Table of contents

A review of electronic portal imaging devices (EPIDs) used in external beam, megavoltage radiation therapy is presented. The review consists of a brief introduction to the definition, role and clinical significance of portal imaging, along with a discussion of radiotherapy film systems and the motivations for EPIDs. This is followed by a summary of the challenges and constraints inherent to portal imaging along with a concise, historical review of the technologies that have been explored and developed. The paper then examines, in greater depth, the two first-generation technologies that have found widespread clinical use starting from the late 1980s. This is followed by a broad overview of the physics, operation, properties and advantages of active matrix, flat-panel, megavoltage imagers, presently being commercially introduced to clinical environments or expected to be introduced in the future. Finally, a survey of contemporary research efforts focused on improving portal imaging performance by addressing various weaknesses in existing commercial systems is presented.

Motexafin lutetium (MLu) is a second-generation photosensitizer for photodynamic therapy (PDT) of cancer. We have developed and applied a diffuse optical reflectance spectrometer for in vivo measurement of MLu uptake, optical properties, haemoglobin concentration and haemoglobin oxygen saturation in normal canine large bowels, kidneys and prostates.

The probe consists of a broadband fibre-optic-coupled light source and detector fibres placed at various distances from the source fibre to collect reflected light. An analysis based on the diffusion approximation of the photon transport equation was used to recover tissue optical properties from the reflectance measurements. The instrumentation and analysis methods were validated using measurements from homogeneous, highly scattering phantoms with known MLu concentrations. The same techniques were then used to estimate chromophore concentrations of normal canine large bowels, kidneys and prostates.

We estimated (mean (standard deviation)) total haemoglobin concentrations of 119 (25), 340 (92) and 51 (11) μM in the large bowels, kidneys and prostates of four dogs, respectively; tissue blood oxygen saturations in these same organs were 75 (15), 76 (21) and 74 (16) per cent, respectively. Tissue MLu concentrations (mg l⁻¹) were estimated from data taken 3.5 h after injection of a 2 mg kg⁻¹ injected dose; data from three dogs gave concentrations of 2.4 (0.4) in large bowels, 6.8 (1.3) in kidneys and 2.2 (1.1) in prostates. The reduced scattering coefficients, μ_s', estimated for large bowels, kidneys and prostates at 730 nm were, respectively: 10.1 (1.3), 19.6 (4.0) and 12.7 (0.6) cm⁻¹.

We observed significant variability in MLu uptake, tissue scattering and haemoglobin concentration between organs and even between the same organ in different dogs. This class of in situ optical property measurement may be desirable to individualize PDT drug and light delivery.

Since the company MDS Nordion Haan GmbH introduced the new afterloading radiation unit GammaMed plus™ in 1998, new HDR Ir-192 sources with a diameter of 0.9 mm (formerly 1.1 mm) and a length of 4.5 mm (formerly 5.5 mm) have been used. With this equipment it is possible to treat peripheral vessels for prophylaxis of restenosis after percutaneous transluminal angioplasty (PTA). In this case a good knowledge of the dose characteristics in the near area is the prerequisite for safe application.

In this study, the dose characteristics for this source type is investigated at close range by means of an ionization chamber. For the description of the dose characteristics, the radial dose distribution and the anisotropy function are used. The measurements were carried out using an ionization chamber, the so-called PinPoint chamber (Type PTW 31006, PTW Freiburg, Germany). The radial dose distribution was examined in the area of 0.26 to 12 cm. The anisotropy function was examined in the range from 0.5 to 5 cm (distance from the effective site of measurement to the source axis).

The radial dose function was compared with the correction function k_as,w(r), published in the German standard DIN 6809 part 2 (1991 Beuth Verlag, Berlin, Germany). In the area up to 4 cm (distance source-axis to measuring point) the deviation maximized up to 2%. The deviation climbs with greater distance from the source with results around 3%.

The anisotropy function was compared with that used in the treatment planning system Abacus 3.0™ of the same manufacturer. In the close range between 0.5 and 1 cm, there were some major variations (up to 10%) from the anisotropy function used in the planning system. The fault reduces itself with greater distance to under 2%.

In view of the results presented in this paper further measurements of the anisotropy function in the close range from 0 to 1.5 cm from the source axis should be made. It can be seen that the function used in the planning system supplies results that are too high in comparison with the results of other authors. This concerns primarily the area of very small and very big angles.

Radiation therapy in the thoracic region is difficult due to the presence of many dose-limiting structures and the large density differences that affect the dose distribution. Conventional irradiation techniques use low-energy photon beams to avoid build-up effects superficially in the tumour and increased lateral scattering of the beams. For deep-seated tumours higher beam energies could have lung-sparing properties that would enable dose escalation.

