Table of contents

This review of different principles used in x-ray computerized tomography (CT) starts with attenuation (transmission) CT. The pros and cons of different geometrical solutions, single-ray, fan-beam and cone-beam, are discussed. Attenuation CT measures the spatial distribution of the linear attenuation coefficient, . The contributions of different interaction processes to have also been used for CT. Fluorescence CT is based on measurements of the contribution, , from an element Z with concentration , to the linear attenuation coefficient. Diffraction CT measures the differential coherent cross section , Compton CT the incoherent scatter cross section . The usefulness of these modalities is illustrated. CT methods based on secondary photons have a competitor in selected volume tomography. These two tomography methods are compared. A proposal to perform Compton profile tomography is also discussed, as is the promising method of phase-contrast x-ray CT.

In the past two decades, the water calorimetry technique for determination of absorbed dose to water in several types of radiation beams has moved significantly closer to being a recognized method. In this paper we summarize the constructional details of a C sealed water calorimeter currently in operation at the University of Gent. This sealed water (SW) calorimeter is of the Domen type and has been improved in several aspects compared with its original design. The relevant correction factors for heat transport and for field perturbation are described. Using relative response measurements in , we experimentally verified the relative heat defect for two distinct chemical systems, using two different detection vessel arrangements. The overall uncertainty on the absorbed dose to water at based on this system amounts to 0.7%.

Dose to water determination in clinical high-energy photon beams with ionization chambers calibrated in terms of absorbed dose to water has been proposed as an alternative to ionization chamber dosimetry based on air kerma calibrations. in the clinical beam is derived using a factor that scales the absorbed dose calibration factor in the reference beam to the absorbed dose calibration factor in the user beam.

In the present study values were determined for the NE2571 chamber in a 5 MV and a 10 MV high-energy photon beam generated at the 15 MeV high-intensity electron linac of the University of Gent. A set of three NE2571 chambers was calibrated relative to the Gent sealed water calorimeter both in and in the linac beam at a depth of 5 cm and a source to detector distance of 100 cm. Two high-purity chemical water systems were used in the detection vessel of the calorimeter, -saturated and Ar-saturated pure water, which are both supposed to give a zero heat defect. and %dd(10) have been evaluated as beam quality specifiers. Simulations using the BEAM/DOSXYZ Monte Carlo system were performed to evaluate potential corrections on the measured beam qualities.

The average values measured for the three NE2571 chambers in the 5 MV and 10 MV photon beams are and respectively. For the three chambers used, the maximum deviation of individual values is 0.2%. The measured beam quality specifiers %dd(10) and are 67.0 and 0.705 for the 5 MV beam and 75.0 and 0.759 for the 10 MV beam. Although our beam design is very different from those used by other investigators for the measurement of values, the agreement with their results is satisfactory showing a slightly better agreement when %dd(10) is used as the beam quality specifier.

SPECT projections are contaminated by scatter, resulting in reduced image contrast and quantitative errors. When tissue is present behind the source, some of the detected photons backscatter via this tissue. Particularly in dual-isotope SPECT and in combined emission-transmission SPECT, backscatter constitutes a major part of the down-scatter contamination in lower-energy windows.

In this paper, the effects of backscatter material were investigated. Planar images of and line sources between varying numbers of Perspex slabs were analysed using the photopeak windows and various scatter windows.

In the photopeak window no significant change in total counts due to backscatter material was measured. In the photopeak window an increase of about 10% in total counts was observed. In the scatter windows an even more explicit influence of backscatter material was measured. For instance, at a forward depth of 10 cm, total counts of a source detected in the 72 keV window eventually doubled with increasing backscatter material, compared with the situation without backscatter material. The backscatter contribution plateaued when more than 5-10 cm of scatter material was placed behind the source.

In conclusion, backscatter should be taken into account, particularly in model-based down-scatter correction methods in dual-isotope SPECT and combined emission-transmission SPECT.

