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Results for “Optimization”

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PubMed2001

Doppler index and plasma level of atrial natriuretic hormone are improved by optimizing atrioventricular delay in atrioventricular block patients with implanted DDD pacemakers.

Doppler index is the sum of isovolumetric contraction time and isovolumetric relaxation time divided by ejection time and has clinical value as an index of combined systolic and diastolic myocardial performance. This crossover study compared the Doppler index and atrial natriuretic hormone (atrial natriuretic peptide) [ANP] between optimal (AV) delay and prolonged AV delay in patients with DDD pacemakers. The study included 14 patients (6 men, 8 women, age 78.4+/-9.3 [SD] years) with AV block with an implanted DDD pacemaker. AV delay was prolonged in a 25-ms, stepwise fashion starting from 125 ms to 250 ms. Pacing rate was set at 70 beats/min. Cardiac output (CO) was assessed by pulsed Doppler echocardiography, and optimal AV delay was defined as the AV delay at which CO was maximum, and an AV delay setting of 250 ms as prolonged AV delay. Plasma level of ANP and Doppler index determined by echocardiography were measured 1 week after programming. AV delay was switched to another AV delay and measurements were repeated after 1 week. Optimal AV delay was 159+/-19 ms. Doppler index was significantly lower at optimal AV delay than at prolonged AV delay (0.68+/-0.26 vs 0.92+/-0.30, P < 0.05). The plasma ANP level was significantly lower at optimal AV delay than at prolonged AV delay (29.0+/-30.7 vs 52.6+/-44.9 pg/mL, P < 0.05). In conclusion, the Doppler index and the plasma ANP level were significantly lower at optimal AV delay than at prolonged AV delay. This study shows the importance of the optimal AV delay setting in patients with an implanted DDD pacemaker, the Doppler index and plasma ANP levels are good indicators for optimizing AV delay.

Aged↗

PubMed1999

Determination of the optimal atrioventricular delay in DDD pacing. Comparison between echo and peak endocardial acceleration measurements.

The goal of this study was to compare two methods determining the optimal atrioventicular delay (AVD) in 19 patients implanted with the BEST-Living system for complete heart block. The definition of the optimal AVD was: the AVD with the echo method that provided the longest diastolic filling time without interruption of the A wave, and the AVD with the peak endocardial acceleration (PEA) method, corresponding to the knee of the PEA curve vs AV delay. The amplitude of the PEA was measured for every AVD programmed via an automatic scanner in steps of 60 to 300 ms (40 ms steps): in the VDD pacing mode with a low base rate, to obtain 100% sensed P waves; in DDD with a base rate = sinus rate + 20%, to obtain 100% paced P waves. Echocardiographic (Echo) measurement of the left ventricular filling time were performed in the same AV delay settings in VDD and DDD as the ones tested in the PEA method, which were manually programmed. The optimal AVDs obtained in DDD and those obtained in VDD were compared in the echo and the PEA tests by a paired Student's t-test. The optimal AVDs obtained by both Echo and by PEA were also compared by a paired Student's t-test in VDD and DDD. The r value of the correlation between the optimal AVDs obtained by Echo and those obtained by PEA was calculated. Similar values of optimal AVD were obtained with both methods. The optimal AVDs given by the Echo technique (179 +/- 25 ms in DDD and 124 +/- 18 ms in VDD) were slightly, but significantly shorter than the ones obtained with the PEA method (202 +/- 21 ms in DDD and 145 +/- 18 ms in VDD, P < 0.05). A highly significant difference between AVD VDD and AVD DDD was found with both methods (P < 0.001). The correlation between the AVDs obtained with the echo and the PEA methods was highly significant (r = 0.78, P < 0.01). Pacemaker software could be modified to determine automatically the optimal AVDs to be applied throughout the heart rate range.

Atrioventricular Node↗

PubMed2001

Optimal atrioventricular delay setting determined by evoked QT interval in patients with implanted stimulus-T-driven DDDR pacemakers.

