Nick Ryckx

Commissioning of the Leksell Gamma Knife® Icon™

Purpose: The Leksell Gamma Knife (LGK) Icon has been recently introduced to provide Gamma Knife technology with frameless stereotactic treatments which use an additional cone‐beam CT (CBCT) imaging system and a motion tracking system (IFMM, Intra‐Fraction Motion Management). The system was commissioned for the treatment unit itself as well as the imaging system. Methods: The LGK Icon was calibrated using an A1SL ionization chamber. EBT3 radiochromic films were employed to independently check the machine calibration, to measure the relative output factors (ROFs) and to collect dose distributions. Coincidence between CBCT isocenter and radiological focus was evaluated by means of EBT3 films. CBCT image quality was investigated in terms of spatial resolution, contrast‐to‐noise ratio (CNR), and uniformity for the two presets available (low dose and high dose). Computed Tomography Dose Index (CTDI) was also measured for both presets. Results: The absolute dose rate of the LGK Icon was 3.86 ± 0.09 Gy/min. This result was confirmed by EBT3 readings. ROF were found to be 0.887 ± 0.035 and 0.797 ± 0.032 for the 8 mm and 4 mm collimators, respectively, which are within 2% of the Monte Carlo‐derived ROF values. Excellent agreement was found between calculated and measured dose distribution with the gamma pass rate >95% of points for the nine dose distributions analyzed with 3%/1 mm criteria. CBCT isocenter was found to be within 0.2 mm with respect to radiological focus. Image quality parameters were found to be well within the manufacturers specifications with the high‐dose preset being superior in terms of CNR and uniformity. CTDI values were 2.41 mGy and 6.32 mGy, i.e. −3.6% and 0.3% different from the nominal values for the low‐dose and high‐dose presets, respectively. Conclusions: The LGK Icon was successfully commissioned for clinical use. The use of the EBT3 to characterize the treatment unit was demonstrated to be feasible. The CBCT imaging system operates well within the manufacturers specifications and provides good geometrical accuracy.

EXPOSURE OF THE SWISS POPULATION BY RADIODIAGNOSTICS: 2013 REVIEW.

In 2013, a nationwide investigation was conducted in Switzerland to establish the populations exposure from medical X rays. A hybrid approach was used combining the Raddose database accessible on-line by the participating practices and the Swiss medical tariffication system for hospitals. This study revealed that the average annual number of examinations is 1.2 per inhabitant, and the associated annual effective dose is 1.4 mSv. It also showed that computed tomography is the most irradiating modality and that it delivers 70 % of the total dose. The annual effective dose per inhabitant registered a 17 % increase in 5 y and is comparable with what was recently reported in neighbouring countries.

PATIENT EXPOSURE OPTIMISATION THROUGH TASK-BASED ASSESSMENT OF A NEW MODEL-BASED ITERATIVE RECONSTRUCTION TECHNIQUE

The goal of the present work was to report and investigate the performances of a new iterative reconstruction algorithm, using a model observer. For that, a dedicated low-contrast phantom containing different targets was scanned at four volume computed tomography dose index (CTDIvol) levels on a Siemens SOMATOM Force computed tomography (CT). The acquired images were reconstructed using the ADMIRE algorithm and were then assessed by three human observers who performed alternative forced choice experiments. Next, a channelised hotelling observer model was applied on the same set of images. The comparison between the two was performed using the percentage correct as a figure of merit. The results indicated a strong agreement between human and model observer as well as an improvement in the low-contrast detection when switching from an ADMIRE strength of 1–3. Good results were also observed even in situations where the target was hard to detect, suggesting that patient dose could be further reduced and optimised.

