Imaging lungs using inert fluorinated gases

Abstract

Rat lungs were imaged by 19F projection MRI of hexafluoroethane, mixed with 20% oxygen to form the inhaled gas. The 3D image had 700 microm resolution, and the data took 4.3 h to acquire. Free induction decays were collected in the presence of steady magnetic field gradients in 686 different directions. To take advantage of fast relaxation (T1 = 5.9 +/- 0.2 ms), the repetition time was 5 ms. To eliminate signal loss from magnetic field inhomogeneities, data were collected within 2 ms of spin excitation (from 80 micros to 2 ms after the 42-micros pi/2 pulses). The singular value decomposition of the transform from frequency to time domain was used to obtain projections despite the absence of data during and immediately after the RF pulses. Inert fluorinated gas imaging may be less expensive than polarized noble gas imaging and is appropriate for imaging steady-state rather than transient gas concentrations.

Dynamic 19F‐MRI of pulmonary ventilation using sulfur hexafluoride (SF6) gas

“…This was achieved by making use of the gradient overdrive of the MR scanner, leading to particularly short gradient ramps by using nonlinear data sampling during the sinusoidal gradient ramps and by using an asymmetric echo. Although the TE achieved with this approach is still significantly longer than that achievable with projection reconstruction pulse sequences (12,22), we obtained sufficient SNR for an ROI evaluation of the MR images. A further advantage of our approach is the fact that image reconstruction can be performed by the conventional fast Fourier transform technique, while projection reconstruction algorithms used in other studies (12,13) may be difficult to implement on some clinical MR systems.…”

“…However, the wash-in of these aerosols critically depends on the size of the aerosol particles (i.e., on the production of the aerosol), which limits quantitative applications of this method as well. Further approaches comprise fluorine-19 ( 19 F) MRI either of 19 F-containing fluids (9,10) or of inert fluorinated gases in the lung (11,12). Whereas the first of these two 19 F MRI approaches has been demonstrated in animals and, most probably, will have a limited role in humans, the second one is of particular interest for human applications.…”

“…Inert fluorinated gases such as tetrafluoromethane (CF 4 ), hexafluoroethane (C 2 F 6 ), or sulfur hexafluoride (SF 6 ) possess very short longitudinal relaxation times T 1 on the order of milliseconds (12). This permits rapid repetition of RF pulses and, hence, a high number of signal averages to compensate for the low spin density of the gases.…”

“…Furthermore, because of the relatively large number of 19 F atoms per molecule, which all resonate at the same Larmor frequency, they also emit a relatively strong MR signal. However, previous applications of this method in rats required acquisition times on the order of hours (12,13). In a recent study (14), we have shown that by making use of gradient hardware available today imaging of SF 6 gas within the pig lung can be performed within a single breath-hold period.…”

19F-MRT der Lungenventilation in Atemanhaltetechnik mittels SF6-Gas

“…Diese Gase besitzen sehr kurze T 1 -Relaxationszeiten im Millisekundenbereich, so dass die geringe Spindichte der Gase durch eine groûe Anzahl von Signalmittelungen kompensiert werden kann [6]. Allerdings war in bisherigen Arbeiten die Messzeit in der Gröûenordnung von Stunden, so dass diese Technik bisher nicht für die Anwendung am Menschen eingesetzt werden konnte [6,7].…”