Cecil W. Thomas

PERCEPTUAL COMPARISON OF PULSED AND CONTINUOUS FLUOROSCOPY

Pulsed fluoroscopy (hereafter called pulsed) at reduced acquisition rates, typically 15 acq/s (pulsed-15), is proposed to reduce x-ray dose in interventional procedures. However, since the human visual system (HVS) acts as a temporal low-pass filter that interacts with such acquisitions, the proper dose for pulsed must be obtained in perception experiments. We determine the dose for low-frame-rate pulsed that gives visualization equivalent to that of conventional 30 acq/s fluoroscopy, hereafter called continuous. Computer-generated phantoms are used. They consist of stationary, low-contrast disks on a flat background containing Poisson noise that mimics quantum noise in fluoroscopy. Image sequences are displayed on the video tachistoscope, a device with considerable display flexibility. Three experimental paradigms are used. (1) In a paired-comparison study, pulsed and continuous are displayed side-by-side on the same monitor, and the visibility of a contrast detail phantom is compared. (2) Using this same display, subjects record the minimally detectable disk contrast (the min-contrast measurement). (3) In a four-alternative forced-choice experiment, a disk is placed in one of four positions, and the subject determines the position of the disk. The methods are complementary--the forced-choice experiment properly eliminates the subjectivity of the observer threshold while the paired-comparison study is much more time efficient. With regard to pulsed and continuous comparisons, remarkable similarity is found between the supra-threshold experiments (1 and 2) and the detectability experiment (3); i.e., the average absolute differences in the equivalent-perception dose as determined by the three measures is approximately 3%. No difference is found between interlaced and noninterlaced display. A relatively small dependence of dose savings on disk size is found with larger disks giving increased dose savings. Average dose savings of 22%, 38%, and 49% are found for pulsed-15, pulsed-10, and pulsed-7.5, respectively.

Electrocardiographic body surface potential mapping in the Wolff-Parkinson-White syndrome. Noninvasive determination of the ventricular insertion sites of accessory atrioventricular connections.

BackgroundA reliable, noninvasive procedure to determine the location of accessory atrioventricular connections in patients with Wolff-Parkinson-White syndrome would add an important diagnostic tool to the clinical armamentarium. Methods and ResultsBody surface potential mapping (BSPM) using 180 electrodes in varioussized vests and displayed as a calibrated color map was used to determine the ventricular insertion site of the accessory atrioventricular (AV) connections in 34 patients with Wolff-Parkinson-White syndrome. Attempts were made to determine the 17 ventricular insertion sites described by Guiraudon et al. All 34 patients had an electrophysiologic study (EPS) at cardiac catheterization, and 18 had surgery so the ventricular insertion sites could be accurately located using EPS at surgery. A number of physiologic observations were also made with BSPM. ConclusionsThe following conclusions were drawn: 1) BSPM using QRS analysis accurately predicts the ventricular insertion site of accessory AV connections in the presence of a delta wave in the electrocardiogram; 2) the ventricular insertion sites of accessory AV connections determined by BSPM and by EPS at surgery were identical or within one mapping site (1.5 cm or less) in all but four of 18 cases; three of the four exceptions had more than one accessory AV connection, and the other had a very broad ventricular insertion; 3) BSPM and EPS locations of the accessory AV connections correlated very well in the 34 cases despite the fact that BSPM determines the ventricular insertion site and EPS determines the atrial insertion site of the accessory AV connection; 4) as suggested by the three cases of multiple accessory AV connections, EPS and BSPM may be complementary since BSPM identified one pathway and EPS identified the other (in the case with a broad ventricular insertion, BSPM and EPS demonstrated different portions of that insertion); 5) BSPM using ST-T analysis is very much less accurate in predicting the ventricular insertion site of accessory AV connections unless there is marked preexcitation; 6) standard electrocardiography using the Gallagher grid methodology (but with no attempt at stimulating maximal preexcitation) was not as accurate as QRS analysis ofBSPM in predicting the ventricular insertion site of the accessory AV connection; however, exact comparison is hampered by the different number and size of the Gallagher and Guiraudon insertion sites; 7) BSPM using QRS analysis appears to be very accurate in predicting right ventricular versus left ventricular posteroseptal accessory AV connections; 8) typical epicardial right ventricular breakthrough, indicative of conduction via the specialized AV conduction system, occurs in all patients with left ventricular free wall accessory AV connections; 9) epicardial right ventricular breakthrough was not observed in cases with right ventricular free wall or anteroseptal accessory AV connections; 10) epicardial right ventricular breakthrough can occur in the presence of posteroseptal accessory AV connections, whether right or left ventricular; and 11) the delay in epicardial right ventricular breakthrough in cases with left ventricular insertion may provide a marker to estimate the degree of ventricular preexcitation. (Circulation 1991;83:886–901)

