PubMed Health⌕ Search

Biomedical subjects

R J Marks

Publications and source records attributed to R J Marks.

17 recordsLinked to original sources
PubMed2000

Hardware-sensitive optimization for intensity modulated radiotherapy.

The multileaf collimator (MLC) hardware constraints are usually neglected in the process of intensity-modulated beam optimization. Consequently, it is not always possible to deliver planned beam modulation using dynamic MLC. Beam optimization is significantly diminished if the results must be approximated due to limitations imposed by the delivery device. To overcome this problem, an inverse beam optimization method which incorporates the hardware constraints has been developed. The hardware constraints, including the leaf velocity, the dose rate and the minimum required gap between opposing and adjacent leaves, were considered. An iterative search for feasible modulation was conducted alternately in the dosimetric space and the MLC position-time space. The optimization algorithm was designed for a unidirectional leaf trajectory and a constant dose rate. A scheme to reduce tongue-and-groove underdosage during optimization was also implemented. Comparisons were made between the solutions produced by this method and conventional optimization disregarding the hardware restrictions. The beam profiles generated by the conventional method were modified to satisfy the hardware specifications. The results indicate that inclusion of MLC constraints during optimization can improve the degree of conformity that is deliverable.

Algorithms↗
PubMed1998

A spherical dose model for radiosurgery plan optimization.

Conventional 3D dose calculations for stereotactic radiosurgery involve integration of individual static beams comprising a set of arcs. For iterative optimization of multiple isocentre treatment, which requires repetitive dose calculations at a large number of sample points, the conventional method is too slow. To overcome this problem spherically symmetric dose distributions are assumed. The authors describe a spherical dose model derived from a parametrized convolution of the collimator width and a dose spread kernel. The method is fast and easy to implement requiring just a single empirically derived value. Furthermore, the model is differentiable with respect to the parameters to be optimized. This property is useful when the optimization strategies rely on gradient information.

Models, Theoretical↗
PubMed1998

Optimization of intensity modulated beams with volume constraints using two methods: cost function minimization and projections onto convex sets.

For accurate prediction of normal tissue tolerance, it is important that the volumetric information of dose distribution be considered. However, in dosimetric optimization of intensity modulated beams, the dose-volume factor is usually neglected. In this paper we describe two methods of volume-dependent optimization for intensity modulated beams such as those generated by computer-controlled multileaf collimators. The first method uses a volume sensitive penalty function in which fast simulated annealing is used for cost function minimization (CFM). The second technique is based on the theory of projections onto convex sets (POCS) in which the dose-volume constraint is replaced by a limit on integral dose. The ability of the methods to respect the dose-volume relationship was demonstrated by using a prostate example involving partial volume constraints to the bladder and the rectum. The volume sensitive penalty function used in the CFM method can be easily adopted by existing optimization programs. The convex projection method can find solutions in much shorter time with minimal user interaction.

Dose-Response Relationship, Radiation↗
PubMed1997

Conformal radiotherapy computation by the method of alternating projections onto convex sets.

Synthesis of beam profiles for a given dose prescription is a central problem in radiotherapy. Care must be taken in the beam design to expose the tumour volume at a high level, to avoid significant irradiation of critical organs, and to minimize exposure of all other tissue. Use of the synthesis procedure known as alternating projections onto convex sets (POCS) is shown to be a viable approach to beam design. POCS is a powerful tool for signal and image restoration and synthesis. Convex sets of signals obeying desired constraint sets are first specified. Then, by repeated projections onto these sets, convergence is to a signal obeying all desired constraints if the constraint sets have a finite intersection. In this paper we apply the method of POCS to conformal radiotherapy dose computation. The performance of the method is shown through three representative examples.

Algorithms↗
PubMed1993

Resolution enhancement of biomagnetic images using the method of alternating protections.

Resolution of biomagnetic images using the technique of the alternating projections is proposed. Our image reconstruction procedure is divided in two steps. First, the biomagnetic inverse problem is solved by use of the projection theorem to reconstruct an initial image of the current distribution from a given magnetic field profile. Although the current distribution thus obtained has poor resolution, it can resemble the original shape of the current distribution. The second step improves the resolution of the reconstructed image by using the method of alternating projections. The procedure assumes that images can be represented by line like elements and involves finding the line like elements based on the initial image and projecting back onto the original solution space. Simulation studies were performed on a set of parallel conductors and a shape of the conductors in the form of letters, UWB@. All conductors were of line like thickness. Restored images closely resemble the original shape of the conductors.

Humans↗
PubMed1991

Predicting outcome in intensive therapy units--a comparison of Apache II with subjective assessments.

In a prospective study 568 patients admitted to a mixed medical and surgical intensive therapy unit (ITU) were assessed using the Apache II severity of illness score to predict outcome. Their outcome was also predicted subjectively by a doctor and nurse on admission. There were 260 deaths in the group. The subjective predictions were compared with the Apache II predictions using logistic regression analysis and receiver-operating-characteristic curve measurement. The subjective assessments were found to be a more powerful predictor of outcome in this group of patients than the Apache II scores and predicted risk of death. Although the predictions could be successfully applied to the population as a whole, none of the tests were suitable for predicting outcome on an individual patient.

Decision Making↗
PubMed1986

Anaesthesia for transoral craniocervical surgery.

Transoral surgery on lesions in the craniocervical region has been performed on 25 patients in this unit. The anaesthetic management is presented. Tracheostomy and nasotracheal intubation have both been used. Peroperative monitoring of cord compression remains an important and only partly solved problem. There were two deaths as a result of anaesthesia and surgery, and these are discussed.

Adolescent↗
PubMed1985

Warming blood before transfusion. Does immersion warming change blood composition?

The practice of warming blood for transfusion by immersion in a water bath has been studied. Blood, 3 days after collection, with CPD-adenine anticoagulant, was warmed at 25 degrees C, 37 degrees C and 45 degrees C for up to 2 hours. The procedure was repeated after 2 and 4 weeks' storage at 4 degrees C. Only trivial biochemical and morphological changes resulted from warming. Within these limits, immersion warming of blood appears to be a reasonable alternative to using a blood-warming coil. Other aspects of immersion warming are discussed.

Blood Chemical Analysis↗
PubMedDate not supplied

Simulation technique for difficult intubation: teaching tool or new hazard?

This investigation evaluated the risks of a simulation drill designed to improve the skill of anesthesia personnel in dealing with an unexpected difficult intubation. In a controlled prospective study, 40 patients with normal airways scheduled to undergo noncardiothoracic surgery were randomized into two groups of 20 patients. In the control group, intubation was performed by standard techniques. In the simulation group, intubation of a difficult airway was simulated and performed with the aid of an endotracheal tube introducer. Heart rate (HR); systolic, mean, and diastolic blood pressures (BPs); and arterial oxygen saturation were measured noninvasively during the preinduction period and 1 minute postintubation. A record was kept of all adverse events, including electrocardiogram (EKG) evidence of myocardial ischemia or cardiac arrhythmias, esophageal intubation, pulmonary aspiration, or tracheal injury. There were no significant differences in percent changes in HR, BP, or oxygen saturation between the two groups. There were five uncomplicated esophageal intubations in the simulation group compared with none in the control group (p = 0.001). No other adverse events were recorded. The potential hazards of esophageal intubation should be considered before this simulation drill is performed.

Adult↗