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.

In-ear vital signs monitoring using a novel microoptic reflective sensor.

Cardiovascular diseases are among the most common causes of death in industrial countries. In order to take preventive actions, it is of great interest, to both physicians and patients, to determine cardiovascular risk factors early. To address this problem, a wearable in-ear measuring system (IN-MONIT) for 24/7 monitoring of vital parameters has been developed. The central component is a microoptic reflective sensor located inside the auditory canal. From the measured photoplethysmographic curves, heart activity and heart rate can be derived. In this paper, we describe the optoelectronic sensor concept and the autonomous design of the IN-MONIT measurement system. For the assessment of heart rate, different algorithms are introduced and the performance of the developed sensor system is evaluated in relation to conventional systems. In addition, the robustness to external artifacts is evaluated and artifact reduction strategies are considered.

A system for assessing motion artifacts in the signal of a micro-optic in-ear vital signs sensor.

Cardiovascular diseases are among the most common causes of death in developed industrial nations. It is of great interest of both physician and patient to determine the cardiovascular risk factors early in order to take preventive measures. To assist these investigations we develop a wearable in-ear measuring system (IN-MONIT) for 24/7 monitoring of heart rate and oxygen saturation (SpO2). The central component is a micro-optic remission/reflection sensor (MORES) located inside the auditory canal. From the measured photoplethysmographic curves the aforementioned vital signs can be derived. In the following we present a recording system for assessing motion artifact influence in the in-ear sensor data. Two accelerometer sensors record posture and motion while at the same time SpO2, heart rate and PPG are measured using both a commercial sensor and the in-ear sensor. The data is transmitted wirelessly to a control PC for storage and further investigation. Using this system we assessed the influence of motion artifacts produced by daily life activities on infrared and red in-ear PPG data and on readings of the reference sensor.

In-ear heart rate monitoring using a micro-optic reflective sensor.

Cardiovascular diseases are among the most common causes of death in western industrial nations. It is of great interest of both physician and patient to determine the cardiovascular risk factors early in order to take preventive measures. To assist the recognition of irregularities in a subject's cardiovascular system, we develop an optic 24/7 inear monitoring system (IN-MONIT). The central component is a micro-optic remission/reflection sensor (MORES), which is placed inside the auditory canal. There the pulsation of blood within the capillaries is measured by means of optical absorption. From the resulting photoplethysmographic curves (pulse plethysmogram, PPG), the heart rate, oxygen saturation (SpO2), respiratory rate and higher order moments can be derived. The optical absorption data are processed locally using a microcontroller and the results are transferred wirelessly to a personal digital assistant (PDA) or PC for sophisticated classification. This paper introduces the IN-MONIT system and two algorithms for heart rate determination from ECG or PPG data. The performance of these algorithms was tested using annotated ECG data from the "MIT-BIH Normal Sinus Rhythm Database", synchronously recorded ECG and pulse oximeter data, and data acquired by the MORES sensor.