A comparison was made for a conventional low photon energy (6 MV) and 50 MV photons for the treatment of a lung tumour. A representative patient geometry was selected, consisting of a small tumour semi-enclosed in lung tissue. Treatment plans were designed using a commercial 3D-pencil beam treatment planning system. The treatment beams designed in the TPS were simulated with the Monte Carlo code EGS4/BEAM and the dose distribution in the phantom created from the patients CT-data was calculated using MCDOSE with identical beam geometry for both energies.

The intrinsic difference between the two photon energies implies a sparing effect of lung that can be utilized for dose escalation. For a treatment with two beams the mean total dose to the tumour could be increased by 5.3% for 50 MV, corresponding to 3.2 Gy for a prescription dose of 60 Gy, with the same complication probability for the treated lung as for 6 MV.

In conclusion, high-energy beams have qualities that can be taken advantage of for irradiation of lung tumours. Optimum solutions would probably require the use of both high- and low-energy beams.

An intensity-modulated beam optimization algorithm is presented which incorporates the delivery constraints into the optimization cycle. The optimization algorithm is based on the quasi-Newton method of iteratively solving minimization problems. The developed algorithm iteratively corrects the incident, pencil-beam-like, fluence to incorporate the delivery constraints. In the present study, the goal of the optimization algorithm is to achieve the best deliverable radiotherapy plan, subject to the constraints of the delivery technique described by a leaf-sequencing algorithm being applied concurrently. In general, if they are applied after, rather than during, the optimization cycle, the delivery constraints associated with the IMRT technique can produce local variations up to 6% in the 'optimized' dose (i.e., distribution without applied constraints) and reduce the degree of conformity, of the dose, to the PTV region.

The optimization method has been applied to three IMRT delivery techniques: dynamic multileaf (DMLC), multiple-static-field (MSF) and slice-by-slice tomotherapy (NOMOS MIMiC). The beam profiles were generated for a prostate tumour with organs at risk being the rectum, bladder and femoral heads. The optimization method described was shown to generate optimum and deliverable IMRT plans for these three delivery techniques. In the case of the DMLC and MSF the optimization converged within 3–5 iterations to a mean PTV dose of 69.60 ± 1.34 Gy and 69.71 ± 1.34 Gy, respectively, while for NOMOS MIMiC approximately 10 iterations were needed to obtain 69.68 ± 1.55 Gy. In addition to this, the IMRT optimization also yielded optimum fluence profiles when clustering was performed concurrently with the leaf-sequencer. An optimum between 8 and 15 clusters of equal fluence 'intensity' was shown to establish the best compromise between the number of fluence levels and the PTV dose coverage.

A simulation model of mammographic x-ray sources with finite size has been developed. The model is based on Monte Carlo methods and it takes into account the electron penetration inside the anode, the anode geometry and material, as well as the resulting heel effect and the spectral and spatial distribution of x-rays. This x-ray source simulation model has been embedded into an earlier developed simulation package of a mammography unit. The main outputs of this model are Monte Carlo generated images that correspond to the irradiation of properly designed phantoms. In this way it is possible to make studies of the influence of x-ray source characteristics on MTF. This paper presents the development of the mammographic x-ray source model, accompanied by a set of simulation studies concerning the influence of magnification effects as well as that of the x-ray spatial and spectral distribution on the mammographic spatial resolution for a certain magnification factor (m = 1.4). The validity level of the model, as well as its limitations and perspectives, rise through comparisons with experimental and theoretical data.

From the standpoint of quality assurance in radiotherapy, it is very important to compare the dose distributions realized by an irradiation system with the distribution planned by a treatment planning system. To compare the two dose distributions, it is necessary to convert the dose distributions on CT images to distributions in a water phantom or convert the measured dose distributions to distributions on CT images. Especially in heavy-ion radiotherapy, it is reasonable to show the biologically equivalent dose distribution on the CT images. We developed tools for the visualization and comparison of these distributions in order to check the therapeutic beam for each patient at the National Institute of Radiological Sciences (NIRS). To estimate the distribution in a patient, the dose is derived from the measurement by mapping it on a CT-image. Fitting the depth–dose curve to the calculated SOBP curve also gives biologically equivalent dose distributions in the case of a carbon beam. Once calculated, dose distribution information can be easily handled to make a comparison with the planned distribution and display it on a grey-scale CT-image.

Quantitative comparisons of dose distributions can be made with anatomical information, which also gives a verification of the irradiation system in a very straightforward way.