Cardiac function is improved by optimizing the atrioventricular (AV) delay. An automatic optimizing function of AV delay may be necessary to achieve the most favourable haemodynamic state in paced patients. The QT interval may change when cardiac function is improved by optimizing the AV delay. The QT or stimulus-T interval is used as a sensor for rate-responsive pacemakers. Evoked (e) QT interval is measured as the time duration from the ventricular pace pulse (stimulus) and the T-sense point that is the steepest point of the intracardiac T wave (stimulus-T interval). The relationship between AV delay, eQT interval and cardiac function was studied in 10 patients (73 +/- 10 (SD) years old) with an implanted stimulus-T-driven DDDR pacemaker. Cardiac output (CO) and pulmonary capillary wedge pressure (PCWP) were measured by Swan-Ganz catheter. The AV delay was prolonged stepwise by 30 ms. Electrocardiogram event markers which indicated ventricular spike and sensed T wave were recorded, and the interval between two event markers was measured as eQT interval. When AV delay was changed from 240 ms to the AV delay at which CO was maximal (172 +/- 33 ms), eQT interval prolonged from 346 +/- 60 to 353 +/- 62 ms (P < 0.01). There was a significant positive correlation between the optimal AV delay at which CO was maximal (172 +/- 33 ms) and the optimal AV delay which was predicted from the maximum eQT interval (179 +/- 37 ms, r = 0.92, P < 0.001). When AV delay was changed from 240 ms to the predicted optimal AV delay, CO increased from 4.2 +/- 0.7 to 4.5 +/- 0.81.min-1 (P < 0.001) and PCWP was decreased from 7.1 +/- 4.0 to 5.7 +/- 3.1 mmHg (P < 0.05). In conclusion, the optimal AV delay can be predicted from the eQT interval which is sensed by an implanted pacemaker. Automatic setting of the optimal AV delay may be achieved by the QT sensor of an implanted pacemaker.

Aged↗

PubMed2001

Optimal continuous positive airway pressure in patients with obstructive sleep apnoea: role of craniofacial structure.

Although nasal continuous positive airway pressure (CPAP) is effective in improving nocturnal obstructive apnoea, daytime sleepiness and well-being in patients with obstructive sleep apnoea syndrome (OSAS), not all patients tolerate this treatment. Since optimal CPAP titration is essential to maintain compliance, it is important to elucidate the factors that help to determine the optimal pressure. However, the determinants of the optimal CPAP level are controversial. The subjects comprised 27 Japanese male patients with OSAS who underwent standard polysomnography (PSG), pulmonary function tests, arterial blood gas analysis, cephalometry and CPAP titration. Twenty normal controls also underwent cephalometric analysis. The apnoea-hypopnoea index (AHI), mean oxygen saturation (mean SaO2) and the lowest SaO2 during sleep were found to be 54.7+/-22.6, 89.0+/-5.6%, and 69.7+/-9.0%, respectively by PSG. The mean optimal CPAP was 9.6+/-1.8 cmH2O. The cephalometric angles (SNA, SNB and NSBa) were similar to those found in the control subjects. but MP-H, and PNS-P were significantly longer than those in the control subjects as shown by cephalometry. The optimal CPAP was correlated with the mean SaO2 (P<0.0001), neck circumference (P<0.05) and three cephalometric variables (NSBa: P<0.01, MP-H: P<0.05, PNS-P: P<0.05). Multiple, step-wise, regression analysis showed that the mean SaO2 and NSBa were independent variables that best predicted the optimal CPAP. These variables accounted for 57.5% of the total variance (R2=0.575, P<0.001). Optimal CPAP was closely correlated with oxygen desaturation during sleep. However, the craniofacial structure had additional effects such as an independent factor in determining the optimal CPAP level.

Adult↗

PubMed2002

Optimization of multiple spin-echo sequences for 3D polymer gel dosimetry.

