[Phi]UN : a region growing phase unwrapping program optimized for MR images

Zsfassung in dt. SpracheEine Besonderheit von Magnet-Resonanz (MR) Daten ist, dass sie, im Gegensatz zu vielen anderen bildgebenden Verfahren, komplexwertig sind. Trotzdem nutzen die meisten Anwendungen nur die Magnitudeninformation. Nichtsdestotrotz gibt es aber Anwendungen, die auf der Phaseninformation basieren, da die Phase sehr sensitiv ist. In diesen Anwendungen stellt das Aliasing (auch Phase-Wrapping genannt), das auf Grund des beschränkten Definitionsbereiches der Phase auftritt, ein schwieriges Problem für die Nachbearbeitung dar. Die Beseitigung der resultierenden Mehrdeutigkeiten wird als Phase Unwrapping bezeichnet.Im Rahmen dieser Masterarbeit wurde ein zweidimensionaler Region-Growing Phase-Unwrapping-Algorithmus für MR-Phasenbilder optimiert. Das Ziel war es eine Methode zu entwickeln, die besonders für suszeptibilitätsgwichtete Bildgebung (SWI) geeignet ist. SWI ist eine spezielle MR Technik, die auf hochaufgelösten Daten basiert, die oft ein niedriges Signal zu Rausch Verhältnis (SNR) aufweisen. Die Implementierung des Algorithmus, PhUN, wurde ausgiebig an Phantom und in vivo Daten getestet und mit einer etablierten Methode verglichen. Gerade bei hochaufgelösten Daten mit niedrigem SNR war PhUN sowohl in Bezug auf Geschwindigkeit als auch im Sinne der Verlässlichkeit der Ergebnisse überlegen.Magnetic Resonance Imaging (MRI) data, unlike the data of many other imaging modalities, are complex valued. Nevertheless, in most applications only the magnitude information is used. However, since the phase is very sensitive, there are techniques that are based on phase images. For data acquired with those techniques aliasing (also known as phase wrapping) due to the limited domain of the phase poses a difficult problem to post processing. Resolving the resulting ambiguities is known as phase unwrapping.For this thesis a two-dimensional region growing phase unwrapping algorithm was optimized for MR phase images. The goal was to develop a method that is especially suited for Susceptibility Weighted Imaging (SWI). SWI is a special MR technique that is based on high-resolution data which often have a low signal-to-noise ratio (SNR). The implementation of the algorithm, PhUN, was evaluated extensively on both phantom and in vivo data, and compared to an established method. PhUN's performance, both in terms of speed and robustness, proved to be superior especially in the case of high resolution, low SNR data, such as SWI data.13

Phase unwrapping of MR images using ΦUN - A fast and robust region growing algorithm

We present a fully automated phase unwrapping algorithm (ΦUN) which is optimized for high-resolution magnetic resonance imaging data. The algorithm is a region growing method and uses separate quality maps for seed finding and unwrapping which are retrieved from the full complex information of the data. We compared our algorithm with an established method in various phantom and in vivo data and found a very good agreement between the results of both techniques. ΦUN, however, was significantly faster at low signal to noise ratio (SNR) and data with a more complex phase topography, making it particularly suitable for applications with low SNR and high spatial resolution. ΦUN is freely available to the scientific community

Combining phase images from multi-channel RF coils using 3D phase offset maps derived from a dual-echo scan

A method is presented for the combination of phase images from multi-channel RF coils in the absence of a volume reference coil. It is based on the subtraction of 3D phase offset maps from the phase data from each coil. Phase offset maps are weighted combinations of phase measurements at two echo times. Multi-Channel Phase Combination using measured 3D phase offsets (MCPC-3D) offers a conceptually and computationally simple solution to the calculation of combined phase images. The dual-echo data required for the phase maps can be intrinsic to the high-resorlution gradient-echo scan to be reconstructed (MCPC-3D-I). Alternatively, a separate, fast, low-resolution dual-echo scan can be used (MCPC-3D-II). Both variants are shown to give near perfect phase matching, yielding images with high SNR throughout and high GM-WM contrast. MCPC-3D is compared with other reference-free phase image crombination methods; high-pass phase filtering, phase difference imaging, and matching using constant offsets (MCPC-C). Multi-Channel Phase Combination using measured 3D phase offsets method does not need an overlap between the signals from individual coils and can be used with parallel imaging, making it ideally suited to multi-channel coils with a large number of elements, and to high and ultra-high field systems

