AJR 2004; 183:79-81
© American Roentgen Ray Society
Retrospective Respiration-Gated MDCT: Initial Results in Canine Models
Kazuo Awai1,
Daisuke Utsunomiya1,
Masanori Imuta1,
Shinya Shiraishi1,
Yasuyuki Yamashita1,
Yasumasa Nishimura2,
Natsuko Sato3 and
Maiko Kudo3
1 Department of Diagnostic Radiology, Graduate School of Medical Sciences,
Kumamoto University, 1-1-1 Honjyo, Kumamoto 860-8556, Japan.
2 Department of Radiology, Kinki University School of Medicine, 377-2
Oono-higashi, Osaka-Sayama City 589-8511, Japan.
3 Philips Medical Systems, Tokyo 108-8507, Japan.
Received November 19, 2003;
accepted after revision February 4, 2004.
Address correspondence to K. Awai.
Introduction
In the field of cardiac CT, development of retrospective ECG-gated MDCT has
made it possible to observe cardiac motion in three dimensions
[1], and use of CT coronary
arteriography has become increasingly common because it provides excellent
spatial resolution. Like the heart beat, respiratory motion is a periodic
motion, and thus retrospective respiration-gated MDCT is theoretically
possible. In healthy persons, however, the respiratory rate (20–25
breaths per minute) is markedly lower than heart rate (60–80 beats per
minute). Therefore, retrospective respiration-gated MDCT would require an
extremely small helical pitch value (
0.2). An added complication is that,
unlike the motion of the heart, much of the motion in the lungs is in the
craniocaudal direction.
The purpose of our study was to verify whether it is possible to observe
respiration using retrospective respiration-gated MDCT in dogs.
Materials and Methods
The experiment was conducted in three beagle dogs, each weighing
approximately 11 kg. First, we placed the dogs under general anesthesia by
administering 25 mg/kg of pentobarbital sodium via a 20-gauge IV catheter
inserted into the radial vein of the right forelimb. After tracheal
intubation, the animals were placed in a supine position on the CT table. We
then administered 1 mL of 0.2% suxamethonium to bring spontaneous breathing to
a standstill and connected a respirator to the intubation tube to provide
artificial respiration at a rate of 30 breaths per minute. The tidal volume
was set at 30 mL for two dogs and 35 mL for one dog.
CT was performed with 16-MDCT scanner (Mx8000/IDT16, Philips Medical
Systems). The scanning parameters were as follows: detector configuration,
0.75 mm x 16; slice thickness, 0.8 mm; rotation time, 0.42 sec; helical
pitch, 0.2; display field of view, 25 cm; 120 kV; 65 mAs (effective tube
current, 30 mAs); and scanning range, 18 cm.
Because a respiration-gating unit was not available, we connected an ECG
signal simulator to the ECG-gating unit of the MDCT scanner to achieve
respiration gating. The ECG signal simulator generated signals for 30 breaths
per minute, in accordance with respiratory rate of each dog.
Axial images obtained in 10 respiratory phases were reconstructed on the
operator console of the scanner by dividing the 2,000 msec respiratory cycle
into 10 phases using the MDCT cardiac reconstruction algorithm. Both the slice
thickness and interval of the axial images were 0.8 mm. The axial image data
were transferred to a workstation (Virtual Place Advance, version 2.0, Medical
Imaging Laboratory). Coronal and sagittal images in each respiratory phase
were then generated on the workstation, also with a slice thickness and
interval of 0.8 mm. Finally, coronal and sagittal maximum-intensity-projection
(MIP) images with a slab thickness of 10 mm were generated from original
coronal and sagittal images.
We viewed the data set of MIP images obtained during all respiratory phases
in the coronal and sagittal planes using cine mode on the workstation. We
evaluated artifacts on the original axial images and on the coronal and
sagittal MIP images in all respiratory phases. Furthermore, we evaluated the
motion of the peripheral pulmonary vessels by comparing the relative length to
the relative width of the adjacent intercostal space in the coronal and
sagittal MIP images. We defined peripheral pulmonary vessels as vessels
located within the area of the thoracic wall. An index of radiation exposure
during CT examination, the CT dose index, was calculated for the previously
described scanning conditions.