Electrocardiographic body surface potential maps of the QRS of normal children

Electrocardiographic body surface potential maps (BSPM), utilizing 180 active dry electrodes imbedded in an inflatable vest, were obtained in 40 normal children, ages 8 to 18. The potential levels of the maps are displayed as different colors. A qualitative analysis indicated that the onset of right ventricular breakthrough could always be recognized in the upstroke of the QRS by a pseudopod from a right shoulder minimum extending into an anterior maximum, at an average of 24.4 +/- 4.2 msec., for an average QRS duration of 75.0 +/- 7.1 msec. However there was considerable normal variation, particularly in the mid and late QRS. At the time of depolarization of the free walls of the ventricles, the maximum often remained anterior, with an extension posterior, even through the Frank system vectorcardiogram invariably was posterior. Most remarkable was the terminal QRS of the BSPM, where the terminal maximum may be right superior anterior, anterior superior, or right posterior, presumably reflecting the right ventricular outflow tract, the superior septum, or the posterior basal left ventricle.

An adaptive reference/test paradigm: Application to pulsed fluoroscopy perception

We developed an adaptive forced-choice method whereby reference and test presentations were alternated in order to minimize effects from variables such as subject attention level. In our demonstration example of an X-ray fluoroscopy perception study, we measured detectability of low-contrast objects in noisy image sequences and determined X-ray dose levels for equivalent detectability of identical contrasts for a new test acquisition method (fluoroscopy at 15 acq/sec) as compared with a reference (conventional fluoroscopy at 30 acq/sec). In preliminary experiments, we found a dose savings with the test method. We derived parameter uncertainties for the adaptive procedure and demonstrated their applicability with Monte Carlo simulations. Repeated experiments on a single subject demonstrated reduced standard errors due to the reduction of day-to-day variations. It is believed that the method can be applied in a variety of situations in which one needs to compare perception measurements.

The Effects of Variations of the Conducting Media Inhomogeneities on the Electroretinogram

A mathematical model was developed to simulate the spatially varying ERG field at an arbitrary instant of time (e. g., the peak of the b-wave). The retina was depicted as a bioelectric source consisting of an axially symmetric double (or dipole) layer covering a little more than a hemisphere and having a spatially uniform strength. The passive volume conductor was represented in the model by electrically homogeneous regions possessing shapes and relative dimensions corresponding to the anatomy of the human eye.

Spectral analysis of the normal electrocardiogram in children and adults

We performed spectral analysis of the X-lead Frank VCG from six normal adults (Group A) and eight normal children (Group B) using a Fast Fourier Transform. The Group A magnitudes were consistently and significantly lower than Group B in the 20--90 Hz region. Similarly, the -40 dB point was 75 Hz for Group A and 107 Hz for Group B. The P waves of the two groups had similar spectra, as did their T waves. When the entire cardiac cycle was analyzed, there was again a consistent and significant difference in the frequencies from 25--80 Hz. Despite the significant differences in the Group A and B spectra, the vast majority of the information contained in the ECG signal was confined to frequencies below 100 Hz. Therefore, a bandwidth of 100 Hz is probably adequate for accurate recording of the ECG signal from children, as well as from adults.