OBJECTIVE TASK-BASED ASSESSMENT OF LOW-CONTRAST DETECTABILITY IN ITERATIVE RECONSTRUCTION

Evaluating image quality by using receiver operating characteristic studies is time consuming and difficult to implement. This work assesses a new iterative algorithm using a channelised Hotelling observer (CHO). For this purpose, an anthropomorphic abdomen phantom with spheres of various sizes and contrasts was scanned at 3 volume computed tomography dose index (CTDIvol) levels on a GE Revolution CT. Images were reconstructed using the iterative reconstruction method adaptive statistical iterative reconstruction-V (ASIR-V) at ASIR-V 0, 50 and 70 % and assessed by applying a CHO with dense difference of Gaussian and internal noise. Both CHO and human observers (HO) were compared based on a four-alternative forced-choice experiment, using the percentage correct as a figure of merit. The results showed accordance between CHO and HO. Moreover, an improvement in the low-contrast detection was observed when switching from ASIR-V 0 to 50 %. The results underpin the finding that ASIR-V allows dose reduction.

MEDICAL PHYSICISTS' IMPLICATION IN RADIOLOGICAL DIAGNOSTIC PROCEDURES: RESULTS AFTER 1 Y OF EXPERIENCE

Since January 2008-de facto 2012-medical physics experts (MPEs) are, by law, to be involved in the optimisation process of radiological diagnostic procedures in Switzerland. Computed tomography, fluoroscopy and nuclear medicine imaging units have been assessed for patient exposure and image quality. Large spreads in clinical practice have been observed. For example, the number of scans per abdominal CT examination went from 1 to 9. Fluoroscopy units showed, for the same device settings, dose rate variations up to a factor of 3 to 7. Quantitative image quality for positron emission tomography (PET)/CT examinations varied significantly depending on the local image reconstruction algorithms. Future work will be focused on promoting team cooperation between MPEs, radiologists and radiographers and on implementing task-oriented objective image quality indicators.

BENCHMARKING OF CT FOR PATIENT EXPOSURE OPTIMISATION

Patient dose optimisation in computed tomography (CT) should be done using clinically relevant tasks when dealing with image quality assessments. In the present work, low-contrast detectability for an average patient morphology was assessed on 56 CT units, using a model observer applied on images acquired with two specific protocols of an anthropomorphic phantom containing spheres. Images were assessed using the channelised Hotelling observer (CHO) with dense difference of Gaussian channels. The results were computed by performing receiver operating characteristics analysis (ROC) and using the area under the ROC curve (AUC) as a figure of merit. The results showed a small disparity at a volume computed tomography dose index (CTDIvol) of 15 mGy depending on the CT units for the chosen image quality criterion. For 8-mm targets, AUCs were 0.999 ± 0.018 at 20 Hounsfield units (HU) and 0.927 ± 0.054 at 10 HU. For 5-mm targets, AUCs were 0.947 ± 0.059 and 0.702 ± 0.068 at 20 and 10 HU, respectively. The robustness of the CHO opens the way for CT protocol benchmarking and optimisation processes.

Swiss survey on hybrid imaging CTs doses in Nuclear Medicine and proposed national dose reference levels

A multidisciplinary working group led by the Swiss Federal Office of Public Health was formed to plan and perform a nationwide survey of patient radiation exposure from computed tomography (CT) in hybrid devices across Nuclear Medicine departments. The survey included 16 departments (of which 5 were university hospitals) and the submitted responses included 10,673 entries for the 33 different protocols proposed (11 in PET and 22 in SPECT). The working group determined the selection and exclusion criteria applied to the analysis. This work presents the survey preparation and data analysis including the exclusion criteria used. The results are used to inform recommendations for National Diagnostic Reference Levels (DRL) for CT procedures in Nuclear Medicine in Switzerland. Of the 33 protocols initially proposed, 10 protocols for both PET and SPECT modalities were retained after exclusion criteria and thresholds were applied. The results obtained in terms of volume-weighted computed tomography dose index (CTDIvol) and dose length product (DLP) have been put forward as recommendations for national Diagnostic Reference Levels for protocols in hybrid imaging devices in Nuclear Medicine in Switzerland and will be published by the Federal Office of Public Health.

