Practical issues and limitations of brain attenuation correction on a simultaneous PET-MR scanner

BACKGROUND

Despite the advent of clinical PET-MR imaging for routine use in 2011 and the development of several methods to address the problem of attenuation correction, some challenges remain. We have identified and investigated several issues that might affect the reliability and accuracy of current attenuation correction methods when these are implemented for clinical and research studies of the brain. These are (1) the accuracy of converting CT Hounsfield units, obtained from an independently acquired CT scan, to 511 keV linear attenuation coefficients; (2) the effect of padding used in the MR head coil; (3) the presence of close-packed hair; (4) the effect of headphones. For each of these, we have examined the effect on reconstructed PET images and evaluated practical mitigating measures.

RESULTS

Our major findings were (1) for both Siemens and GE PET-MR systems, CT data from either a Siemens or a GE PET-CT scanner may be used, provided the conversion to 511 keV μ-map is performed by the PET-MR vendor's own method, as implemented on their PET-CT scanner; (2) the effect of the head coil pads is minimal; (3) the effect of dense hair in the field of view is marked (> 10% error in reconstructed PET images); and (4) using headphones and not including them in the attenuation map causes significant errors in reconstructed PET images, but the risk of scanning without them may be acceptable following sound level measurements.

CONCLUSIONS

It is important that the limitations of attenuation correction in PET-MR are considered when designing research and clinical PET-MR protocols in order to enable accurate quantification of brain PET scans. Whilst the effect of pads is not significant, dense hair, the use of headphones and the use of an independently acquired CT-scan can all lead to non-negligible effects on PET quantification. Although seemingly trivial, these effects add complications to setting up protocols for clinical and research PET-MR studies that do not occur with PET-CT. In the absence of more sophisticated PET-MR brain attenuation correction, the effect of all of the issues above can be minimised if the pragmatic approaches presented in this work are followed.

Specific recommendations for accurate and direct use of PET-CT in PET guided radiotherapy for head and neck sites

PURPOSE

To provide specific experience-based guidance and recommendations for centers wishing to develop, validate, and implement an accurate and efficient process for directly using positron emission tomography-computed tomography (PET-CT) for the radiotherapy planning of head and neck cancer patients.

METHODS

A PET-CT system was modified with hard-top couch, external lasers and radiotherapy immobilization and indexing devices and was subject to a commissioning and quality assurance program. PET-CT imaging protocols were developed specifically for radiotherapy planning and the image quality and pathway tested using phantoms and five patients recruited into an in-house study. Security and accuracy of data transfer was tested throughout the whole data pathway. The patient pathway was fully established and tested ready for implementation in a PET-guided dose-escalation trial for head and neck cancer patients.

RESULTS

Couch deflection was greater than for departmental CT simulator machines. An area of high attenuation in the couch generated image artifacts and adjustments were made accordingly. Using newly developed protocols CT image quality was suitable to maintain delineation and treatment accuracy. Upon transfer of data to the treatment planning system a half pixel offset between PET and CT was observed and corrected. By taking this into account, PET to CT alignment accuracy was maintained below 1 mm in all systems in the data pathway. Transfer of structures delineated in the PET fusion software to the radiotherapy treatment planning system was validated.

CONCLUSIONS

A method to perform direct PET-guided radiotherapy planning was successfully validated and specific recommendations were developed to assist other centers. Of major concern is ensuring that the quality of PET and CT data is appropriate for radiotherapy treatment planning and on-treatment verification. Couch movements can be compromised, bore-size can be a limitation for certain immobilization techniques, laser positioning may affect setup accuracy and couch deflection may be greater than scanners dedicated to radiotherapy. The full set of departmental commissioning and routine quality assurance tests applied to radiotherapy CT simulators must be carried out on the PET-CT scanner. CT image quality must be optimized for radiotherapy planning whilst understanding that the appearance will differ between scanners and may affect delineation. PET-CT quality assurance schedules will need to be added to and modified to incorporate radiotherapy quality assurance. Methods of working for radiotherapy and PET staff will change to take into account considerations of both parties. PET to CT alignment must be subject to quality control on a loaded and unloaded couch preferably using a suitable emission phantom, and tested throughout the whole data pathway. Data integrity must be tested throughout the whole pathway and a system included to verify that delineated structures are transferred correctly. Excellent multidisciplinary team communication and working is vital, and key staff members on both sides should be specifically dedicated to the project. Patient pathway should be clearly devised to optimize patient care and the resources of all departments. Recruitment of a cohort of patients into a methodology study is valuable to test the quality assurance methods and pathway.

Recommendations for the use of PET and PET-CT for radiotherapy planning in research projects

With the increasing use of positron emission tomography (PET) for disease staging, follow-up and therapy monitoring in a number of oncological indications there is growing interest in the use of PET and PET-CT for radiation treatment planning. In order to create a strong clinical evidence base for this, it is important to ensure that research data are clinically relevant and of a high quality. Therefore the National Cancer Research Institute PET Research Network make these recommendations to assist investigators in the development of radiotherapy clinical trials involving the use of PET and PET-CT. These recommendations provide an overview of the current literature in this rapidly evolving field, including standards for PET in clinical trials, disease staging, volume delineation, intensity modulated radiotherapy and PET-augmented planning techniques, and are targeted at a general audience. We conclude with specific recommendations for the use of PET in radiotherapy planning in research projects.