Tracking of illusory target motion: differences between gaze and head responses.

We compared ocular and eye-head tracking responses to an illusion of diagonal motion produced when vertical movement of a small visual target was synchronized to horizontal movement of a background display. In response to sinusoidal movement, smooth ocular pursuit followed vertical target motion, with only a small horizontal component. In response to regular stepping movement, all anticipatory saccades were in the direction of the illusion; these erroneous oblique movements were followed by corrective horizontal saccades. When the head was free to move, it usually showed a diagonal trajectory that, for both sinusoidal and stepping target motion, was always in the direction of the illusion; no corrective movements were present. Thus, for our illusory stimuli, eye and head tracking showed qualitative differences that imply that ocular tracking was ultimately controlled by actual target motion but head tracking was controlled by illusory target motion.

The spectrum of right bundle branch block as manifested in electrocardiographic body surface potential maps

A wide spectrum of types of right bundle branch block (RBBB) were studied utilizing the body surface potential maps (BSPMs) of 37 children. Although the spectrum varied from very advanced RBBB to minimal partial RBBB, a common diagnostic feature was the absence of evidence for right ventricular breakthrough in the maps of all patients. Evidence for left ventricular breakthrough was usually seen, the exceptions being five patients with partial RBBB and one of 29 with advanced RBBB. The appearance of evidence for activation of the right ventricle by way of the septum was late in onset. In addition, especially in advanced RBBB, the BSPM pattern reflecting right ventricular activation was prolonged in such a manner that it appeared that utilization of right ventricular Purkinje tissue was minimal and inefficient. The BSPMs during ST-T, which were of inverse polarity, reflect repolarization that is determined by the sequence of depolarization to a greater degree than in the normal. In general, the more extensive the surgery, the more advanced the RBBB (as reflected in the BSPM), although there were exceptions. The one parameter that linked all patients with RBBB together was the absence of evidence for right ventricular epicardial breakthrough.

Age effects in coding tasks: componential analysis and test of the sensory deficit hypothesis.

Multiple forms of a symbol-digit substitution task were used to provide a componential analysis of age differences in coding task performance. The results demonstrated age differences in feature encoding, memory, and visual search. A 2nd experiment was conducted with young adults to investigate a sensory deficit as a locus of age differences. The spatial contrast sensitivity deficit of older adults was simulated on forms by applying a digital filter. Persons in the age-simulated contrast condition performed worse than those in the normal contrast condition. The stimulus degradation effect was linked to visual search speed. The study illustrates the utility of componential analysis and offers direct support for the hypothesis that sensory deficits affect performance on tasks used to assess intelligence.

Electrocardiographic body surface potential maps of the QRS and T of normal young men. Qualitative description and selected quantifications

A qualitative and quantitative analysis of the Body Surface Potential Maps (BSPM) of 40 young men, ages 19-41, is presented utilizing a 180 electrode system, with 135 anterior leads and 45 posterior leads. Evidence for epicardial right ventricular breakthrough was demonstrated in 36/40 at 27.9 +/- 6.8 ms, whereas our previous studies on normal children (average age 12.5 years) have demonstrated evidence for epicardial right ventricular breakthrough at 25.0 +/- 8.9 ms. The peak-to-peak magnitude at maximal potential (at 42.3 +/- 4.8 ms) was not significantly different from that of the children (4,430 +/- 1,165 microV), and the peak-to-peak magnitude of ST-T was virtually the same as that of the children (1,182 +/- 376.2 microV). The terminal activation pattern of late QRS on the body surface map appeared in the true posterior, anterior superior, posterior right superior and/or right anterior superior positions, in order of frequency. There were other regions appearing less frequently. In contrast, this pattern in children was seen only in the anterior superior, right anterior superior, posterior right superior, and true posterior in order of frequency. In 18/40, the body surface manifestation of repolarization was seen an average of 9.4 +/- 4.8 ms before the end of the QRS. A new pseudocolor display with 31 color levels representing body surface potentials allowed excellent resolution of isopotential detail.