusion and gated imaging can be performed on half-count data
with diagnostic accuracy, acceptable image quality, diagnostic
confidence, similar defect extent and severity to full-time
acquisitions, and a low perceived need for rest imaging. Performing
studies using stress-only may increase patient acceptance, further
reduces radiation doses (compared to stress/rest half dose imaging).6
to Dr. Ebert, “The positive rate in stress-only imaging is 10% or 11%.
So that means 90% of the time that we get these scans, the stress-only
portion is normal, and you can argue that the rest of the portion was
unnecessary. I think there’s a future in stress-only imaging.”
advantage of stress imaging, as Dr. O’Donnell points out, is “you can
save the radiation dose.” “When [patients] have the test, if it looks
normal in the stress images, you can save the radiation dose and the
time,” he explained.
According to Dr. Ebert’s assessment, “The
new iterative reconstruction techniques, such as FIT, have made
improvements; together they have all really helped us maintain the
lowest dose that we can possibly get and still maintain image quality.”
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- Image gently, SNM launch
initiative to standardize radiation dose in pediatric nuclear medicine.
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- Miller JC. Hybrid SPECT/CT:
Enhancing Nuclear Medicine. Radiology Rounds.
Accessed November 7, 2011.
- Hansis E, Da Silva A, Hines H.
BrightView XCT – flat-panel cone-beam CT with iterative reconstruction
for localization and attenuation correction. Whitepaper. Koninklijke Philips Electronics N.V. 2011.
- Johnston J, Killion J, Veale B, Comello R. U.S. Technologists’ Radiation Exposure Perceptions and Practices. Radiol Technol. 2011;82:311-320.
Heller GV, Bateman TM, Cullom SJ, Hines HH. Improved clinical
performance of myocardial perfusion SPECT imaging using Astonish
iterative reconstruction. Netforum community. Astonish reconstruction.
Accessed October 7, 2011.
- Borras C. Radiation Protection in Diagnostic Radiology. Radiological Physics and Health Services Consultant. Washington, DC.