Fifth Finnish Medical Physics and Medical Engineering Day
Abstracts of the Awarded Master's Graduate Theses

Ossi Riekkinen, University of Kuopio
Influence of Overlying Soft Tissues on Trabecular Bone Acoustic Measurement at Various Ultrasound Frequencies

In osteoporosis degenerative changes in bone composition and structure lead to diminished bone strength and increased fracture risk. Ultrasound measurements have been introduced as promising tools for osteoporosis diagnostics. However, soft tissues overlying bones may impair reliability of these techniques. In this study the effect of overlying adipose and lean tissue on bone ultrasound measurements was investigated at ultrasound frequencies of 0.2 - 6.7 MHz. In addition, the effect of sound frequency on the ultrasound ability to predict mechanical properties of trabecular bone was assessed.

In experimental measurements, human trabecular bone samples (n = 25, thickness = 7.5 mm) were investigated using mechanical, acoustic and dual energy X-ray absorptiometry (DXA) techniques. Ultrasound attenuation, speed, reflection from bone surface and backscattering were determined from acoustic measurements, conducted for bone samples without and with overlying adipose (n = 25, thickness = 10 - 20 mm) and/or lean (n = 25, thickness = 10 - 20 mm) tissue. Further, acoustic parameters were also determined for adipose and lean tissue layers only. As the thickness and attenuation in adipose/lean tissue were determined from pulse-echo data, their effects on bone (with soft tissue) measurements were numerically analysed. Thereby, corrected values for bone ultrasound parameters were derived and compared with those obtained from the measurements of bone without overlying soft tissues.

The errors induced by the soft tissues on the ultrasound measurement were typically reduced by ~50% after introduction of the numerical correction technique, even when adipose to lean tissue ratio was incorrect. Speed of sound was exception; the error induced by the soft tissue was diminished only when adipose to lean tissue ratio was correct. After soft tissue correction, ultrasound parameters (0.5 - 3.8 MHz) were significant predictors of bone ultimate strength (r = 0.58 - 0.76, p < 0.01).

Results of present study suggest that the frequency range used in clinical ultrasound devices (0.2 -0.6 MHz) may not be optimal. In the present study, ultrasound parameters measured from 0.5 - 3.8 MHz frequency range were the strongest predictors of mechanical properties of trabecular bone samples. Soft tissue correction technique, as presented in the present study, improves accuracy of ultrasound results substantially, as compared to analysis with soft tissue ignored.

Jukka Kortelainen, University of Oulu
Visualization and Removal of ECG Motion Artifacts

Real-time monitoring of the heart's electrical activity has proven to be very useful for patients with a probable coronary heart disease. However, the effects of respiration and body position changes (BPC) on the morphology of electrocardiogram (ECG) make it very difficult to interpret the signal measured in ambulatory conditions. This work focuses on the visualization and removal of these distortions, known as the motion artifacts of ECG.

For the illustration of motion artifacts, a software simulation environment EMAS (ECG Motion Artifact Simulator) was created. With the designed software the user is able to simulate the effect of self-defined respiration and BPCs on vectorcardiogram (VCG) and ECG. The simulation is possible to carry out in a static manner, assuming that the respiration phase and body position are constant, or dynamic, in which the motion artifacts vary continuously as a function of time.

The second objective of the study was the removal of motion artifacts. For this purpose, ambulatory VCG measurements were performed for 25 volunteers. During the recordings the volunteers were supposed to change their respiration depth and body position following a predefined schedule, and thus contaminating the signal with motion artifacts. A novel data alignment method for beat-to-beat variability reduction was then applied to the measurement data. The aim was to test the method and adjust its parameters to be optimal with the signals containing varying amounts of noise. For comparison, alignment was performed with two other methods presented in the literature. The new method was able to reduce the variability of whole the cardiac cycles, QRS complexes and T waves by factors of 0.79, 0.47 and 0.91, respectively, whereas the corresponding numbers for the other methods were 0.89, 0.64 and 0.96 at best.

The results show that the morphological beat-to-beat variation of ambulatory ECG can be reduced considerably by the new method. Due to the method's low complexity, the variation reduction can be performed in real-time. This makes it possible to interpret continuously monitored ECG with a higher accuracy and hence decreases the amount of misclassifications.

Petteri Lapinlampi, Helsinki University of Technology
A Novel Measure for EEG/EMG Responsiveness May Indicate the Level of Sedation in ICU Patients

Sedation is a process in which the patient's level of consciousness is lowered by anesthetic agents in order to help him/her tolerate the treatment in an intensive care unit (ICU). The goal is to keep the patient calm and free of pain while still arousable by speech. However, the administration of the correct amount of sedative drugs to achieve this is difficult. The traditionally used methods based on the patient's weight often lead to oversedation especially if the patient has compromised renal and/or liver function. Both over- and undersedation should be avoided in order to minimize the patient's length of stay in the ICU and to prevent other complications. Level of sedation in ICU patients is difficult to assess, and relies on clinical assessments such as the Ramsay score. These tests discriminate deep sedation poorly. Therefore, an objective and automated method for measuring the depth of sedation is needed.