Persistence of the video signal between TV frames, an effect also known as image lag, can lead to anomalously good contrast–detail test results for fluoroscopy systems. In this practical paper, a simple method is described which quantifies lag in fluoroscopy systems and corrects for its effect on threshold contrast. A digital framestore was used to acquire temporally contiguous fluoroscopy images. Correlation of the variance between an initial base TV frame and successive later frames was then measured via the correlation coefficient. Plotted against time, this function defines a time constant which characterizes the rate at which the initial variance pattern is replaced by incoming quantum noise. A survey of seven fluoroscopy units incorporating vacuum TV camera tubes found a mean time constant of 0.06 s.

The relative change in contrast–detail performance was then measured as a function of applied digital frame averaging for two separate fluoroscopy units. A time constant was found for each frame averaging mode using the correlation of variance between frames. These measurements were used to derive a function which corrects contrast–detail results obtained for a unit with a measured nominal time constant to the typical vacuum camera tube time constant of 0.06 s. The correction is shown to significantly reduce the spread of contrast–detail results obtained over a range of temporal filtration settings.

We study the iterative solution of dense linear systems that arise from boundary element discretizations of the electrostatic integral equation in magnetoencephalography (MEG). We show that modern iterative methods can be used to decrease the total computation time by avoiding the time-consuming computation of the LU decomposition of the coefficient matrix. More importantly, the modern iterative methods make it possible to avoid the explicit formation of the coefficient matrix which is needed when a large number of unknowns are used. To study the convergence of iterative solvers we examine the eigenvalue distributions of the coefficient matrices. For the sphere we show how the eigenvalues of the integral operator are approximated by the eigenvalues of the coefficient matrix when the collocation and Galerkin methods are used as discretization methods. The collocation method approximates the eigenvalues of the integral operator directly. The Galerkin method produces a coefficient matrix that needs to be preconditioned in order to maintain optimal convergence speed. With the ILU(0) preconditioner iterative methods converge fast and independent of the number of discretization points for both the collocation and Galerkin approaches. The preconditioner has no significant effect on the total computational time.

The conventional rationale that uses per cent diameter reduction to assess diffuse coronary artery disease is not appropriate because no normal reference segments exist. In a recent publication, we have proposed a theoretical model based on physical principles that relate the various morphological and haemodynamic parameters (cross-sectional area, length, volume and flow) of the normal coronary arterial tree. The model was validated using haemodynamic simulations based on detailed morphological data of the pig coronary arterial tree. This paper extends the model validation to in vivo swine studies. Coronary arteriography was performed in five swine (15–18 kg body weight) after power injection of contrast material into the coronary artery. Coronary arterial length was obtained using a 3D reconstruction technique. The arterial volume, cross-sectional area and blood flow were measured using videodensitometry. The proposed relationships between these quantities were validated. Furthermore, a sensitivity analysis was demonstrated based on a simulation of diffuse coronary artery disease (approximately 40% reduction in cross-sectional area). The results of a sensitivity analysis based on a simulation of diffuse coronary artery disease suggest that the relationships between arterial volume, cross-sectional area, blood flow and the distal arterial length can be utilized to quantify moderate levels of diffuse coronary artery disease.

The longitudinal (R₁) and transverse (R₂) relaxivities of the clinically used contrast agents Gd(DTPA)²⁻, Gd(DOTA)⁻ and Gd(DTPA-BMA) have been determined in mixed aqueous metabolite solutions for choline, creatine and N-acetylaspartate. Measurements were performed at 1.5 T using a STEAM sequence on 25 mM metabolite solutions at pH = 7.4 and 22 °C. The data showed that for all the contrast agents and metabolites, R₁ ∼ R₂. The largest range of relaxivity values was found for Gd(DTPA)²⁻, where R₂ = 6.8 ± 0.3 mM⁻¹ s⁻¹ for choline and 1.5 ± 0.4 mM⁻¹ s⁻¹ for N-acetylaspartate. Variation in relaxivity values was attributed primarily to differences between the charges of the paramagnetic agent and metabolite. The maximum potential influence of the contrast agents on in vivo metabolite signals was calculated using the measured relaxivities.

In this note we describe and evaluate the performance of a novel approach to information recovery that involves consecutive projection onto convex sets (POCS). The method is applied to a time series of medical image data and the results are compared to images reconstructed using the standard POCS reconstruction method. The consecutive POCS method converges in a desired step-wise manner producing reconstructed images of superior quality compared to the standard scheme and can speed up the reconstruction process. The proposed method is of value for many finite sampling imaging problems including, in particular, fast-scan magnetic resonance imaging applications.