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1 Department of Medicine, University of California, San Diego, La Jolla, California, United States
2 Department of Radiology, Mail Code 8756, UC-San Diego Medical Center, San Diego, California, United States
3 School of Medicine, University of California, San Diego, La Jolla, California, United States
4 San Diego, California, United States; Department of Radiology, Mail Code 8756, UC-San Diego Medical Center, San Diego, California, United States
5 Department of Medicine, University of California, San Diego, La Jolla, California, United States; Department of Radiology, Mail Code 8756, UC-San Diego Medical Center, San Diego, California, United States
* To whom correspondence should be addressed. E-mail: shopkins{at}ucsd.edu.
In-vivo radioactive tracer and microsphere studies have differing conclusions as to the magnitude of the gravitational effect on the distribution of pulmonary blood flow. We hypothesized that some of the apparent vertical perfusion gradient in-vivo is due to compression of dependent lung increasing local lung density and therefore perfusion/volume. To test this, 6 normal subjects underwent functional MRI with arterial spin labeling during a breath-hold at functional residual capacity, and perfusion quantified in non-overlapping 15mm sagittal slices covering most of the right lung. Lung proton density was measured in the same slices using a short echo 2D-FLASH sequence. Mean perfusion was 1.7±0.6 ml/min/cm3 and was related to vertical height above the dependent lung (slope=-3%/cm, p<0.0001). Lung density averaged 0.34±0.08 g/cm3, and was also related to vertical height (slope=-4.9%/cm, p<0.0001). By contrast when perfusion was normalized for regional lung density, the slope of the height-perfusion relationship was not significantly different from zero (p=0.2). This suggests that in-vivo variations in regional lung density affect the interpretation of vertical gradients in pulmonary blood flow and is consistent with a simple conceptual model: that the lung behaves like a Slinky®, a deformable spring distorting under its own weight. The greater density of lung tissue in the dependent regions of the lung is analogous to a greater number of coils in the dependent portion of the vertically oriented spring. This implies that measurements of perfusion in-vivo will be influenced by density distributions and will differ from excised lungs where density gradients are reduced by processing.
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