Ultrasonic contrast agents are used to enhance backscatter from blood and thus aid in delineating blood from surrounding tissue. However, behaviour of contrast agents in an acoustic field is nonlinear and leads to harmonic components in the backscattered signal. Various research groups have investigated second-harmonic emissions. In this work, the subharmonic emission from contrast agents is investigated with a view towards potential use in imaging. It is shown that the microbubbles with various surface properties, such as contrast agents, generate significant subharmonics under various insonating conditions. Theoretical results as well as experimental results using Optison indicate the generation of strong subharmonics with burst insonation at twice the resonant frequency of the microbubble. It is suggested that subharmonic imaging may provide a better modality than second-harmonic imaging to delineate blood from tissue and will be of significant importance for imaging deep vessels, such as in echocardiography and vascular diseases, due to the high signal-to-clutter ratio of the subharmonic imaging.

The use of primates for examining the effects of electromagnetic radiation on behavioural patterns is well established. Rats have also been used for this purpose. However, the monkey is of greater interest as its physiological make-up is somewhat closer to that of the human. Since the behavioural effects are likely to occur at lower field strengths for resonant absorption conditions for the head and neck, the need for determination of resonance frequencies for this region is obvious. Numerical techniques are ideal for the prediction of coupling to each of the organs, and accurate anatomically based models can be used to pinpoint the conditions for maximum absorption in the head in order to focus the experiments. In this paper we use two models, one of a human male and the other of a rhesus monkey, and find the mass-averaged power absorption spectra for both. The frequencies at which highest absorption (i.e. resonance) occurs in both the whole body and the head and neck region are determined. The results from these two models are compared for both E-polarization and k-polarization, and are shown to obey basic electromagnetic scaling principles.

Understanding the limitations of Monte Carlo codes is essential in order to avoid systematic errors in simulations, and to suggest further improvement of the codes. MCNP and EGS4, Monte Carlo codes commonly used in medical physics, were compared and evaluated against electron depth dose data and experimental backscatter results obtained using clinical radiotherapy beams. Different physical models and algorithms used in the codes give significantly different depth dose curves and electron backscattering factors. The default version of MCNP calculates electron depth dose curves which are too penetrating. The MCNP results agree better with experiment if the ITS-style energy-indexing algorithm is used. EGS4 underpredicts electron backscattering for high- Z materials. The results slightly improve if optimal PRESTA-I parameters are used. MCNP simulates backscattering well even for high- Z materials. To conclude the comparison, a timing study was performed. EGS4 is generally faster than MCNP and use of a large number of scoring voxels dramatically slows down the MCNP calculation. However, use of a large number of geometry voxels in MCNP only slightly affects the speed of the calculation.

Hydrogen kerma factors and their uncertainties are deduced on an experimental basis, starting from our previously measured differential cross sections completed with available data from the literature, in the incident neutron energy range 25 to 75 MeV. The deduced experimental kerma factors are compared with theoretical predictions. A simple to use parametrization of the hydrogen kerma factor values in the incident neutron energy range 0.3 to 100 MeV is also proposed.

This study tests a multiwavelength frequency-domain near-infrared oximeter (fdNIRS) in an in vitro model of the human brain. The model is a solid plastic structure containing a vascular network perfused with blood in which haemoglobin oxygen saturation was measured by co-oximetry, providing a standard for comparison. Plastic shells of varying thickness (0.5-2 cm), with a vascular network of their own and encircling the brain model, were also added to simulate extracranial tissues of the infant, child and adult. The fdNIRS oximeter utilizes frequency-domain technology to monitor phaseshifts at 754 nm, 785 nm and 816 nm relative to a 780 nm reference to derive through photon transport and Beer-Lambert equations. We found a linear relationship between fdNIRS and co-oximetry with excellent correlation that fitted the line of identity in all experiments ( n = 7). The bias of fdNIRS oximetry was -2% and the precision was 6%. Blood temperature and fdNIRS source-detector distance did not affect fdNIRS oximetry. Low haemoglobin concentration altered the fdNIRS versus co-oximetry line slope and intercept, producing a 15% error at the extremes of . The infant- and child-like shells overlying the brain model did not alter fdNIRS oximetry, whereas the adult-like shell yielded an error as high as 32%. In conclusion, fdNIRS accurately measures in an in vitro brain model, although low haemoglobin concentration and extracranial tissue of adult thickness influence accuracy.