The acoustic properties, centered on 20 MHZ, of an IEC agar-based tissue-mimicking material and its temperature, frequency and age dependence

The purpose of this study was to characterize the ultrasonic properties of agar-based tissue-mimicking materials (TMMs) at ultrasound frequencies centered around 20 MHz. The TMM acoustic properties measured are the amplitude attenuation coefficient alpha (dB cm(-1)MHz(-1)), the speed of sound (ms(-1)) and the backscattered power spectral density (distribution of power per unit frequency normalized to the total received power) characteristics of spectral slope (dB MHz(-1)), y-axis intercept (dB) and reflected power (dB). The acoustic properties are measured over a temperature range of 22 to 37 degrees C. An intercomparison of results between two independent ultrasound measurement laboratories is also presented. A longitudinal study of the acoustic properties over a period of two years is also detailed, and the effect of water immersion on the acoustic properties of TMM is measured. In addition, the physical parameters of mass density rho (kg m(-3)) and specific heat capacity C (J kg(-1) K(-1)) are included. The measurement techniques used were based on the substitution technique using both broadband and narrowband pulses centered on 20 MHz. Both the attenuation coefficient and speed of sound (both group and phase) showed good agreement with the expected values of 0.5 dB cm(-1) MHz(-1) and 1540 ms(-1), respectively, with average values over the three-year period of 0.49 dBcm(-1)MHz1 (SD +/- 0.05) and 1540.9 ms(-1) (SD +/- 8.7). These results also showed agreement between the two independent measurement laboratories. Speed of sound and attenuation coefficient were shown to change with temperature with rates of + 2.1 m s(-1) degrees C(-1) and -0.005 dB cm(-1) MHz(-1) degrees C(-1), respectively. Attenuation changed linearly with frequency at the high frequency range of 17 to 23 MHz, and speed of sound was found to be independent of frequency in this range. The spectral slope of relative backscattered power for the material increased with frequency at typically 1.5 dB MHz(-1). This compared favorably with theoretical spectral slope values, calculated for a variety of scatterer sizes, albeit at a lower frequency range. It is also noticed that, on extrapolation back to lower frequencies, the backscatter is comparable with that measured at 7 MHz. Overall, this non-commercial agar-based TMM is shown to perform as expected at the higher frequency range of 17 to 23 MHz and is seen to retain its acoustic properties of attenuation and speed of sound over a three-year period.

Effect of multislice scanners on patient dose from routine CT examinations in East Anglia

As part of the dose optimization process, the Ionising Radiation (Medical Exposure) Regulations 2000 include requirements relating to the assessment of patient dose, and the setting and subsequent review of diagnostic reference levels. In East Anglia, audits of effective dose in CT have been carried out in 1996, 1999 and 2002. In the 2002 audit, nine of the 14 scanners assessed had been replaced since the previous audit. Eight of the new scanners were multislice scanners, acquiring up to 16 slices in a single rotation. The objective of the 2002 audit was to investigate the effect of the introduction of these multislice scanners on patient doses from routine CT examinations. Exposure parameters were collected for 10 different types of routine CT examination. In excess of 550 sets of patient data were obtained. For each of these, effective doses were calculated using the results of Monte Carlo simulations published by the National Radiological Protection Board. Averaged across all 10 examinations, regional mean effective doses are 34% higher than in 1999. The multislice scanners in the region give, on average, 35% more effective dose than the single-slice scanners. The effect of collimation in multislice scanners makes these effective dose differences most notable for examinations that use narrow slice widths. Further optimization of exposures on multislice scanners has the potential to reduce the differences observed between single-slice and multislice doses. However, when taken in combination with the increased use of CT in many hospitals, the effective dose increases observed are likely to result in a significant increase in the already substantial collective radiation dose from CT.

PERINAL--a new no-touch spray to relieve the symptoms of pruritus ani

A new no-touch aqueous spray has been developed for the topical treatment of pruritus ani. It contains 0.2% hydrocortisone and 1.0% lignocaine hydrochloride, and has been shown in studies involving 83 patients to constitute a most convenient and clinically effective preparation for alleviating the symptoms of this distressing chronic condition.

Value of written health information in the general practice setting

The value of a library for patients as a way of providing written health information in the general practice setting has been investigated using a questionnaire. In 15 months, 243 books were borrowed from one library. Each book contained a questionnaire and 163 questionnaires were returned (67%). It was found that 106 respondents (65%) would not have sought the information elsewhere and 159 respondents (98%) found the books they had read to be very useful or of some use. The perceived level of anxiety after reading was raised in nine respondents (6%) but reduced in 71 (44%). Patients read books on 53 separate topics overall, although 73 respondents (45%) read on 10 recurring topics. A patients' library thus enables most patients to gain useful information from their general practice without increasing their anxiety.