Bjoern Heismann

Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA).

Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k‐space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k‐space is filled with radial half‐projections, whereas the centre is measured single pointwise on a Cartesian trajectory. This hybrid sequence combines the features of single point imaging with radial projection imaging. No hardware changes are required. Using this method, 3D images with an isotropic resolution of 1 mm can be obtained in less than 3 minutes. The differences between PETRA and the ultrashort echo time (UTE) sequence are evaluated by simulation and phantom measurements. Advantages of pointwise encoding time reduction with radial acquisition are shown for tissue with a T2 below 1 ms. The signal to noise ratio and Contrast‐to‐noise ratio (CNR) performance, as well as possible limitations of the approach, are investigated. In‐vivo head, knee, ankle, and wrist examples are presented to prove the feasibility of the sequence. In summary, fast imaging with ultrashort echo time is enabled by PETRA and may help to establish new routine clinical applications of ultrashort echo time sequences. Magn Reson Med, 2012.

Correcting slice selectivity in hard pulse sequences.

Many MRI sequences use non-selective hard pulse excitation in the presence of imaging gradients. In this work, we investigate to which extent the sinc-shaped frequency excitation profiles of the pulse can be used for imaging without the generation of artefacts. A correction algorithm is proposed that eliminates the influence of the excitation profile. Phantom as well as in vivo measurements prove that enhanced image quality can be obtained as long as the first minimum of the excitation profile lies outside the imaged object.

Spectral ρZ-Projection Method for Characterization of Body Fluids in Computed Tomography

RATIONALE AND OBJECTIVES The identification of body fluids in computed tomography poses a major diagnostic challenge. The chemical composition of body fluids deviates only slightly from water with very similar computed tomographic (CT) values, which typically range from 0 to 100 HU. The aim of this study was to assess physical and chemical properties of different body fluids in an ex vivo setting. MATERIALS AND METHODS A total of 44 samples of blood, blood mixed with pus, pus, bile, and urine obtained during diagnostic and therapeutic punctures were scanned at 80 and 140 kV. Data was quantitatively assessed using the spectral rhoZ-projection algorithm, which converts dual-energy CT scans into mass density (rho) and effective atomic number (Z(eff.)) information. RESULTS Attenuation values measured at 80 and 140 kV were largely overlapping. CT values allowed, to some degree, for the differentiation of bile or pus from blood or the blood/pus mixture. By applying the rhoZ-projection, most substances, except for urine, were distinguishable with only small standard deviations ranging between 0.003 and 0.007 g/cm(3) for mass density and between 0.020 and 0.043 for Z(eff.). CONCLUSION The rhoZ-projection method is suited to quantitatively assess mass density and effective atomic number of ex vivo body fluid samples. In clinical routine, this technique might be useful for identifying unclear fluid collections even in unenhanced computed tomography.

Ray Contribution Masks for Structure Adaptive Sinogram Filtering

The patient dose in computed tomography (CT) imaging is linked to measurement noise. Various noise-reduction techniques have been developed that adapt structure preserving filters like anisotropic diffusion or bilateral filters to CT noise properties. We introduce a structure adaptive sinogram (SAS) filter that incorporates the specific properties of the CT measurement process. It uses a point-based forward projector to generate a local structure representation called ray contribution mask (RCM). The similarities between neighboring RCMs are used in an enhanced variant of the bilateral filtering concept, where the photometric similarity is replaced with the structural similarity. We evaluate the performance in four different scenarios: The robustness against reconstruction artifacts is demonstrated by a scan of a high-resolution-phantom. Without changing the modulation transfer function (MTF) nor introducing artifacts, the SAS filter reduces the noise level by 13.6%. The image sharpness and noise reduction capabilities are visually assessed on in vivo patient scans and quantitatively evaluated on a simulated phantom. Unlike a standard bilateral filter, the SAS filter preserves edge information and high-frequency components of organ textures well. It shows a homogeneous noise reduction behavior throughout the whole frequency range. The last scenario uses a simulated edge phantom to estimate the filter MTF for various contrasts: the noise reduction for the simple edge phantom exceeds 80%. For low contrasts at 55 Hounsfield units (HU), the mid-frequency range is slightly attenuated, at higher contrasts of approximately 100 HU and above, the MTF is fully preserved.

Parallel imaging-based reduction of acoustic noise for clinical magnetic resonance imaging.

ObjectivesThe objective of this study was to demonstrate the feasibility of improving perceived acoustic comfort for a standard clinical magnetic resonance imaging protocol via gradient wave form optimization and validate parallel imaging as a means to achieve a further reduction of acoustic noise. Materials and MethodsThe gradient wave forms of a standard T2 axial turbo spin-echo (TSE) sequence in head examinations were modified for acoustic performance while attempting to keep the total acquisition and inter-echo spacing the same. Parallel imaging was then used to double the inter-echo spacing and allow further wave form optimization. Along with comparative acoustic noise measurements, a statistical analysis of radiologist scoring was conducted on volumes from standard and modified sequences acquired from 10 patients after informed consent was obtained. ResultsCompared with TSE, significant improvement of acoustic comfort was measured for modified-sequences quiet TSE and quiet TSE with generalized autocalibrating partially parallel acquisitions (P = 0.0034 and P = 0.0003, respectively), and no statistically significant difference in diagnostic quality was observed without the use of parallel imaging. ConclusionsStandard clinical magnetic resonance imaging protocols can be made quieter through adequate gradient wave form optimization. In scans with high signal-to-noise ratio, parallel imaging can be used to further reduce acoustic noise.

Technology and image results of a spectral CT system

We report the implementation and first test results of a two-channel spectral Computed Tomography (CT) prototype. We use an energy-resolving CT detector with a sandwich-like two layer set-up. Compared to dual-energy approaches with tube voltage switching, it yields a low and a high energy channel in a one shot measurement. We explain the basic set-up of the system and its calibration. The effects of spectral weighting are examined and the weighting functions w(E) of the detector channels are calculated. We present spectral image data of a water phantom, a set of calibration materials and an organic sample. Finally, we show how the data can be used for quantitative CT measurements. The system is work in progress and currently not available in the United States.

Quiet diffusion-weighted head scanning: Initial clinical evaluation in ischemic stroke patients at 1.5T.

To compare the quality and diagnostic value of routine single‐shot, echo‐planar imaging, diffusion‐weighted imaging (ss‐EPI‐DWI) to those of quiet readout segmented EPI‐DWI (q‐DWI) in magnetic resonance imaging (MRI) of acute stroke.