Holography with high energy x-rays is now feasible due to the coherence properties of third generation synchrotron sources. Simple in-line holographic techniques can be used to generate edge-enhanced images which for many samples can be interpreted without direct phase retrieval. The coherence properties of such sources and their exploitation for phase-contrast microimaging are demonstrated. The technique can easily be combined with computed microtomography (CMT) data collection and reconstruction strategies for three-dimensional imaging. A dramatically improved image contrast, as compared with absorption CMT, was obtained when imaging a wet human coronary artery specimen. In the tomograms, previously invisible detail could be visualized with absorbed doses below the level where radiation damage impedes the imaging. The results indicate the considerable potential of the in-line holographic CMT method in three-dimensional biomedical microscopy.

In this study we analysed the accuracy of computed tomography (CT) measurements in assessing cortical bone. We determined the dependency of thickness and density measurements on the true width and density of the cortex and on the spatial resolution in the CT images using two optimized segmentation methods. As a secondary goal, we assessed the ability of CT to reflect small changes in cortical thickness.

Two different bone-mimicking phantoms with varying cortical thickness were scanned with single-slice CT on a Somatom Plus 4 scanner. Images were reconstructed with both a standard and a high-resolution convolution kernel. Two special operator-independent segmentation methods were used to automatically detect the edges of the cortical shell. We measured cortical thickness and density and compared the phantom measurements with theoretical computations by simulating a cross-sectional shape of the cortical shell. Based on the simulations, we calculated CT's power to detect small changes in cortical thickness.

Simulations and phantom measurements were in very good agreement. Cortical thickness could be measured with an error of less than 10% if the true thickness was larger than 0.9 (0.7) mm for the standard (high-resolution) kernel which is close to the full width at half maximum (FWHM) of the point spread functions for these kernels and our scanner. Density measurements yielded errors of less than 10% for true cortical thickness values above two to three times the FWHM corresponding to 2.5 (2) mm in our case. The simulations showed that a 10% change in cortical width would not be detected with satisfying probability in bones with a cortical shell thinner than 1.2 mm.

An accurate determination of the cortical thickness is limited to bones with a thickness higher than the FWHM of the scanner's point spread function. Therefore, the use of a high-resolution reconstruction kernel is crucial. Cortical bone mineral density can only be measured accurately in bones two to three times thicker than this number. In thinner bones, the measured density becomes dependent on the thickness. Changes in cortical thickness can only be assessed if the change is rather large or if the measured bone has sufficient thickness. Therefore, assessing density or thickness of the vertebral shell by CT should be treated with caution.

Computation of physiologically relevant kinetic parameters from dynamic PET or SPECT imaging requires knowledge of the blood input function. This work is concerned with developing methods to accurately estimate these kinetic parameters blindly; that is, without use of a directly measured blood input function. Instead, only measurements of the output functions - the tissue time-activity curves - are used. The blind estimation method employed here minimizes a set of cross-relation equations, from which the blood term has been factored out, to determine compartmental model parameters. The method was tested with simulated data appropriate for dynamic SPECT cardiac perfusion imaging with Tc-teboroxime and for dynamic PET cerebral blood flow imaging with O water. The simulations did not model the tomographic process. Noise levels typical of the respective modalities were employed. From three to eight different regions were simulated, each with different time-activity curves. The time-activity curve (24 or 70 time points) for each region was simulated with a compartment model. The simulation used a biexponential blood input function and washin rates between 0.2 and and washout rates between 0.2 and . The system of equations was solved numerically and included constraints to bound the range of possible solutions. From the cardiac simulations, washin was determined to within a scale factor of the true washin parameters with less than 6% bias and 12% variability. Tc-teboroxime washout results had less than 5% bias, but variability ranged from 14% to 43%. The cerebral blood flow washin parameters were determined with less than 5% bias and 4% variability. The washout parameters were determined with less than 4% bias, but had 15-30% variability. Since washin is often the parameter of most use in clinical studies, the blind estimation approach may eliminate the current necessity of measuring the input function when performing certain dynamic studies.

