Patient Scan Protocol

Imatron C-100XL CT Scanning Protocol

Tube Current

630 mA

Tube Voltage

130 kV

Image Thickness

1 mm with 0.5 mm overlap

Contrast

None

Lung Volume Control

Just before scanning, the patient should be instructed to hold their breath at the selected lung volume and to maintain the breath hold through out the scanning procedure. The most repeatable lung volume for the subject to achieve is Functional Residual Capacity (end expiration). Kalender has discussed a method using a spirometer to monitor air flow at the mouth and to calibrate air flow to % vital capacity (difference between residual volume and total lung capacity).[1].

In our implementation of this approach to lung volume control during scanning, we are utilizing a Hans Rudolph pneumotach coupled with a two way balloon occlusion valve (Hans Rudolph Model 9317). CT scanner with volume controller hardware. The occlusion valves have a response time of 50 msec. Air flow will be monitored via a dedicated PC and integrated to keep track of air volume. We have modified a respiratory mouth piece to accommodate a Millar MPC500 tepaflex catheter manometer to monitor airway pressure. Signals from the Millar catheter and the pneumotachare digitized, recorded and monitored via a dedicated PC. The airway pressure signal is fed back into the scanner room so that the patient can visualize his/her signal so as to maintain a non-fluctuating pressure achieved by relaxing against the closed airway valve. We wish to assure that the patient does not work to contract or expand the chest cage during scanning. As preparation to scanning, we have developed a protocol whereby the subject is trained to maintain respiration through a mouth piece with the nose clamped. The patient is asked to expire to residual volume (RV: full expiration) and inspire to total lung capacity (TLC: full inspiration) while we keep track of air volume (integrated flow) during this maneuver and during normal breathing. Atthe time of scanning we close the airway valve at 50% of vital capacity (50% difference between RV and TLC). [note: other lung volume may also be desirable depending upon the reason for the study] The subject is instructed to relax against the closed valve and the constancy of this relaxation is monitored via airway pressure stability. Maximum breath holds are patient dependent, but range around 15 seconds after which the subject is allowed to breath and then again the valve is closed at the onset of the subsequent scanning periods. This allows for the acquisition of enough slices to span the full extent of the area of interest.

If one wishes to evaluate an airway area at multiple lung volumes, then the stack of slices scanned must span at least the cephalo-caudal distance which one might expect the airway to travel between maneuvers. Likewise, if one wishes to compare the same airway location with intervening periods of time where the patient has walked away from the scaning table, then one most likely needs to scan the airway in three-dimensions so that landmarks can be found to re-locate the same airwy anatomy between the two scans. This is discussed in greater detail in Sundaramoorthy et al [4] and Sonka et al [3] and will not be considered further here since this tutorial is primarily related to 2-D measurements.

Slice Thickness and Timing Considerations

Our use of the term High Resolution CT is slightly different than that described by other authors. When Muller [2] and others refer to HRCT, an important aspect of their definition is thin slices, meaning 1mm. Our use of term has includes the acquisition of 3mm thick slices via the Electron Beam CT (Imatron) scanner. The increased slice thickness does add partial volume effects not present in the more conventional HRCT mode of other scanners. However, our "HRCCT" technique adds high temporal resolution which would be expected to reduce blur due to motion since movement of the airways would be minimal during scanning. Wei et al. [5] have previously demonstrated that significant cardiogenic oscillations of the lung parenchyma can cause changes in regional lung volume at some locations as great as with a tidal breath.

References

Kalender, W.A. and R. Reinmuller, H. Fichte, J. Behr, T. Beinert, W. Seissler, et al., "Spirometrically gated CT measurement of lung density and structure," Radiology, 185: 354, 1992.
Muller N. L. and R.R. Miller. Computed tomography of chronic diffuse infiltrative lung disease. Am. Rev. Respir. Dis. 142: 206-215, 1990.
Sonka M. and G. Sundaramoorthy, and E.A. Hoffman: Knowledge-based segmentation of intrathoracic airways from multidimensional high-resolution CT images. SPIE Medical Imaging, Vol 2168 pp.73-85, 1994.
Sundaramoorthy, G. and E.A. Hoffman, T. Mitsa, J. Qian, and M. Sonka: Knowledge-based intrathoracic airway tree segmentation: fractal model and physical phantom based validation. SPIE Medical Imaging. Vol. 2168 pp. 86-97, 1994.
Wei, J. H. and E.A. Hoffman, E.L. Ritman, AND E.H. Wood. Cardiogenic motion of right lung parenchyma in anesthetized intact dogs. J. Appl. Physiol. 58: 384-9, 1985.

Last modified: Wed Apr 3 10:26:03 CST