Toivo Katila

Neuronal responses to magnetic stimulation reveal cortical reactivity and connectivity

MOTOR and visual cortices of normal volunteers were activated by transcranial magnetic stimulation. The electrical brain activity resulting from the brief electromagnetic pulse was recorded with high-resolution electroencephalography (HR-EEG) and located using inversion algorithms. The stimulation of the left sensorimotor hand area elicited an immediate response at the stimulated site. The activation had spread to adjacent ipsilateral motor areas within 5–10 ms and to homologous regions in the opposite hemisphere within 20 ms. Similar activation patterns were generated by magnetic stimulation of the visual cortex. This new non-invasive method provides direct information about cortical reactivity and area-to-area neuronal connections.

A review of cardiac image registration methods

In this paper, the current status of cardiac image registration methods is reviewed. The combination of information from multiple cardiac image modalities, such as magnetic resonance imaging, computed tomography, positron emission tomography, single-photon emission computed tomography, and ultrasound, is of increasing interest in the medical community for physiologic understanding and diagnostic purposes. Registration of cardiac images is a more complex problem than brain image registration because the heart is a nonrigid moving organ inside a moving body. Moreover, as compared to the registration of brain images, the heart exhibits much fewer accurate anatomical landmarks. In a clinical context, physicians often mentally integrate image information from different modalities. Automatic registration, based on computer programs, might, however, offer better accuracy and repeatability and save time.

Model extraction from magnetic resonance volume data using the deformable pyramid

A general framework for automatic model extraction from magnetic resonance (MR) images is described. The framework is based on a two-stage algorithm. In the first stage, a geometrical and topological multiresolution prior model is constructed. It is based on a pyramid of graphs. In the second stage, a matching algorithm is described. This algorithm is used to deform the prior pyramid in a constrained manner. The topological and the main geometrical properties of the model are preserved, and at the same time, the model adapts itself to the input data. We show that it performs a fast and robust model extraction from image data containing unstructured information and noise. The efficiency of the deformable pyramid is illustrated on a synthetic image. Several examples of the method applied to MR volumes are also represented.

Magnetocardiographic QT Interval Dispersion in Postmyocardial Infarction Patients with Sustained Ventricular Tachycardia: Validation of Automated QT Measurements

T dispersion is a measure of heterogeneity in ventricular repolarization. Increased ECG QT dispersion is associated with life‐threatening ventricular arrhythmias. We studied if magnetocardiographic (MCG) measures of QT dispersion can separate postmyocardial infarction patients with and without susceptibility to sustained VT. Manual dispersion measurements were compared to a newly adapted automatic QT interval analysis method. Ten patients with a history of sustained VT (VT group) and eight patients without ventricular arrhythmias (Controls) were studied after a remote myocardial infarction. Single‐channel MCGs were recorded from 42 locations over the frontal chest area and the signals were averaged. QT dispersion was defined as maximum — minimum or standard deviation of measured QT intervals. VT group showed significantly more QT and JT dispersion than Controls. QTapex dispersions were 127 ± 26 versus 83 ± 21 ms (P = 0.004) and QTend dispersions 130 ± 37 versus 82 ± 37 ms (P = 0.013), respectively. Automatic method gave comparable values. Their relative differences were 9% for QTapex and 27% for QTend dispersion on average. In conclusion, increased MCG QT interval dispersion seems to be associated with a susceptibility to VT in postmyocardial infarction patients. MCG mapping with automated QT interval analysis may provide a user independent method to detect nonhomogeneity in ventricular repolarization.

Detection of Exercise‐Induced Myocardial Ischemia by Multichannel Magnetocardiography in Single Vessel Coronary Artery Disease

Background: Detection of myocardial ischemia was studied with multichannel exercise magneto‐cardiography (MCG). A surface gradient method was applied to analyze cardiac magnetic fields.

Instrumentation and calibration methods for the multichannel measurement of phase and amplitude in optical tomography

In this article, we describe the multichannel implementation of an intensity modulated optical tomography system developed at Helsinki University of Technology. The system has two time-multiplexed wavelengths, 16 time-multiplexed source fibers and 16 parallel detection channels. The gain of the photomultiplier tubes (PMTs) is individually adjusted during the measurement sequence to increase the dynamic range of the system by 104. The PMT used has a high quantum efficiency in the near infrared (8% at 800nm), a fast settling time, and low hysteresis. The gain of the PMT is set so that the dc anode current is below 80nA, which allows the measurement of phase independently of the intensity. The system allows measurements of amplitude at detected intensities down to 1fW, which is sufficient for transmittance measurements of the female breast, the forearm, and the brain of early pre-term infants. The mean repeatability of phase and the logarithm of amplitude (lnA) at 100MHz were found to be 0.08° and 0.004, res...

Recording locations in multichannel magnetocardiography and body surface potential mapping sensitive for regional exercise-induced myocardial ischemia.