The overall performance of polymer gel dosimeters for three-dimensional radiation dosimetry is determined by the temporal and spatial stability of the gels, dose sensitivity and image quality with respect to both systematic and stochastic deviations. The dose resolution (D(p)delta) is determined by the dose sensitivity and the signal-to-noise ratio (SNR) in the dose images. The dose sensitivity can be altered by changing the chemical composition of the polymer gel. The SNR is determined by the scanner and the imaging sequence. In the dose verification of conformal radiotherapy treatments the chosen number of slices may reach a number of 10-20. For these experiments, to obtain a sufficient SNR within a reasonable measurement time using a certain MR scanner, the imaging sequence should be optimized. A few other studies have emphasized the importance of optimizing the imaging sequence with respect to dose resolution (D(p)delta) or SNR but do not give quantitative values for the optimal sequence parameters for scanning a polymer gel dosimeter in three dimensions. In this paper, it is proved that a multiple spin-echo sequence is preferable to a single spin-echo sequence. It is also shown that when using a multiple spin-echo sequence it is not the inter-echo time that should be optimized but the number of echoes. An algebraical expression is derived for the dose resolution in terms of sequence parameters. A mathematical formalism and look-up tables are provided that can be used to optimize both a single and a slice-selective multiple spin-echo sequence to acquire a set of dose images at various locations. The use of the optimization protocol is illustrated by some examples. The optimization protocol enables the user to derive the optimal sequence parameters to acquire a set of dose maps obtained by quantitative T2 imaging for each polymer gel dosimeter within the shortest time possible.

Acrylic Resins↗

PubMed2001

Constrained global optimization for estimating molecular structure from atomic distances.

Finding optimal three-dimensional molecular configurations based on a limited amount of experimental and/or theoretical data requires efficient nonlinear optimization algorithms. Optimization methods must be able to find atomic configurations that are close to the absolute, or global, minimum error and also satisfy known physical constraints such as minimum separation distances between atoms (based on van der Waals interactions). The most difficult obstacles in these types of problems are that 1) using a limited amount of input data leads to many possible local optima and 2) introducing physical constraints, such as minimum separation distances, helps to limit the search space but often makes convergence to a global minimum more difficult. We introduce a constrained global optimization algorithm that is robust and efficient in yielding near-optimal three-dimensional configurations that are guaranteed to satisfy known separation constraints. The algorithm uses an atom-based approach that reduces the dimensionality and allows for tractable enforcement of constraints while maintaining good global convergence properties. We evaluate the new optimization algorithm using synthetic data from the yeast phenylalanine tRNA and several proteins, all with known crystal structure taken from the Protein Data Bank. We compare the results to commonly applied optimization methods, such as distance geometry, simulated annealing, continuation, and smoothing. We show that compared to other optimization approaches, our algorithm is able combine sparse input data with physical constraints in an efficient manner to yield structures with lower root mean squared deviation.

Algorithms↗

PubMed2000

Which factors determine the optimal pedaling rate in sprint cycling?

INTRODUCTION: Mechanical power output in sprint cycling depends on pedaling rate, with an optimum at around 130 revolutions per minute (rpm). In this study, the question is addressed if this optimal pedaling rate can be understood from a Hill-type description of muscular dynamics. In particular, it is investigated how 1) the power-velocity relationship that follows from Hill's force-velocity relationship and 2) activation dynamics (from the perspective of which the optimal pedaling rate is near-zero) affect the optimal pedaling rate. METHODS: A forward dynamics modeling/simulation approach is adopted in this study. The skeletal model is a 2D linkage of rigid segments; it is actuated by eight Hill-type "muscles." Input of the model is the neural stimulation of the muscles, output is the resulting movement and variables dependent thereupon, such as pedal forces. For a wide range of isokinetic pedaling rates, the neural stimulation is optimized with respect to the average mechanical power output. RESULTS: Correspondence between experimental data and simulation results regarding 1) the (pedaling-rate dependent) muscle phasing, 2) pedal forces, and 3) the power-pedaling rate relationship is good. At the optimal pedaling rate predicted by the model (120 rpm), muscles contract at velocities well below those that maximize their power output. Finally, when a model is considered that lacks activation dynamics, it is found that both the optimal pedaling rate and the maximal power output increase substantially. DISCUSSION: From the results pertaining to the standard model, it is concluded that the optimal pedaling rate is not uniquely specified by the power-velocity relationship of muscle, as suggested in literature. From the results pertaining to the model lacking activation dynamics, it follows that activation dynamics plays a surprisingly large role in determining the optimal pedaling rate. It is concluded that the pedaling rate that maximizes mechanical power output in sprint cycling follows from the interaction between activation dynamics and Hill's power-velocity relationship.