Improved elimination of phase effects from background field inhomogeneities for susceptibility weighted imaging at high magnetic field strengths

To enhance susceptibility-related contrast of magnetic resonance images, the phase of susceptibility weighted data needs to be corrected for background inhomogeneities and phase wraps caused by them. Current methods either use homodyne filtering or a combination of phase unwrapping and high pass filtering. The drawback of homodyne filtering is incomplete elimination of phase wraps in areas with steep phase topography produced by background inhomogeneities of the static magnetic field. The disadvantage of phase unwrapping is that it requires subsequent high pass filtering, which introduces artifacts in areas with very steep transitions, such as areas near interfaces between parenchyma and bone or air. A method is proposed that reduces the artifacts associated with high pass filtering without sacrificing the advantages of phase unwrapping. This technique is demonstrated with phantom data at 1.5 T and with human data at 1.5, 3 and 7 T

Radiology Case Reports

Detection of multiple intracranial hemorrhages in a child with acute lymphocytic leukemia (ALL) by susceptibility weighted imaging (SWI

Quality Assurance Investigations and Impurity Characterization during Upscaling of [¹⁷⁷Lu]Lu-PSMA^I&T

[177Lu]Lu-PSMAI&T is widely used for the radioligand therapy of metastatic castration-resistant prostate cancer (mCRPC). Since this kind of therapy has gained a large momentum in recent years, an upscaled production process yielding multiple patient doses in one batch has been developed. During upscaling, the established production method as well as the HPLC quality control were challenged. A major finding was a correlation between the specific activity and the formation of a pre-peak, presumably caused by radiolysis. Hence, nonradioactive reference standards were irradiated with an X-ray source and the formed pre-peak was subsequently identified as a deiodination product by UPLC-MS. To confirm the occurrence of the same deiodinated side product in the routine batch, a customized deiodinated precursor was radiolabeled and analyzed with the same HPLC setup, revealing an identical retention time to the pre-peak in the formerly synthesized routine batches. Additionally, further cyclization products of [177Lu]Lu-PSMAI&T were identified as major contributors to radiochemical impurities. The comparison of two HPLC methods showed the likelihood of the overestimation of the radiochemical purity during the synthesis of [177Lu]Lu-PSMAI&T. Finally, a prospective cost reduction through an optimization of the production process was shown

Contrast-enhanced, high-resolution, susceptibility-weighted magnetic resonance imaging of the brain: dose-dependent optimization at 3 Tesla and 1.5 Tesla in healthy volunteers

OBJECTIVES: We sought to determine the optimal dose of a contrast agent with known high relaxivity on 1.5 and 3 Tesla scanners that would achieve the best compromise between image quality and scan time for the clinical application of contrast-enhanced susceptibility-weighted imaging (CE-SWI). METHODS: Pre- and postcontrast SWI was performed with different contrast agent doses (0.05, 0.1, and 0.2 mmol/kg gadobenate dimeglumine) at both 1.5 and 3 T in 6 healthy volunteers, resulting in 72 examinations. Venograms were created from minimum intensity projection reconstructions over specified deep white matter volumes to enhance the visual appearance of connected venous structures. Three independent radiologists blindly rated the visibility of the veins on a continuous scale of 1 to 10. A general linear model was used for statistical evaluation, with fixed effects of the contrast agent dose, the field strength, the rater and the patients as a random effect. RESULTS: With CE-SWI, we found significant differences in the visibility of the deep veins dependent on the contrast media dose (P = 0.02). At 3 T, the visibility of deep venous vessels, with regard to susceptibility effect, image quality, and scan time reduction after a standard contrast agent dose 0.1 mmol/kg was significantly better than that achieved with 0.05 mmol/kg. The visibility was considered equal with 0.1 mmol/kg of the contrast agent to the precontrast images and a dose of 0.2 mmol/kg. At 1.5 T, no significant difference was found between the 4 contrast agent doses. We found no difference in the visibility of the veins with the shorter sequences at 3 T compared with the sequences at 1.5 T. CONCLUSIONS: Only a standard dose (0.1 mmol/kg) of gadobenate dimeglumine is required to achieve the optimum susceptibility effect and image quality at 3 T, together with a reduced scan time. This result can be attributed to the higher relaxivity of gadobenate dimeglumine, compared with conventional gadolinium chelates