Estimation of organ and effective doses resulting from cone beam CT imaging for radiotherapy treatment planning.

In this study, organ doses were measured for various kilovoltage cone beam CT exposures on the Varian Acuity simulator and an alternative method of dose estimation was also assessed. Organ doses were measured by distributing thermoluminescent dosimeters (TLDs) throughout an anthropomorphic phantom, and effective doses were calculated using International Commission on Radiological Protection (ICRP) 60 and ICRP 103 tissue-weighting factors. The ImPACT CT patient dosimetry calculator was also used to estimate doses for comparison with the TLD results. Effective doses of 15.3 mSv (19.4 mSv), 14.3 mSv (9.7 mSv) and 2.8 mSv (3.2 mSv) were calculated from the TLD measurements and ICRP 60 (ICRP 103) weighting factors for breast, pelvis and head acquisitions, respectively. When a 10 cm pencil ionisation chamber was used to measure the CT dose index, the ImPACT calculator was found to provide an adequate estimation of dose when compared with the TLD results. However, the doses for half-fan exposures were found to be overestimated, with the extent of overestimation depending on the radiosensitive organs irradiated. The organ and effective doses reported provide information for justification and optimisation of cone beam CT procedures, and are compared with doses delivered by other imaging devices. The ImPACT calculator may be used to estimate doses from cone beam CT procedures, if the potential for overestimation is acknowledged.

Comparison of the acoustic streaming in amniotic fluid and water in medical ultrasonic beams.

AIM: Acoustic streaming in amniotic fluid has been investigated under a variety of conditions relevant to the diagnostic use of ultrasound. METHOD: An ultrasonic Doppler method has been used for measurement. Streaming velocities have been compared with those generated in water for the same exposure conditions. Beams were generated by laboratory equipment simulating beams from clinical systems. The fluids were insonated IN VITRO using 3.5 MHz, 5 MHz and 7.5 MHz transducers in continuous wave (CW) and pulsed mode. RESULTS: Acoustic streaming was measured in both amniotic fluid and water at the power levels 50 mW and 140 mW. Enhancement of velocities due to non-linear effects in high amplitude pulses was demonstrated for amniotic fluid as well as for water. The potential and limitations of present numeric methods for the prediction of acoustic streaming were explored. CONCLUSION: Pulsed ultrasound caused similar streaming velocities in amniotic fluid and water while continuous wave beams induced significantly faster streaming in amniotic fluid than in water.

Measurement of acoustic streaming using magnetic resonance.

Magnetic resonance imaging (MRI) has been used to explore acoustic streaming caused in water under ultrasonic exposure conditions similar to those used for diagnostic applications. Streaming was established in an enclosed tube with acoustically transparent end windows, using a pulsed, weakly-focused transducer of acoustic frequency 3.5 MHz. Phase-detection MRI was used to image and quantify streaming profiles in the region of the acoustic focus. Acoustic powers in the range 0.4 mW to 100 mW were used. The sensitivity of the technique enabled streaming velocities down to 0. 1 mm s(-1) to be measured, generated by acoustic power less than 1 mW. In addition, acoustic streaming generated within open meshes with minimum pore dimensions of 3.0 mm and 2.0 mm was measured. The flow velocity in the coarser mesh reached 0.9 mm s(-1) at 95 mW total acoustic power. These observations demonstrate that acoustic streaming is probably a much more general phenomenon in diagnostic ultrasound (ultrasound) than previously recognised. The combination of magnetic resonance and ultrasound shows promise as a diagnostic method for the differentiation of cystic lesions in vivo, and for their characterisation, with sensitivity significantly greater than using ultrasound alone.

Radiation protection of the ovaries in young scoliosis patients.

