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Respiratory gating in PET/CT: A step in the right direction
Received Aug
Respiratory gating in PET/CT: A step in the right direction
Tinsu Pan 0
0 Reprint requests: Tinsu Pan , Houston, TX , USA
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Respiratory motion is inevitable in cardiac imaging
with PET as the rest or stress myocardial perfusion
imaging (MPI) takes about 6 minutes with 82Rb and 10
to 15 minutes with 13N-ammonia and viability imaging
10 to 30 min with 18F-FDG.1 Figure 1 illustrates an
example of respiratory motion between the
end-inspiration and end-expiration phases in an 18F-FDG scan.
All newer PET scanners are with CT and without
transmission line sources.2 CT has shortened the time
for the transmission scan for attenuation correction of
the PET data from minutes to seconds and could also
provide the important information of calcium scores and
contrast-enhanced coronary artery CT images to help
diagnose heart disease. CT, however, introduces a new
problem of potential mis-registration of the CT and PET
data due to its fast scan speed resulting in each CT
image being a snapshot or a single phase of the heart in
respiratory motion. A series of continuous respiratory
phases of snapshot CT images may not be suitable for
attenuation correction of the PET data. It is important to
mitigate the impact of respiratory motion to improve
cardiac PET image quality.
In the article by Ichikawa et al, the authors
proposed an abdominal restriction technique by applying
an abdominal belt to restrict respiratory motion in
MPIPET. They wrapped a 20-cm abdominal belt around the
abdomen of 8 male healthy volunteers (BMI 23±2 and
ages 55±17 years) at end-expiration to restrict their
respiratory motion. They reported that the motion of
the heart in the cranio-caudal direction was
significantly reduced from 12.1 ± 6.1 mm without the belt to
8.1 ± 7.1 mm with the belt and that this restriction was
well tolerated in the 8 healthy volunteers and
subsequent 53 patients (ages 70 ± 10). The authors also
reported a small but statistically significant
improvement of a qualitative visualization score of 0.42 (0.29
to 0.71) and 0.71 (0.33 to 1.04) in the anterior and
inferior walls, respectively, and no change in the lateral
walls in a scale of 0 = normal to 4 = defect.
Reduction of respiratory motion did not have an impact on the
assessment of cardiac functions of ejection fraction or
left ventricular end-diastolic or end-systolic volumes.
In conclusion, the abdominal belt technique was
suggested to be an effective way of reducing the impact of
respiratory motion in MPI-PET or even MPI-SPECT
imaging.
Some limitations were noted. The data were only
from rest MPI-PET, not stress MPI-PET; there was no
quantitative blood flow measurement; there was no
standard for adjusting the abdominal belt other than ‘the
belt was tightened as much as possible without causing
discomfort to the patient.’ It was not clear if this
‘tolerable’ procedure can be translated into a ‘comfortable’
procedure, which may be the best indicator for success
of an MPI-PET scan, and which can be best represented
by the regular or normal breathing patterns recorded, but
not reported in the study. The belt may need to be
loosened between the rest and stress MPI studies to
improve patient comfort when the total duration of the
rest and stress studies can last over 40 minutes for
13Nammonia and many patients cannot keep still after 20
minutes on the imaging couch.
In addition to the proposed abdominal restriction
technique, there are other techniques from radiation
therapy that are used to mitigate the respiratory motion
in treating mobile tumors in the thorax or abdomen.
Abdominal compression, similar to the proposed
abdominal belt to restrict respiratory motion, can
restrict the abdominal motion by applying an
abdominal plate against the abdomen under a controlled
pressure.3 However, the device is bulky, needs to be
setup on a flat-couch, and may not be suitable in an
MPI-PET session with EKG leads and wires. Another
way of reducing the respiratory motion is to have the
patient wear goggles to watch a real-time video of the
patient’s respiration level and to ask the patient to
exercise respiration in a predefined range. Some may
also play a recording of breathing instructions to coach
the patient to breath in a shallow manner and in a
certain frequency to make respiratory motion regular.
Abdominal compression, wearing goggles, and playing
breathing recordings have been practiced in radiation
therapy. However, it may be impractical to expect all
patients to follow any or a combination of these
somewhat intrusive procedures. Some may tolerate one
procedure better than another, and some may not be
able to tolerate any procedure without increasing
anxiety to cause unintended increase in motion. A patient
could also do perfectly during a training session and do
very poorly during the imaging session or vise versa.
The best results may come from the patients without
any breathing instructions or restrictive devices
because most patients breathe shallowly in their (...truncated)