The use of out-of-plane high Z patient shielding for fetal dose reduction in computed tomography: Literature review and comparison with Monte-Carlo calculations of an alternative optimisation technique

When performing CT examinations on pregnant patients, great effort should be dedicated towards optimising the exposure of the mother and the conceptus. For this purpose, many radiology departments use high-Z garments to be wrapped around the patients lower abdomen for out-of-plane organ shielding to protect the fetus. To assess their current protection efficiency, we performed a literature review and compared the efficiencies mentioned in the literature to Monte-Carlo calculations of CT protocols for which the overall scan length was reduced. We found 11 relevant articles, all of them reporting uterus exposure due to CT imaging performed for exclusion of pulmonary embolism, one of the leading causes of peripartum deaths in western countries. Uterus doses ranged between 60 and 660 µGy per examination, and relative dose reductions to the uterus due to high-Z garments were between 20 and 56%. Calculations showed that reducing the scan length by one to three centimetres could potentially reduce uterus dose up to 24% for chest imaging, and even 47% for upper abdominal imaging. These dose reductions were in the order of those achieved by high-Z garments. However, using the latter may negatively influence the diagnostic image quality and even interfere with the automatic exposure control system thus increasing patient dose if positioned in the primary beam, for example in the overranging length in helical acquisition. We conclude that efforts should be concentrated on positioning the patient correctly in the gantry and optimising protocol parameters, rather than using high-Z garments for out-of-plane uterus shielding.

Task-based quantification of image quality using a model observer in abdominal CT: a multicentre study

ObjectiveWe investigated the variability in diagnostic information inherent in computed tomography (CT) images acquired at 68 different CT units, with the selected acquisition protocols aiming to answer the same clinical question.MethodsAn anthropomorphic abdominal phantom with two optional rings was scanned on 68 CT systems from 62 centres using the local clinical acquisition parameters of the portal venous phase for the detection of focal liver lesions. Low-contrast detectability (LCD) was assessed objectively with channelised Hotelling observer (CHO) using the receiver operating characteristic (ROC) paradigm. For each lesion size, the area under the ROC curve (AUC) was calculated and considered as a figure of merit. The volume computed tomography dose index (CTDIvol) was used to indicate radiation dose exposure.ResultsThe median CTDIvol used was 5.8 mGy, 10.5 mGy and 16.3 mGy for the small, medium and large phantoms, respectively. The median AUC obtained from clinical CT protocols was 0.96, 0.90 and 0.83 for the small, medium and large phantoms, respectively.ConclusionsOur study used a model observer to highlight the difference in image quality levels when dealing with the same clinical question. This difference was important and increased with growing phantom size, which generated large variations in patient exposure. In the end, a standardisation initiative may be launched to ensure comparable diagnostic information for well-defined clinical questions. The image quality requirements, related to the clinical question to be answered, should be the starting point of patient dose optimisation.Key Points• Model observers enable to assess image quality objectively based on clinical tasks.• Objective image quality assessment should always include several patient sizes.• Clinical diagnostic image quality should be the starting point for patient dose optimisation.• Dose optimisation by applying DRLs only is insufficient for ensuring clinical requirements.

PATIENT DOSE ASSESSMENT AFTER INTERVENTIONAL CARDIOLOGY PROCEDURES: A MULTI-CENTRIC APPROACH TO TRIGGER OPTIMISATION.

As the number and complexity of fluoroscopically guided interventions increase, a serious effort has to be put on the optimisation of the X-ray dose delivered to the patient. In order to set up this optimisation process, the clinical practice for a given cardiology centre has to be analysed with relevant statistical power and compared with the data at local or national level. Data from 8 Swiss cardiology centres for 10 different vascular and heart rhythm procedures have been collected. The collected dose indicators were, when available, cumulated air kerma, cumulated dose-area product, fluoroscopy time and the number of images per procedure. Data analysis was performed using an in-house software solution in terms of the first, second and third quartiles. This kind of large-scale analysis could yield some onsets towards local practice optimisation based on anonymous dose indicator cross-comparison. Further effort should nevertheless be made in order to proceed towards an operator-based data analysis, thus allowing for an individual practice optimisation.