Concerns in clinical practice arose over the amount of ovarian irradiation received from X-ray examinations in females with scoliosis. This study was instigated to assess the adequacy of ovarian protection in this young and genetically vulnerable group of patients. A total of 283 plain films in 20 patients with scoliosis were reviewed. If the area immediately adjacent to the medial wall of the acetabulum was clearly seen, then this was taken as indicative of ovarian irradiation. In a separate study, the radiation dose in the centre of the X-ray field on the surface of a tissue-equivalent anthropomorphic phantom was measured using thermoluminescent dosimeters. Standard conditions for scoliosis X-ray examination were used. The average age of patients was 21.5 years. The mean number of single X-ray exposures per patient was 14.1 over a mean of 44 months. The mean measured entrance dose to the skin in the 20 patients was 0.08 mGy (equivalent dose = 0.08 mSv). The mean percentage of examinations without lead protection was 18% per patient (range 0-40%). This would have resulted in a mean equivalent dose to the surface of the abdomen of 0.1 mSv per year per patient from the unprotected examinations. The maximum dose received in 1 year was 0.6 mSv. The maximum dose to the unprotected ovary was estimated to be 0.05 mSv from a single examination. The mean total cumulative ovarian dose was calculated as 180 microSv per patient (range 45-355 microSv) over the time period studied. The findings of this study indicate that ovarian protection should be improved. Reasons for this and suggestions for improvement are discussed.

Studies of acoustic streaming in biological fluids with an ultrasound Doppler technique.

Acoustic streaming generated by diagnostic ultrasound fields is an important area for study both for safety reasons and because of its potential application as a diagnostic tool. A method of investigating streaming in biological fluids is reported. A number of fluids were insonated using a 3.5 MHz weakly focused single element transducer which was driven in pulsed mode. Streaming was detected in each fluid using an 8 MHz continuous wave Doppler system. The maximum streaming velocity was obtained by spectral analysis of the Doppler signal. Using this system longitudinal streaming profiles were measured. At an acoustic power of 150 mW the maximum streaming velocities detected were: 9.3 cm s-1 in water, 6.8 cm s-1 in 4.5% human serum albumin (HSA) solution and 4.9 cm s-1 in blood, when transmission was through a water path of approximately 10 cm into a 3 cm sample of fluid. When measurements were made in the biological fluids alone, without a water path, the maximum streaming velocities were reduced.

Film viewing conditions in mammography.

A requirement for a minimum viewing box brightness of 3000 cd m-2 for reading mammograms has been widely advocated. Some recent work has challenged that opinion by reporting no significant variation in visibility of low contrast and fine detail objects over a wide range of brightness levels. This paper provides further experimental evidence to support the latter conclusion, at least over the range 1340-4190 cd m-2, and suggests that the currently recommended minimum viewing box brightness levels need to be revised. The importance of reducing room lighting levels is fully confirmed.

A study of the heating capabilities of diagnostic ultrasound beams.

A simple device for the experimental study of the heating capabilities of diagnostic ultrasound beams is described. Some results are reported that demonstrate the manner in which the device may be used to explore the heating potential of any particular commercial transducer, operating over the full range of output conditions. The heat generated in the base of a polyethylene container, filled with water, was measured using a fine-wire thermocouple, attached externally. The majority of measurements were carried out in beams generated by a curved array operating with a modern commercial scanner (Doppler, 2.5 MHz: imaging 3 MHz). A temperature rise in excess of 30 degrees C was generated by a pulsed Doppler beam, when the water path and scanner controls were set appropriately. Comparable temperatures were measured at comparable intensities generated by Doppler beams of other scanners. Of the imaging beams studied, the greatest temperature rise observed was less than 2 degrees C, when the highest frame rate and line density were selected. The greatest temperature rise in colour Doppler mode was 7.8 degrees C. It was observed that the position of the fixed (nonelectronic) focus was significant in controlling the heating profile with depth, for scanned beams. As expected, there was a strong dependence of temperature rise on axial time-average intensity. A weak dependence on -6 dB beam area was observed over a range of beam area of about 7 to 70 mm2. A strong dependence on finite amplitude effects was observed, resulting from energy loss associated with acoustic shock propagation.

A comparison of ultrasound exposure in therapy and pulsed Doppler fields.

A detailed comparison of the ultrasound exposure in water from a therapeutic beam and a pulsed Doppler beam was carried out. A significant overlap in acoustic power was found between therapy intensity levels used clinically and the upper end of the diagnostic range, between approximately 100 mW and 200 mW. In addition, pulse pressure amplitudes in the range 0.5-1.0 MPa were measured close to the transducer on both units. It is common to use physiotherapy equipment at pulse average intensities of 0.5 W/cm2 or less, and at these levels exposures of similar magnitude may be obtained with beams currently defined as therapeutic and those available from pulsed Doppler equipment.

