Erich Hell

Feasibility of ultra-short echo time (UTE) magnetic resonance imaging for identification of carious lesions.

The objective of this study was to investigate the potential of ultra short echo time imaging for the assessment of caries lesions and early demineralization. 12 patients with suspected caries lesions underwent a dental magnetic resonance imaging investigation comprising ultra short echo time imaging (echo time = 50 μs) and spin echo imaging. Before the dental magnetic resonance imaging, all patients underwent a conventional clinical dental investigation including visual assessment of the teeth as well as dental x‐ray imaging. All lesions identifiable in the x‐ray could be clearly identified in the ultra short echo time images, but only about 19% of the lesions were visible in the spin echo images. In 19% of all lesions, the lesions could be more clearly delineated in the ultra short echo time images than in the x‐ray images. This was especially the case for secondary lesions. In direct comparison with the x‐ray images, all lesions appeared substantially larger in the dental magnetic resonance imaging data. The presented data provide evidence that caries lesions can be identified in ultra short echo time magnetic resonance imaging with high sensitivity. The apparent larger volume of the lesions in dental magnetic resonance imaging may be attributed to fluid accumulation in demineralized areas without substantial breakdown of mineral structures. Magn Reson Med, 2011.

A Small Surrogate for the Golden Angle in Time-Resolved Radial MRI Based on Generalized Fibonacci Sequences

In golden angle radial magnetic resonance imaging a constant azimuthal radial profile spacing of 111.246...° guarantees a nearly uniform azimuthal profile distribution in k-space for an arbitrary number of radial profiles. Even though this profile order is advantageous for various real-time imaging methods, in combination with balanced steady-state free precession (SSFP) sequences the large azimuthal angle increment may lead to strong image artifacts, due to the varying eddy currents introduced by the rapidly switching gradient scheme. Based on a generalized Fibonacci sequence, a new sequence of smaller irrational angles is introduced ( 49.750...°, 32.039...°, 27.198...°, 23.628...°, ... ). The subsequent profile orders guarantee the same sampling efficiency as the golden angle if at least a minimum number of radial profiles is used for reconstruction. The suggested angular increments are applied for dynamic imaging of the heart and the temporomandibular joint. It is shown that for balanced SSFP sequences, trajectories using the smaller golden angle surrogates strongly reduce the image artifacts, while the free retrospective choice of the reconstruction window width is maintained.

Golden ratio sparse MRI using tiny golden angles

The combination of fully balanced SSFP sequences with iterative golden angle radial sparse parallel (iGRASP) MRI leads to strong image artifacts due to eddy currents caused by the large angular increment of the golden angle ordering. The purpose of this work is to enable the combination of iterative golden angle radial sparse parallel MRI with balanced SSFP using the recently presented tiny golden angles.

Segmentation-free empirical beam hardening correction for CT.

PURPOSE The polychromatic nature of the x-ray beams and their effects on the reconstructed image are often disregarded during standard image reconstruction. This leads to cupping and beam hardening artifacts inside the reconstructed volume. To correct for a general cupping, methods like water precorrection exist. They correct the hardening of the spectrum during the penetration of the measured object only for the major tissue class. In contrast, more complex artifacts like streaks between dense objects need other techniques of correction. If using only the information of one single energy scan, there are two types of corrections. The first one is a physical approach. Thereby, artifacts can be reproduced and corrected within the original reconstruction by using assumptions in a polychromatic forward projector. These assumptions could be the used spectrum, the detector response, the physical attenuation and scatter properties of the intersected materials. A second method is an empirical approach, which does not rely on much prior knowledge. This so-called empirical beam hardening correction (EBHC) and the previously mentioned physical-based technique are both relying on a segmentation of the present tissues inside the patient. The difficulty thereby is that beam hardening by itself, scatter, and other effects, which diminish the image quality also disturb the correct tissue classification and thereby reduce the accuracy of the two known classes of correction techniques. The herein proposed method works similar to the empirical beam hardening correction but does not require a tissue segmentation and therefore shows improvements on image data, which are highly degraded by noise and artifacts. Furthermore, the new algorithm is designed in a way that no additional calibration or parameter fitting is needed. METHODS To overcome the segmentation of tissues, the authors propose a histogram deformation of their primary reconstructed CT image. This step is essential for the proposed algorithm to be segmentation-free (sf). This deformation leads to a nonlinear accentuation of higher CT-values. The original volume and the gray value deformed volume are monochromatically forward projected. The two projection sets are then monomially combined and reconstructed to generate sets of basis volumes which are used for correction. This is done by maximization of the image flatness due to adding additionally a weighted sum of these basis images. sfEBHC is evaluated on polychromatic simulations, phantom measurements, and patient data. The raw data sets were acquired by a dual source spiral CT scanner, a digital volume tomograph, and a dual source micro CT. Different phantom and patient data were used to illustrate the performance and wide range of usability of sfEBHC across different scanning scenarios. The artifact correction capabilities are compared to EBHC. RESULTS All investigated cases show equal or improved image quality compared to the standard EBHC approach. The artifact correction is capable of correcting beam hardening artifacts for different scan parameters and scan scenarios. CONCLUSIONS sfEBHC generates beam hardening-reduced images and is furthermore capable of dealing with images which are affected by high noise and strong artifacts. The algorithm can be used to recover structures which are hardly visible inside the beam hardening-affected regions.

Automatic detection of patient motion in cone-beam computed tomography

Some computed tomography (CT) applications, for example micro- or dental-CT, have long acquisition sequences and consequently motion of the object is likely to occur. The common motion correction method, not using optical techniques for patient motion measurement, is data-driven motion-correction (DDMC). This method is based on subdivision of the projection data into motion free subsections. Therefore, motion positions have to be determined. In this work, an approach for motion position detection in cone-beam CT data is described. Distance metric values, computed from two successive projection images, provide information of the incorporation of movement. Quantitative evaluation of motion detection is possible due to utilization of CT data with known movement positions, using generated dental cone-beam CT datasets. The proposed method uses nothing but information contained in the cone-beam projections. Therefore, it is generally applicable for motion detection in cone-beam CT. A correct detection rate of 99.89% is achieved by using a structural similarity index as a distance measure.

A self-gating method for time-resolved imaging of nonuniform motion.

To develop a self‐gating method capable of assessing nonuniform motion, e.g., in cardiovascular magnetic resonance imaging of patients with severe arrhythmia, or for imaging of the temporomandibular joint.

The application of ultra short echo time MRI (UTE) for the structural assessment of dental hard tissue components

The application of MRI as non-invasive imaging modality for dental diagnosis has not entered clinical routine due to its limited performance in assessment of dental hard tissues, due to the related very short T2/T2* - relaxation times of well below 200 is. With the recently introduced ultra-short echo time (UTE) MRI techniques, image acquisitions with echo times as low as a few is became possible. The objective of this work is to investigate the applicability of ultra-short echo time (UTE) MRI for the assessment of structural changes in the enamel, the dentin and the pulpa tissue. The UTE technique has been evaluated in-vitro in extracted human teeth and in-vivo in volunteers. Furthermore, the impact of dental filling materials on the resulting image quality has been assessed in-vitro for a variety of different filling materials.