Bicycling↗

PubMed1997

Electrical defibrillation optimization: an automated, iterative parallel finite-element approach.

To date, optimization of electrode systems for electrical defibrillation has been limited to hand-selected electrode configurations. In this paper we present an automated approach which combines detailed, three-dimensional (3-D) finite-element torso models with optimization techniques to provide a flexible analysis and design tool for electrical defibrillation optimization. Specifically, a parallel direct search (PDS) optimization technique is used with a representative objective function to find an electrode configuration which corresponds to the satisfaction of a postulated defibrillation criterion with a minimum amount of power and a low possibility of myocardium damage. For adequate representation of the thoracic inhomogeneities, 3-D finite-element torso models are used in the objective function computations. The CPU-intensive finite-element calculations required for the objective function evaluation have been implemented on a message-passing parallel computer in order to complete the optimization calculations in a timely manner. To illustrate the optimization procedure, it has been applied to a representative electrode configuration for transmyocardial defibrillation, namely the subcutaneous patch-right ventricular catheter (SP-RVC) system. Sensitivity of the optimal solutions to various tissue conductivities has been studied. Results for the optimization of defibrillation systems are presented which demonstrate the feasibility of the approach.

Algorithms↗

PubMed2001

Optimization of wide-band linear arrays.

An optimization method is proposed for linear arrays to be used in ultrasound systems under wide-band operation. A fast algorithm, the threshold accepting, has been utilized to determine the element positions and weight coefficients of a linear array that generates a desired beam pattern. To reduce the computational burden in the optimization procedure, an efficient numerical routine for the beam pattern evaluation has been implemented. We address the optimization problem of both dense and sparse wide-band arrays. In the first case, the goal is to minimize the side-lobe energy by varying the element weights; we compare the optimized beam pattern with that obtained with classical shading functions, showing that better results can be achieved with a wide-band optimization. We also consider the optimization of the layout (positions and weights) of a sparse linear array to achieve a desired beam pattern with a fixed or minimum number of array elements. The comparison of the proposed method with a narrow-band optimization algorithm is presented, showing that better performances (about -7 dB further reduction of the side-lobe level) can be achieved with a wide-band sparse array optimization. Further numerical simulations are given, showing that the proposed method yields better results than wide-band sparse random arrays and periodic arrays with the same aperture width.

Journal Article↗

PubMed2000

A dose-volume histogram based optimization algorithm for ultrasound guided prostate implants.

The task of treatment planning for prostate implants is to find an optimal seed configuration, comprising the target coverage and dosimetric consideration of critical structures such as the rectum and urethra. An efficient method to accomplish this is to use an inverse planning technique that derives the optimized solution from a prescribed treatment goal. The goal can be specified in the voxel domain as the desired doses to the voxels of the target and critical structures, or in the dose volume representation as the desired dose volume histograms (DVHs) of the target and critical structures. The DVH based optimization has been successfully used in plan optimization for intensity-modulated radiation therapy (IMRT) but little attention has been paid to its application in prostate implants. Clinically, it has long been known that some normal structure tolerances are more accurately assessed by volumetric information. Dose-volume histograms are also widely used for plan evaluation. When working in the DVH domain for optimization one has more control over the final DVHs. We have constructed an objective function sensitive to the DVHs of the target and critical structures. The objective function is minimized using an iterative algorithm, starting from a randomly selected initial seed configuration. At each iteration step, a trial position is given to a randomly selected source and the trial position is accepted if the objective function is decreased. To avoid being trapped in a less optimal local minimum, the optimization process is repeated. The final plan is selected from a pool of optimized plans obtained from a series of randomized initial seed configurations.

Algorithms↗

PubMedDate not supplied

Optimized dynamic rotation with wedges.

