Hyperpolarized (129)Xe human imaging and spectroscopy are very promising methods to provide functional information about the lung.

Purpose To implement and characterize a single-breath xenon transfer contrast (SB-XTC) method to assess the fractional diffusive gas transport F in the lung: to study the dependence of F and its uniformity as a function of lung volume; to estimate local alveolar surface area per unit gas volume SA/VGas from multiple diffusion time measurements of F; to evaluate the reproducibility of the measurements and the necessity of B1 correction in cases of centric and sequential encoding. Materials and Methods In SB-XTC three or four gradient echo images separated by inversion/saturation pulses were collected during a breath-hold in eight healthy volunteers, allowing the mapping of F (thus SA/VGas) and correction for other contributions such as T1 relaxation, RF depletion and B1 inhomogeneity from inherently registered data. Results Regional values of F and its distribution were obtained; both the mean value and heterogeneity of F increased with the decrease of lung volume. Higher values of F in the bases of the lungs in supine position were observed at lower volumes in all volunteers. Local SA/VGas (with a mean ± standard deviation of SA/VGas¯=89±30cm-1) was estimated in vivo near functional residual capacity. Calibration of SB-XTC on phantoms highlighted the necessity for B1 corrections when k-space is traversed sequentially; with centric ordering B1 distribution correction is dispensable. Conclusion SB-XTC technique is implemented and validated for in vivo measurements of local SA/VGas.

Purpose: To measure T 1 and T 2 of the fine structures of the in vivo eye. Materials and Methods:Involuntary saccades make it difficult to obtain artifact-free images. Using a method recently reported (Bert et al, Acad Radiol 2006;12:368 -378), near artifact-free spin-echo images were obtained. Both an isolated enucleated eye and eight human subjects were studied at 1.5T. Spin-echo variable TR/TE data was acquired for T 1 /T 2 determination. Average relaxation times were calculated two ways. First, an arithmetic average over different subjects was computed. Second, all data was normalized using the fitted amplitudes of each data set and pooled to obtain a single least squares fit. BECAUSE OF THE very small size of many structures in the human eye, as well as the difficulty with involuntary patient motion of the eye, high resolution and high signal-to-noise ratio (SNR) MRI of the human eye is technically difficult. There are, however, many reports of ocular MRI in the early literature (1-13). For example, in 1987, Atlas et al (13) retrospectively reviewed their initial experience in 59 patients with MRI of the orbit concluding that there were five specific areas in which MRI ocular imaging provided important information. Imaging at a high field, which was 1.5T in 1987, together with a surface coil and short TR values (600 -800 msec) were very important in increasing SNR so that the acquisition time (using 128 phase encodes) could be kept less than two minutes. For longer values of TR, considerable motion artifacts were observed from both head and eye motion. A number of limitations were noted, including poor specificity. In 1991, Wilms et al (14) reported the results of ocular MRI in 41 patients. Their conclusions were that MRI was superior to computed tomography (CT) in depicting ocular lesions because of multi-planar imaging and higher contrast resolution. Critical determinations of T 1 and T 2 values for ocular structures were not reported.From a technical standpoint, other than implementing surface coils and going to high field, it is useful to employ fast spin-echo (FSE) techniques that reduce total acquisition time by measuring multiple lines of k-space for each acquisition (15). In 2003, Simon et al (16) reported on the use of a three-dimensional T2-weighted FSE method to obtain a spatial resolution of 0.703 mm (readout), 0.938 mm (phase encode), and 1.2-1.6 mm (slice), over an 18 cm field of view (FOV) that included the eye. Total imaging time, however, was fairly long at nine minutes 52 seconds. Clinical results with this method were reported by McCaffery et al (17) and showed that with the thinner slice sections ob-

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