Introduction This study aimed to identify the optimal locations in multichannel magnetocardiography (MCG) and body surface potential mapping (BSPM) to detect exercise-induced myocardial ischemia. Methods We studied 17 healthy controls and 24 coronary artery disease (CAD) patients with stenosis in one of the main coronary artery branches: left anterior descending (LAD) in 11 patients, right (RCA) in 7 patients, and left circumflex (LCX) in 6 patients. MCG and BSPM signals were recorded during a supine bicycle stress test. The capability of a recording location to separate the groups was quantified by subtracting the mean signal amplitude of the normal group from that of the patient group during the ST segment and at the T-wave apex, and dividing the resulting amplitude difference by the corresponding standard deviation within all subjects. Results In MCG the optimal location for ST depression was at the right inferior grid for the RCA, at the mid-inferior grid for the LCX, and in the middle of these locations for the LAD subgroup (mean ST amplitudes: CAD −80 ± 360fT, controls 610 ± 660fT; p < 0.001). In BSPM it was on the left upper anterior thorax for the LAD, left lower anterior thorax for the RCA, and on the lower back for the LCX subgroup (mean ST amplitudes: CAD −39 ± 61 μV and controls 38 ± 38 μV; p < 0.001). In MCG the optimal site for T-wave amplitude decrease was the same as the one for the ST depression. In BSPM it was on the middle front for the LAD, on the back for the LCX and on the left abdominal area for the RCA group. In accordance with electromagnetic theory, the largest ST segment and T-wave amplitude changes took place in MCG in locations orthogonal to those in BSPM. Conclusion This study identified magnetocardiographic and BSPM recording locations which are sensitive for detecting transient myocardial ischemia by evaluation of the ST segment as well as the T-wave. These locations strongly depend on ischemic regions and are outside the conventional 12-lead ECG recording sites.

A 3-D model-based registration approach for the PET, MR and MCG cardiac data fusion

In this paper, a new approach is presented for the assessment of a 3-D anatomical and functional model of the heart including structural information from magnetic resonance imaging (MRI) and functional information from positron emission tomography (PET) and magnetocardiography (MCG). The method uses model-based co-registration of MR and PET images and marker-based registration for MRI and MCG. Model-based segmentation of MR anatomical images results in an individualized 3-D biventricular model of the heart including functional parameters from PET and MCG in an easily interpretable 3-D form.

Predictive Value of Wavelet Correlation Functions of Signal-Averaged Electrocardiogram in Patients After Anterior Versus Inferior Myocardial Infarction

OBJECTIVES This study sought to evaluate the prognostic value of wavelet correlation functions of the signal-averaged electrocardiogram (ECG) for arrhythmic events in patients after myocardial infarction. BACKGROUND Wavelet transform of the signal-averaged ECG has been shown to be a nonstationary analysis technique describing the time evolution of frequency spectra throughout the QRS complex. To quantify the wavelet transform, we introduced the new concept of the wavelet correlation function. METHODS The relation among wavelet correlation functions, ventricular late potentials and the site of infarction was investigated in 769 men < 66 years old who survived the acute phase of myocardial infarction (351 [46%] anterior, 418 [54%] inferior infarctions). Signal-averaged ECG recordings were obtained 2 to 3 weeks after infarction. During 6 months of follow-up, 33 patients (4.3%) experienced a malignant arrhythmic event. Wavelet correlation functions of the signal-averaged ECG were evaluated in a time-frequency plane ranging from 25 ms before QRS onset to 25 ms after QRS offset in the frequency range between 40 and 100 Hz. RESULTS Patients with an anterior infarction had lower mean wavelet correlation coefficients (p < 0.001) and a lower incidence of ventricular late potentials than patients with an inferior infarction (32.3% vs. 42.7%, p = 0.003). The combination of wavelet correlation functions and late potentials increased the total predictive accuracy from 52% to 72% for inferior and from 64% to 76% for anterior infarctions. CONCLUSIONS Spectral changes in the signal-averaged QRS complex are more prominent in anterior than inferior infarctions. Combination of late potential analysis and wavelet correlation functions increases the prognostic value for serious arrhythmic events after myocardial infarction.

Instrumentation for the accurate measurement of phase and amplitude in optical tomography

A single-channel prototype for a frequency-domain optical tomography system is presented. The two main goals in the design of the system were the measurement of phase with minimal systematic errors and a high enough signal-to-noise ratio to detect the small changes in the absorption of brain tissue during brain activity. Although the system inherently is an imaging system, the aspects of the system that relate to multichannel operation will be published separately, as this part of the system is not yet finished. The instrument is described in detail, including the radio-frequency system, the light detection system, and the light source. Factors that affect the accuracy of the measured phase include phase drift, radio-frequency coupling between the source and detector electronics, phase-amplitude cross talk, and others. To increase the range of intensities that can be measured, the gain of the detector is adjusted while keeping the mean anode current small compared with the quiescent current through the vo...