Developments in positron emission tomography (PET) technology have resulted in systems with finer detector elements designed to further improve spatial resolution. However, there is a limit to what extent reducing detector element size will improve spatial resolution in PET. The spatial resolution of PET imaging is limited by several other factors, such as annihilation photon non-collinearity, positron range, off-axis detector penetration, detector Compton scatter, undersampling of the signal in the linear or angular directions for the image reconstruction process, and patient motion. The overall spatial resolution of the systems is a convolution of these components. Of these other factors that contribute to resolution broadening, perhaps the most uncertain, poorly understood, and, for certain isotopes, the most dominant effect is from positron range. To study this latter effect we have developed a Monte Carlo simulation code that models positron trajectories and calculates the distribution of the end point coordinates in water for the most common PET isotopes used: F, N, C and O. In this work we present some results from these positron trajectory studies and calculate what effect positron range has on the overall PET system spatial resolution, and how this influences the choice of PET system design parameters such as detector element size and system diameter. We found that the fundamental PET system spatial resolution limit set from detector size, photon non-collinearity and positron range alone varied from nearly 1 mm FWHM (2 mm FWTM) for a 10-20 cm diameter system typical for animal studies with F to roughly 4 mm FWHM (7 mm FWTM) for an 80 cm diameter system typical for human imaging using O.

The basic principles of a non-contact, near-infrared technique for the mapping of layered tissues are discussed theoretically and verified experimentally. The propagation properties of diffuse photon-density waves in tissues depend on the optical properties of the tissue. When a layered medium is irradiated by amplitude modulated light, the difference in optical properties between the layers is evident in the phase and amplitude of the diffuse reflection coefficient, which is a result of the interference of the partial waves propagating in the different layers. Thus, diffuse photon-density waves are applicable to the analysis of the structure of layered tissue. The probing depth is determined by the modulation frequency of the incident light. For modulation frequencies between several hundred megahertz and a few gigahertz, this allows us to analyse the properties of muscle tissue of up to 4-8 mm below the surface. Experimental results based on chicken breast muscle are given. As an example, the technique might be of use for evaluating the depth of necrosis and the blood volume fraction in deep burns.

We present a SQUID susceptometer with a non-homogeneous magnetizing field which is null at the sensing coil and increases towards the patient position with a constant gradient plus a cubic term at large distances. Compared with the magnetizing fields of similar instruments described in the literature, our gradient field enhances the signal due to internal organs with respect to the signal due to superficial tissue. Preliminary measurements have been performed on phantoms of known magnetic susceptibility. The advantage of using a non-homogeneous field compared with a uniform field has been investigated in the case of a double-layer phantom.

The lack of well established dosimetry protocols for HDR sources is a point of great concern regarding the uniformity of procedures within a particular country and worldwide. The main objective of this paper is to report the results from ten institutions of an intercomparison of calibration procedures for HDR sources currently in use in Brazil.

The treatment irradiator of one institution was calibrated by a reference system and used by all participants with their own measuring electrometers and ionization chambers under the same experimental conditions. Two methods were used: the calibration jig and the well-type ionization chamber. Each participant was allowed to use their own method and formalism.

The results of this exercise were very positive since this was the first time in Brazil that a group of users gathered to share their experience and openly discuss the physical concepts behind the calibration procedures. The results were all within %, except one case where -4.6% was observed and later identified as a problem with the value for x-rays.

Though the magnitude of the deviations found was generally acceptable considering the diversity of formalisms currently in use, a proposal is now being prepared to be adopted as a national protocol.

The identification of the institutions was left out for the sake of confidentiality.