Forces acting in the direction of propagation in pulsed ultrasound fields.

This paper considers some non-thermal effects resulting from absorption of acoustic energy from an ultrasound beam. An experimental investigation of the location of the 'source pump', responsible for the generation of streaming in high amplitude diagnostic fields in water, is reported. Acoustically transparent membranes were inserted in the ultrasound field in order to restrict the streaming volume. It is shown that the major contribution to an acoustic stream is generated in the region near to the focus of a transducer where the intensity in the beam and the degree of non-linear distortion are both high. In the second part of the paper a simple model of non-linear propagation is used to predict the magnitude of the maximum pressure gradient induced in a medium by the absorption of acoustic energy from a beam. Propagation in water, in tissue and in amniotic fluid are considered. Within the limitations of this model it is shown that the pressure gradients induced in pulsed acoustic fields do not result in the ultimate shear stress of tissue being exceeded.

Surface heating of diagnostic ultrasound transducers.

Surface temperatures of a variety of transducers used with common commercial ultrasonic diagnostic equipment have been measured. Transducers operating in imaging mode, in both continuous and pulsed Doppler modes, and in mixed modes were investigated. A total of 30 transducers and scan-heads used with equipment from 10 manufacturers were examined, including a range of array types, mechanical sectors and continuous-wave Doppler transducers. Measurements were made using an infrared radiometer, or a thermocouple probe, with the transducers operating in air. Surface temperatures of 13 transducers operating in imaging mode were found to be in the range 0.0-13.1 degrees C above ambient after 5 min operation. Some transducers operating in pulsed Doppler mode reached considerably higher temperatures. The most extreme example increased the surface temperature by 36.5 degrees C after 1 min and reached a steady-state temperature of almost 80 degrees C. Transducers operating at these temperatures cannot be retained on the skin of a conscious subject without pain, and will cause skin burns within a brief period of time. A linear relationship has been demonstrated between temperature increase and spatial-average acoustic intensity. The rate of increase in air was found to be about 10 times greater for pulsed arrays than for continuous-wave Doppler transducers.

An experimental investigation of streaming in pulsed diagnostic ultrasound beams.

Streaming is shown to occur in water in the focused beams produced by a number of medical pulse-echo devices. The use of hot film anemometry to measure the streaming velocity is described and velocities measured in water using commercial equipment are quoted. The highest velocities occur in pulsed Doppler mode with a maximum velocity of 14 cm s-1 being observed. An experimental set-up was used to investigate the parameters affecting streaming and it was found that the harmonic content of the pulse waveform had a major effect on the streaming velocity. The time taken for a stream to become established at the focus of the acoustic beams studied was typically approximately 0.5 s.

A survey of the acoustic output of ultrasonic Doppler equipment.

Measurements of the acoustic output generated by a variety of clinical ultrasonic Doppler instruments have been carried out. The instruments surveyed include continuous wave Doppler units for cardiovascular investigations, fetal monitors, stand-alone pulsed Doppler equipment and 'duplex' scanners working in Doppler mode. Acoustic measurements have been made using a calibrated PVDF membrane hydrophone, and a milliwatt radiation force balance. Almost all the pulsed Doppler and duplex systems investigated could generate spatial-peak, temporal-average intensities in water which exceeded 100 mW cm-2, with a maximum of 825 mW cm-2 measured. These intensities were also reached by some continuous-wave Doppler systems, particularly those incorporated into duplex scanning systems. Fetal monitoring equipment was found to operate typically at lower intensities. Doppler units have been found to show a very wide variation in pulse length, repetition frequency and spatial peak pressure, and a wider range of pulse average intensities than previously reported. Units were found to vary considerably in their control of total acoustic power whilst operating parameters such as gate width and range were altered.

The development of harmonic distortion in pulsed finite-amplitude ultrasound passing through liver.

