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Purpose: Gamma Knife (GK) planning is mostly based upon MRI. Due to the high accuracy required for SRS treatments, geometric distortions in these images should be minimal. MRI Manufacturer provided first order geometric distortions correction algorithms may not reduce distortions to a level that may not be acceptable for treatments requiring high positional accuracy. The purpose of this work is to evaluate the residual, radiosurgery-relevant geometric distortions on VIBE, MPRAGE, GRE and CISS sequences. Methods: The 3D rectilinear phantom, GRID3D, was scanned in the Leksell Frame-G with different MRI axial sequences: MPRAGE, VIBE, GRE and CISS, on a Siemens SKYRA 3T MR unit. Siemens 3D distortion correction was applied to all sequences. The residual distortion map of the phantom's 2002 vertices was evaluated by comparing the expected positions of the vertices, with the actual measured locations using Modus’ image distortion analysis software (QUASARTM). Distortions were specified as X, Y, Z, and radial deviations. As an independent verification, the MRI sequences were co-registered with CT images of the phantom in GammaPlan. Results: All sequences showed mean radial residual distortions greater than 1 mm. Maximum radial distortion ranged between 3 and 8 mm. Z axis distortions were the highest for GRE and CISS sequences, and lowest for VIBE. Y distortions were lowest for the VIBE and MPRAGE sequences. X distortion was lowest for GRE and CISS sequences. Almost equal X and Y distortions were measured for CISS. Larger distortions were mainly observed in peripheral regions of the phantom for all sequences. CT-MR co-registration in GammaPlan confirmed these results. The residual distortions were deemed unacceptable for MR based GK treatments. Conclusion: The in-phantom analysis of MR sequences using the Modus phantom allows for effective quantitative evaluation of residual distortion in MR images. MR-CT co-registration facilitates qualitative assessment of MRI distortions.

The rotational spectrum of methylenecyclopropene has been measured in the 180–650 GHz frequency range. One hundred and forty-three new lines have been assigned and a centrifugal distortion analysis is presented. The experimental values of the centrifugal distortion constants confirm the rigidity of this molecule related to the high ring strain.

The NMR spectra of the nonaromatic protons in several benzo-substituted five- and six-membered rings have been analyzed at 60- and 90-MHz. The vicinal coupling constants from theCH2CH2fragments give a quantitative assessment of the degree of ring flattening. The distortions are discussed in terms of the structural components of the molecules.

The microwave spectrum of cyclopropene has been investigated in the region of 18−40 GHz. The two monosubstituted 13C species have been assigned and a centrifugal distortion analysis carried out on the main isotopic species. Structural parameters obtained are r (C1C2) = 1.509±0.001 A, r (C2C2′) = 1.2959±0.0004 A, &C2C1C2′ = 50.84±0.05°, r (CH ethylenic) = 1.072±0.001 A, r (CH methylenic) = 1.088±0.002 A, &C2C2′H = 149.85±0.08°, &HC1H = 114.57±0.19°.

The microwave spectrum of cyclopropyl cyanide has been re‐examined and a centrifugal distortion analysis has been carried out. Also, the two isotopic species containing 13C in the ring have been assigned allowing a substitution determination of the ring structure. Parameters found are r12 = 1.528±0.005 A, r23 = 1.500±0.002 A, &C2C1C3 = 58.8±0.3°.

The internal rotation fine structure of the microwave spectrum of ethyl fluoride CH3CH2F was resolved and analyzed in the ground state. The assignment of the high J lines was checked by a centrifugal distortion analysis. The barrier height V3 determined from the ground state is 3349±4 cal/mol.