Dynamic rotation is a computer-controlled therapy technique utilizing an automated multileaf collimator in which the radiation beam shape changes dynamically as the treatment machine rotates about the patient so that at each instant the beam shape matches the projected shape of the target volume. In simple dynamic rotation, the dose rate remains constant during rotation. For optimized dynamic rotation, the dose rate is varied as a function of gantry angle. Optimum dose rate at each gantry angle is computed by linear programming. Wedges can be included in the optimized dynamic rotation therapy by using additional rotations. Simple and optimized dynamic rotation treatment plans, with and without wedges, for a pancreatic tumor have been compared using optimization cost function values, normal tissue complication probabilities, and positive difference statistic values. For planning purposes, a continuous rotation is approximated by static beams at a number of gantry angles equally spaced about the patient. In theory, the quality of optimized treatment planning solutions should improve as the number of static beams increases. The addition of wedges should further improve dose distributions. For the case studied, no significant improvements were seen for more than 36 beam angles. Open and wedged optimized dynamic rotations were better than simple dynamic rotation, but wedged optimized dynamic rotation showed no definitive improvement over open beam optimized dynamic rotation.

Humans↗

PubMed1988

Matching between feline left ventricle and arterial load: optimal external power or efficiency.

We tested the hypothesis that the feline left ventricle normally works at optimal external power as opposed to optimal efficiency by (re)analyzing data from five isolated, blood-perfused cat hearts and 39 open-thorax cats. In the isolated hearts, we measured pump function, external steady power, myocardial oxygen consumption, and efficiency. Optimal external power and optimal efficiency were found at different left ventricular outputs (6.94 +/- 0.33 and 8.35 +/- 0.37 ml/s, respectively; P less than 0.001). In the in situ cat hearts the working point was found at an output of 4.72 +/- 0.32 ml/s, whereas optimal external power was found at 4.84 +/- 0.26 ml/s. These values were not significantly different. Assuming that the point of optimal efficiency was located at the same fraction of the maximal unloaded left ventricular output (Fmax) as in the isolated hearts, i.e., 0.7, we found the point of optimal efficiency for the in situ heart at a flow of 5.83 +/- 0.32 ml/s, which was significantly different (P less than 0.001) from the flow in the working point. Our data therefore indicate that the left ventricle in the open-thorax cat is matched to the arterial load such that its external power output rather than efficiency is optimized.

Algorithms↗

PubMed1993

Signal-averaged electrocardiogram. Improved identification of patients with ventricular tachycardia using a 28-lead optimal array.

BACKGROUND: Although the signal-averaged ECG (SAECG) is currently the best noninvasive test to identify patients with ventricular tachycardia (VT) following myocardial infarction (MI), it is still a relatively insensitive test. Body surface mapping has improved the sensitivity of ECG in detecting various cardiac diseases. This study applied body surface mapping to the SAECG in the form of a clinically practical, 28-lead optimal array and compared its sensitivity and specificity with those of an orthogonal array. METHODS AND RESULTS: Two hundred twenty-three patients with previous MI (82 with inducible VT) underwent SAECG using 28 surface electrodes from which were obtained a three-lead orthogonal array and a 28-lead optimal array (optimal). From the orthogonal array, two QRS durations (QRSd) were obtained using the combined vector magnitude method (CVM) and the earliest onset to latest offset of the three individually filtered leads (individual). From the optimal array, 28 QRSd were obtained, each defined as the duration from the earliest onset of any of the 28 leads to the offset of each individually filtered lead. QRSd > 120 msec in > or = 3 leads was considered abnormal. For CVM and individual, QRSd of > 120 msec were considered abnormal. While the specificity of each method was comparable (84%, 86%, and 84% for CVM, individual, and optimal, respectively), the sensitivity of optimal (70%) was significantly greater than the sensitivity of CVM (54%) (p = 0.001) or individual (59%) (p = 0.004). The magnitude of improvement in sensitivity, 16% and 15%, respectively, was equal for anterior (n = 120) and inferior (n = 103) infarctions. CONCLUSIONS: Body surface mapping using the 28-lead optimal array significantly improved the sensitivity of the SAECG without loss of specificity. The increased sensitivity was of equal magnitude for inferior and anterior infarctions. The superiority and practicality of the 28-lead optimal array make it worth pursuing as an option for further refinement in SAECG:

Aged↗

PubMedDate not supplied

Evaluation of nonlinear optimization for scheduling of follow-up cystoscopies to detect recurrent bladder cancer. The Bladder Cancer follow-up Group.