We evaluated a novel measure, EEG/EMG responsiveness, and compared it to Ramsay scores. EEG/EMG responsiveness was calculated by applying an algorithm that detects abrupt changes in the measured EEG/EMG signal indicative of a patient's arousal or activation. The results were compared to those obtained using the EEG entropy. Thirty consenting general ICU patients with non-neurologic primary ICU diagnosis were investigated for a maximum of 72 hours. The Response Entropy (RE) and State Entropy (SE) values as well as the EEG signal from which the EEG/EMG responsiveness was calculated were recorded from forehead electrodes using the Entropy™ Module (GE Healthcare). When possible, Ramsay score was evaluated every 30 minutes by a single trained observer, amounting to a total of 370 assessments.

The obtained prediction probability PK = 0.86 for EEG/EMG responsiveness to distinguish deep Ramsay levels 4-6 from levels 1-3 was significantly higher than the corresponding PK values 0.80 and 0.79 for RE and SE, respectively (p<0.05). In the subgroup of 18 patients judged clinically to have no acute brain disorders, the PK values for EEG/EMG responsiveness, RE, and SE (0.91, 0.85, and 0.84, respectively) were significantly higher (p<0.001) than those in the remaining subgroup of 12 patients with encephalopathy (0.78, 0.73, and 0.75).

The novel measure of EEG/EMG responsiveness shows promise as an indicator for the level of sedation. As an objective and automatic method it provides valuable information for the nursing staff in order to titrate an optimal amount of sedative drugs to each patient.

Virva Lepomäki, University of Turku
Optical Cell-Reader

The structure of an optical cell-reader is similar to that of an ordinary microscope but operation of a cell-reader is fully automated. The purpose of this work is to determine limits for a two photon excitation based optical cell-reader.

Parameters of two photon excitation based optical cell-reader are studied using theoretical analysis and measurements. In this work we use imaging speed to determine limits for a two photon excitation based optical cell-reader. Special emphasis has been paid on the effects of focus depth end excitation power to imaging speed.

From theoretical analysis and from measurements it can be seen that it is possible to measure samples very quickly. The imaging speed is restricted by the laser used to excite the label molecules, concentration of label molecules, background signal intensity and focus depth.

Iina Tarnanen, Helsinki University of Technology
Electroencephalography in a 3-T Magnetic Resonance Imaging Scanner

Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) are two noninvasive functional brain imaging methods with complementary characteristics. Combined, they offer the highest temporal and spatial resolution attainable to date. However, the acquisition of data using simultaneous EEG and fMRI is challenging. Three aspects need to be considered: image quality, EEG quality, and safety. This study addresses the first two.

First, the influence of the components of a commercial EEG system (BrainAmp MR Plus, BrainProducts GmbH, Germany) on MR image quality, obtained with a 3-T MRI scanner (GE Signa 3.0 T with Excite) was examined. The components were placed on a standard gel phantom, and echo planar images (EPI) used in fMRI were obtained. The extent of the consequent susceptibility artefacts into the phantom was measured on a pixel-by-pixel basis from the images. Second, the effect of the static magnetic field on EEG quality was evaluated. Alpha activity, which is represented by 8-13 Hz oscillatory activity in the posterior parts of the brain, was recorded from one subject at 3 T in order to identify the extent of cardiac-cycle related artefacts enhanced by the static field. The frequency spectrum was determined using Hanning-windowed Fourier transform.

The results showed that the largest image artefacts, which extended several centimetres into a phantom, were found to originate from electrode lead connectors. The size of the artefacts decreased with increased distance from the phantom. Other EEG system components perturbed the images to a lesser extent. EEG signal was heavily tainted by intense artefacts. However, alpha activity could be observed. In addition, unexpected interference was found in the EEG signal recorded within the MRI scanner bore. The frequency distribution of the interference was characterized by several large peaks of minimal variance centred around 24 Hz.

In order to obtain good quality data, both the image and EEG artefacts need to be confined to minimum. Thus, the EEG lead connectors could be either detached from the skin or replaced by alternative non-magnetic connectors to reduce the image artefact to an acceptable level. The interference in the EEG was deduced to originate from the cryogenic pump of the MRI scanner. By turning off the pump during EEG recording and applying artefact removal methods, good quality EEG can be obtained. However, before simultaneous EEG and fMRI experiments can be commenced with human subjects, safety aspects need to be considered.