The progressive development of finite-amplitude distortion of ultrasonic pulses has been investigated in excised bovine liver using pulsed focused ultrasonic beams at nominal frequencies of 2.5 and 3.5 MHz. Both the transducers and the powers used were those which may be encountered with clinical imaging equipment. Significant distortion of the waveform was observed to occur, particularly at higher powers. For example, at 2.5 MHz, with a mean input pressure (p0) of 0.58 MPa, the second harmonic in the pulse spectrum showed a maximum value of 10.5 dB below the fundamental and the highest third harmonic component was 19 dB below the fundamental. These particular observations illustrate that finite-amplitude distortion may be of considerable significance in the transmission through tissue of ultrasonic pulses during diagnostic scanning.

The locations of peak pressures and peak intensities in finite amplitude beams from a pulsed focused transducer.

The effect of finite-amplitude distortion on the positions of peak pressures and peak intensities in beams generated in water by pulsed focused transducers has been investigated experimentally. The pulses generated by three single-element, circular, focused transducers with nominal frequencies of 2.25, 3.5, and 5.0 MHz have been investigated with pressures at the transducer P0 being varied over the range 10 kPa to 1.4 MPa. Measurements were made using a 9 micron thick polyvinylidene difluoride membrane hydrophone. In all cases the locations of peak pressures were not stationary as the field strength was altered. As P0 increased, the distance from the transducer to the positive peak initially increased, and then decreased. The distance to the negative, or decompression peak decreased monotonically with increasing P0. The location of peak pulse intensity integral was found to alter slightly with power, taking a position between the peak positive and negative pressures.

The output of pulse-echo ultrasound equipment: a survey of powers, pressures and intensities.

A survey of the powers, pressures and intensities generated by ultrasonic pulse-echo equipment in clinical use has been carried out. Three conventional B-scanners, four linear-array scanners and four mechanically sectored scanners were included in the study. Measurements were made on a total of 22 transducers covering the nominal frequency range 2.25-7.5 MHz. On those instruments where an output power control was provided, two measurements were made: one at the maximum available power and a second at a lower power. On arrays with a variable transmit focus control, measurements were made at all available focus settings. In all, measurements were made on 38 separate focused pulsed ultrasonic fields. The measurements were carried out using a calibrated ultrasonic force balance, and a calibrated polyvinylidene difluoride (PVdF) membrane hydrophone. A very wide range of maximum powers, pressures and intensities were found. Powers from 0.5-80 mW were measured; spatial-average temporal-peak positive pressures at the transducer varied between 30 kPa and 1.15 MPa, and spatial-peak pulse-average intensities were in the range 3.6 X 10(3)-1.1 X 10(7) Wm-2.

Evidence for ultrasonic finite-amplitude distortion in muscle using medical equipment.

Finite-amplitude distortion of ultrasonic waves from medical equipment has been observed to occur following transmission through calf muscle in human volunteers. Measurements were made using both dynamic pulse-echo imaging equipment and physiotherapy equipment. In both cases irradiation was carried out under operating conditions commonly used clinically. Pressure waveforms were measured at the skin surface using a broadband polyvinylidene difluoride membrane hydrophone. Using a pulsed, weakly focused 2.5-MHz beam with input peak pressure of 0.8 MPa and a pressure gain of 5.3 at the focus, the mean second harmonic peak magnitude (16 measurements) was 17 dB below the fundamental peak. A 1.1-MHz continuous wave therapy set with input peak pressure of 0.5 MPa showed mean second harmonic magnitude 23 dB below the fundamental.

Acoustic shock generation by ultrasonic imaging equipment.

The pulses generated by ultrasonic imaging equipment have been observed to form acoustic shocks in water within a range of a few centimetres under normal operating conditions. The commonly held view of pulse propagation from ultrasonic imaging equipment is that the acoustic pulse has the form of a damped sine wave which will project largely unchanged in waveform. Any waveform changes which do occur result from diffraction effects and from the scattering and attenuation properties of tissue. The theory on which this understanding is based assumes that propagation laws are linear. This paper presents experimental evidence that this assumption is quite invalid at the pressures generated by commercial pulse-echo imaging equipment in common use. Measurements in water of the pulse waveforms using a calibrated broad-band polymer hydrophone have demonstrated that pulse distortion and shock formation commonly occur due to the inherent non-linearity of the propagation medium. This fact must be considered during the calibration of pulse-echo equipment. In addition, the conditions under which shock formation might occur during normal clinical procedures should be reviewed and any associated biological effects assessed.