The aim of the study was to develop and test a dynamic phantom simulating radionuclide renography. The phantom consisted of five partly lead covered plastic containers simulating kidneys, heart, bladder and background (soft tissues, liver and spleen). Dynamics were performed with multiple movable steel plates between containers and a gamma camera. Control of the plates is performed manually with a stopwatch following exact time schedules. The containers were filled with activities (Tc) which produce count rates close to clinical situations. Count rates produced by the phantom were compared with ten clinical renography cases: five Tc MAG3 and five Tc DTPA examinations. Two phantom simulations were repeated three times with separate fillings, acquisitions and analyses. Precision errors as a coefficient of variation (CV) of repeated measurements were calculated and theoretical values were compared with the corresponding measured ones. A multicentre comparison was made between 19 nuclear medicine laboratories and three clinical cases were simulated with the phantom.

Correlations between count rates produced by the phantom and clinical studies were r = 0.964 for Tc MAG3 ( p < 0.001) and 0.961 for Tc DTPA ( p < 0.001). The precision error was % and the percentage difference between theoretical and measured values for was %. Images and curves of the scanned phantom were close to a real patient in all 19 laboratories but calculated parameters varied: the difference between theoretical and measured values for was %. The difference between laboratories is most probably due to variations in acquisition protocols and analysis programs: 19 laboratories with 18 different protocols and 8 different programs.

The dynamics were found to be repeatable and suitable for calibration purposes for radionuclide renography programs and protocols as well as for multicentre comparisons.

.

Fundamental to achieving the desired clinical outcome in radiotherapy are accurate and traceable dosimetry calibrations. In radiotherapy departments, clinical dosimetry for both electron and photon treatments is reliant upon accurate measurements performed using ionization chambers. These instruments have been the mainstay of clinical dosimetry for decades and require calibration using a traceable chain of dosimeters. Calibration is generally based upon absorbed dose to water.

Critical to this traceable dosimetry chain is the development of an accurate and reproducible standard for absorbed dose to water, by the national primary standards laboratory. National primary standards laboratories across the world have adopted different approaches to the development of an absorbed dose to water standard. Fricke dosimeters and graphite calorimeters have been used in this context.

Several complex scientific problems confront standards laboratories in the development of an absorbed dose to water standard based upon calorimetry. These include establishing conversion factors for the calorimeter and the methods of heat transfer internally within the calorimeter.

In this issue of Physics in Medicine and Biology there are two papers, both originating from the Department of Biomedical Physics at the University of Gent, Belgium, which make significant contributions to the field of calorimetry.

The first paper, by Seuntjens and Palmans entitled `Correction factors and performance of a 4 °C sealed water calorimeter', describes the construction of an improved sealed water calorimeter of the Domen type. The paper describes the relevant correction factors for both heat transport and field perturbation. Various aspects of the paper are of particular interest:

• The studies of heat loss in the calorimeter in relation to excess and profile heat loss.

• Establishing a correction factor to make an allowance for the radiation field.

• The overall 1σ uncertainty was 0.7% on the absorbed dose to water (⁶⁰Co).

The second paper is entitled `Absorbed dose beam quality correction factors k_Q for the NE2571 chamber in a 5 MV and a 10 MV photon beam' and was written by Palmans, Mondelaers and Thierens. k_Q is a correction factor for beam quality dependence and references the absorbed dose to water calibration factor in a reference beam to the absorbed dose to water calibration factor in the user's beam quality. Of particular note in the paper are:

• The quality of the experimental study.

• k_Q values were 0.995 and 0.979 for 5 MV and 10 MV photon beams from a linear accelerator. These values are an average for three chambers.

• The maximum deviation on individual values of k_Q was 0.2%.

• k_Q values can be applied to beams with the same beam quality parameters.

These two studies, when taken together, represent a significant progress in the development of primary dosimetry standards based on calorimetry. In turn, they will contribute to improved clinical dosimetry.

References

Seuntjens J and Palmans H 1999 Correction factors and performance of a 4 °C sealed water calorimeter Phys. Med. Biol. 44 627-46

Palmans H, Mondelaers W and Thierens H 1999 Absorbed dose beam quality correction factors k_Q for the NE2571 chamber in a 5 MV and a 10 MV photon beam Phys. Med. Biol. 44 647-63

K Faulkner Quality Assurance Reference Centre Newcastle General Hospital Westgate Road Newcastle upon Tyne NE4 6BE UK