Results
No conspicuous artifacts were seen in the images of respiratory motion
obtained with a tidal volume of 30 mL in two dogs and with a tidal volume of
35 mL in one dog. On coronal images, the periphery of the pulmonary vessels
moved by approximately one intercostal space around the lung base, whereas the
pulmonary vessels hardly moved in relation to the ribs at the lung apex (Fig.
1A,
1B,
1C,
1D). No difference in the
evaluations of the images was found between the two radiologists. The CT dose
index was 4.3 mGy during scanning.
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Fig. 1A. —Retrospective respiration-gated MDCT scans of canine lungs.
Axial scans were obtained at level of right and left ventricles during tidal
inspiration (A) and tidal expiration (B) phases.
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Fig. 1B. —Retrospective respiration-gated MDCT scans of canine lungs.
Axial scans were obtained at level of right and left ventricles during tidal
inspiration (A) and tidal expiration (B) phases.
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Fig. 1C. —Retrospective respiration-gated MDCT scans of canine lungs.
Coronal maximum-intensity-projection images were obtained at level of right
and left ventricles during tidal inspiration (C) and tidal expiration
(D) phases. Right costophrenic angle moved from level of ninth rib
(arrow, C) to level of 10th rib (arrow, D)
between tidal inspiration and tidal expiration.
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Fig. 1D. —Retrospective respiration-gated MDCT scans of canine lungs.
Coronal maximum-intensity-projection images were obtained at level of right
and left ventricles during tidal inspiration (C) and tidal expiration
(D) phases. Right costophrenic angle moved from level of ninth rib
(arrow, C) to level of 10th rib (arrow, D)
between tidal inspiration and tidal expiration.
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Discussion
Clinical Applications
This study confirmed that retrospective respiration-gated MDCT is possible
and is a promising method for future analysis of respiratory motion. The
technique has clinical applications in the following five areas.
Assessment of the chest wall and mediastinal invasion by lung
cancer.—Some authors have reported using CT scans of respiratory
motion to assess the chest wall and mediastinal invasion by tumor
[2,
3]. However, these authors used
methods that required rescanning the lesion after routine scanning of both
entire lungs. Our method makes it possible to obtain both conventional CT
images and respirationgated images with one scan. In addition, our method
makes it easy to observe the craniocaudal motion of the lungs by constructing
coronal and sagittal views. MRI assessment of the chest wall and mediastinal
invasion by lung cancer has been reported
[4], but CT allows the lesion
to be analyzed more closely because of its much higher spatial resolution.
Observation of diaphragmatic motion in advanced pulmonary
emphysema.—Volume reduction surgery such as resection of bulla and
emphysematous lungs is frequently performed in patients with bullous emphysema
to improve respiratory motion. In these patients, diaphragmatic motion is
often observed on MRI as an index of respiratory motion
[5]. However, lesion
distribution in the lungs must also be examined on CT because MRI cannot
visualize the lungs themselves. We can observe diaphragmatic motion and lesion
distribution in the lungs at the same time on retrospective respiration-gated
dynamic MDCT.
Diagnosis of airway diseases.—In patients with airway
diseases, we can use retrospective respiration-gated dynamic MDCT to prove air
trapping by comparing the reconstructed images obtained during the expiratory
phase with those obtained during the inspiratory phase.
Radiotherapy for lung cancer.—Retrospective
respiration-gated dynamic MDCT makes it possible to capture 3D images of a
tumor as respiration causes it to move and thus could allow accurate targeting
of the tumor for irradiation.
Routine CT of patients who are unable to
breath-hold.—Reconstructing images obtained during the maximal
inspiration phase is sufficient for establishing a diagnosis in patients who
cannot breath-hold. However, our method cannot be adapted for patients who
have severe respiratory conditions that cause difficulties in regular
breathing.
Radiation Dose
The CT dose index was 4.3 mGy during our examination. In our CT scanner, CT
dose index is equivalent at any helical pitch with the same tube voltage (kV)
and tube current (mAs per slice). With the scanning parameters of 120 kV and
200 mAs per slice, the CT dose index is estimated to be 13.3 mGy for both
routine chest CT performed with a helical pitch of 1.2 and retrospective
respirationgated MDCT performed with helical pitch of 0.2 or less. Therefore,
using retrospective respiration-gated MDCT in place of routine CT of the chest
is valid.
Conclusion
Retrospective respiration-gated MDCT is feasible, and its possible clinical
applications are promising.
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Introduction
Materials and Methods