Standard recommendations for patients who have had superficial bladder cancer are inspection by cystoscopy quarterly for a year or two after tumor removal, then half-yearly and yearly. The authors assessed the potential for improvement in scheduling cystoscopies according to probabilistic optimization techniques. Eight hypothetical practices were created, based on retrospective analysis of 918 bladder-cancer-patient charts. Standard and alternative recommendations for the interval to next cystoscopy were compared. The alternatives were derived from patient-specific predictions of future tumor risks (based on the patient's prior recurrence rate and tumor stage and grade) and a nonlinear optimization approach to allocation of the same number of cystoscopies as were available for standard follow-up. The optimization proposed longer intervals between visits for low-risk patients and shorter intervals for high-risk patients. Overall, optimization reduced expected tumor detection delays by 30%, from 12.6 to 8.7 weeks. When optimization intervals were shorter than standard, cancer was found more often at subsequent cystoscopies (34% vs 27%, p less than 0.05), suggesting that the optimization was a better predictor of cancer recurrence. If reduction in tumor-detection delay is the goal of follow-up for recurrent cancers, then urologists can improve monitoring by using probabilistic optimization methods for scheduling cystoscopies. Further understanding of the accuracy of predictive models for bladder-cancer recurrence rates is desirable. Subsequently, the optimization method developed here may be tested prospectively.

Aftercare↗

PubMed2001

Optimal wave-front reconstruction strategies for multiconjugate adaptive optics.

We propose an optimal approach for the phase reconstruction in a large field of view (FOV) for multiconjugate adaptive optics. This optimal approach is based on a minimum-mean-square-error estimator that minimizes the mean residual phase variance in the FOV of interest. It accounts for the C2n profile in order to optimally estimate the correction wave front to be applied to each deformable mirror (DM). This optimal approach also accounts for the fact that the number of DMs will always be smaller than the number of turbulent layers, since the C2n profile is a continuous function of the altitude h. Links between this optimal approach and a tomographic reconstruction of the turbulence volume are established. In particular, it is shown that the optimal approach consists of a full tomographic reconstruction of the turbulence volume followed by a projection onto the DMs accounting for the considered FOV of interest. The case where the turbulent layers are assumed to match the mirror positions [model-approximation (MA) approach], which might be a crude approximation, is also considered for comparison. This MA approach will rely on the notion of equivalent turbulent layers. A comparison between the optimal and MA approaches is proposed. It is shown that the optimal approach provides very good performance even with a small number of DMs (typically, one or two). For instance, good Strehl ratios (greater than 20%) are obtained for a 4-m telescope on a 150-arc sec x 150-arc sec FOV by using only three guide stars and two DMs.

Journal Article↗

PubMed1999

Geometric optimization of a tissue pattern for semilunar valve reconstruction.

BACKGROUND AND AIM OF THE STUDY: A novel geometric trefoil pattern has been suggested for semilunar valve reconstruction. Optimization of the geometry must rely on an appreciation of normal anatomy and knowledge of the mechanical properties of the tissue used for the reconstruction. METHODS: Computer-assisted design (CAD) was used to create an optimized leaflet geometry based on published dimensions for normal human aortic valves. The optimized leaflet geometry was subjected to finite element analysis (FEA) to study stress distribution with pressure loading of the leaflet. In vitro function of the optimized trefoil tissue pattern is being studied by static testing initially, with physiological saline. RESULTS: An optimized leaflet geometry has been developed by CAD, and further refined by FEA. Static testing of the optimized trefoil tissue pattern shows near-normal anatomy, with no prolapse or pin wheeling, and full valve competence to 90 mmHg pressure. CONCLUSIONS: An initial optimized geometry has been developed for a two-dimensional tissue pattern that can be used to reconstruct diseased semilunar heart valves with human pericardium. Optimization studies are based on the mechanical properties of the tissue, CAD to mimic normal anatomy, FEA to study stress distribution, and static load testing to confirm function.

