The influence of lung airways branching structure and diffusion time on measurements and models of short-range 3He gas MR diffusion

Reply: On the Use of 3He Diffusion Magnetic Resonance as Evidence of Neo-Alveolarization during Childhood and Adolescence

“…In fact, it concludes that their findings were due to neo-alveolarization. d. In contrast to the model proposed by Parra-Robles and Wild (12,13), ADC correlates to histological measures of alveolar dimensions in animal and human lungs (17)(18)(19)(20).…”

“…The diameter of alveoli based on previous histological studies is of the order of 0.25 to 0.3 mm (9-11). For a diffusion time of 2 ms, suggested by Parra-Robles and Wild (12,13), the value for s in Equation 1 is 0.57 mm, assuming D = 0.8 cm 2 /s for helium in air. For longer diffusion times, this changes by a factor of ( ffiffi t p ) (s is only 1.58 times larger for t = 5 ms).…”

“…Parra-Robles and Wild base their critique heavily on their simulations based on their model alveolar duct (12,13). We have some concerns about the assumptions underlying the simulation.…”

“…There have been reports of a reduction of the anterior to posterior gradient in 3 He ADC found in patients with cystic fibrosis (11). Similarly, this gradient is reduced when the lung becomes more inflated (e.g., at TLC it disappears [12]). Could it be possible that in the cases of extreme preterm subjects who have lower FRC (and body weight/size) that their ADCs were obtained at a higher inflation volume (NB the same volume of gas was inhaled by all subjects) than the normal ones, hence showing a reduced gravitational gradient (and lower standard deviation)?…”

Pulmonary MRI morphometry modeling of airspace enlargement in chronic obstructive pulmonary disease and alpha‐1 antitrypsin deficiency

“…As previously described (23), r and R maps were generated by fitting Equations [1] to [9] to multiple b-value measurements by using a custombuilt IDL 6.4 (ITT Visual Information Solutions, Boulder, CO, USA) algorithm on a voxel-by-voxel basis. Two conditions also determined the maximum b-value (16) such that D L and D T were estimated:…”