Animals↗

PubMed2000

Optimizing replacement decisions for Finnish dairy herds.

The purposes of the study were to determine how "an optimal herd" would be structured with respect to its calving pattern, average herdlife and calving interval, and to evaluate how sensitive the optimal solution was to changes in input prices, which reflected the situation in Finland in 1998. The study used Finnish input values in an optimization model developed for dairy cow insemination and replacement decisions. The objective of the optimization model was to maximize the expected net present value from present and replacement cows over a given decision horizon. In the optimal solution, the average net revenues per cow were highest in December and lowest in July, due to seasonal milk pricing. Based on the expected net present value of a replacement heifer over the decision horizon, calving in September was optimal. In the optimal solution, an average calving interval was 363 days and average herdlife after first calving was 48.2 months (i.e., approximately 4 complete lactations). However, there was a marked seasonal variation in the length of a calving interval (it being longest in spring and early summer) that can be explained by the goal of having more cows calving in the fall. This, in turn, was due to seasonal milk pricing and higher production in the fall. In the optimal solution, total replacement percentage was 26, with the highest frequency of voluntary culling occurring at the end of the year. Seasonal patterns in calving and replacement frequencies by calendar month and variation in calving interval length or herdlife did not change meaningfully (< 1%-2% change in the output variables) with changes in calf, carcass or feed prices. When the price of a replacement heifer decreased, average herdlife was shorter and replacement percentage increased. When the price increased, the effect was the opposite.

Animal Feed↗

PubMed2002

Radiation dose and image quality in diagnostic radiology. Optimization of the dose-image quality relationship with clinical experience from scoliosis radiography, coronary intervention and a flat-panel digital detector.

UNLABELLED: X-rays are known to cause malignancies, skin damage and other side effects and they are thus potentially dangerous. Therefore, it is essential and in fact mandatory to reduce the radiation dose in diagnostic radiology as far as possible. This is also known as the ALARA (as low as reasonably achievable) principle. However, the dose is linked to image quality and the image quality may not be lowered so far that it jeopardizes the diagnostic outcome of a radiographic procedure. The process of reaching this balance between dose and image quality is called optimization. The aim of this thesis was to propose and evaluate methods for optimizing the radiation dose-image quality relationship in diagnostic radiography with a focus on clinical usefulness. The work was performed in three main parts. OPTIMIZATION OF SCOLIOSIS RADIOGRAPHY: In the first part, two recently developed methods for digital scoliosis radiography (digital exposure and pulse fluoroscopy) were evaluated and compared to the standard screen-film method. Radiation dose was measured as kerma area-product (KAP), entrance surface dose (ESD) and effective dose; image quality was assessed with a contrast-detail phantom and through visual grading analysis. Accuracy in angle measurements was also evaluated. The radiation dose for digital exposure was nearly twice as high as the screen-film method at a comparable image quality while the dose for pulsed fluoroscopy was very low but with a considerably lower image quality. The variability in angle measurements was sufficiently low for all methods. Then, the digital exposure protocol was optimized to a considerably lower dose with a slightly lower image quality compared to the baseline. FLAT-PANEL DETECTOR: In the second part, an amorphous-silicon direct digital flat-panel detector was evaluated using a contrast-detail phantom, measuring dose as entrance dose. The flat-panel detector yielded a superior image quality at a lower dose than both storage phosphor plates and screen-film. Equivalent image quality compared to storage phosphor plates was reached at about one-third of the dose. OPTIMIZATION OF PERCUTANEOUS CORONARY INTERVENTION (PCI): In the third part, influence of various settings on radiation dose and image quality in coronary catheterisation and PCI was investigated. Based on these findings, the dose rate for fluoroscopy was reduced to one-third. The dose reduction was evaluated in a clinical series of 154 PCI procedures before and 138 after the optimization. Through this optimization, the total KAP was significantly reduced to two-thirds of the original value. IN SUMMARY: This thesis indicates the possibility of dose reduction in diagnostic radiology through optimization of the radiographic process.

Coronary Angiography↗