Calculating interfractional prostate motion during radiation therapy course using fiducial markers.

PURPOSE: In this study we evaluated the day to day prostate displacement during radiation therapy by using implanted radiopaque fiducials and daily image guided position verification. METHODS: The data of 10 patients that received radiation therapy to the prostate were analyzed. Three fiducial markers were implanted in the prostate before treatment initiation for everyday verification of the target's position. Daily X ray images (kilovolt/KV films) of the pelvis were acquired for verification and were matched with baseline images produced during treatment preparation using bony structures and fiducials as landmarks. We calculated the mean difference between the two methods and the prostate displacement derived from these measurements. RESULTS: A total of 208 KV films were obtained. Our results showed a non-uniform prostate motion, with most of the displacements observed in the caudal direction followed by anterior, posterior, cranial, right and left. The mean target motion in each of the above directions was 3.5 mm, 3.5 mm, 3.3 mm, 3.9 mm, 2 mm and 2.4 mm. Based on the cumulative frequency of the target's displacement, a margin of 8 mm, 7mm, 5 mm, 4 mm, 9 mm and 7 mm in the anterior, posterior, left, right, cranial and caudal direction respectively would account for 95% of prostate's motion, provided that every day KV image guidance is performed. CONCLUSION: A non-isotopic margin of 8 mm, 7mm, 5 mm, 4 mm, 9 mm and 7 mm around the prostate can be considered safe for treatment delivery.

Optimization Model for the Distribution of Fiducial Markers in Liver Intervention.

The distribution of fiducial markers is one of the main factors affected the accuracy of optical navigation system. However, many studies have been focused on improving the fiducial registration accuracy or the target registration accuracy, but few solutions involve optimization model for the distribution of fiducial markers. In this paper, we propose an optimization model for the distribution of fiducial markers to improve the optical navigation accuracy. The strategy of optimization model is reducing the distribution from three dimensional to two dimensional to obtain the 2D optimal distribution by using optimization algorithm in terms of the marker number and the expectation equation of target registration error (TRE), and then extend the 2D optimal distribution in two dimensional to three dimensional to calculate the optimal distribution according to the distance parameter and the expectation equation of TRE. The results of the experiments show that the averaged TRE for the human phantom is approximately 1.00 mm by applying the proposed optimization model, and the averaged TRE for the abdominal phantom is 0.59 mm. The experimental results of liver simulator model and ex-vivo porcine liver model show that the proposed optimization model can be effectively applied in liver intervention.

A multi-subject accuracy study on granular jamming for non-invasive attachment of fiducial markers to patients.

PURPOSE: This short communication describes experimental evaluation of a new granular jamming cap (GJC) recently introduced in Wellborn et al. (Int J Comput Assist Radiol Surg 12(6):1069-1077, 2017). The contributions beyond [8] are (1) to evaluate accuracy across multiple human subjects, and (2) to determine how much of the accuracy improvement is attributable to improved fiducial marker arrangement alone, and how much is due to granular jamming. The motivation for this GJC is to improve the accuracy of image-guidance interfaces in transnasal skull base surgery. Accuracy depends on a rigid connection between tracked fiducial markers and the patient. By molding itself to the unique contours of the individual patient's head and then solidifying, the GJC can firmly attach fiducial markers to a patient, increasing accuracy in the presence of disturbances. METHODS: A multi-subject study ([Formula: see text]) was performed to evaluate the accuracy of the GJC compared to a clinically used headband-based fixation device, in the presence of simulated accidental bumping (light force and impact events) that could occur in a real-world operating room. RESULTS: The GJC reduced the average target registration error at the pituitary gland by 66% in our force experiments and 78% in our impact experiments, which were statistically significant reductions ([Formula: see text]). Maximum target registration error was similarly reduced by 55% and 78% in the same two perturbation tests. CONCLUSION: The GJC increases the accuracy of transnasal image-guidance under force and impact perturbations by more firmly, yet non-invasively, attaching fiducial markers to the patient. We find that granular jamming provides accuracy improvement beyond that associated with improved fiducial marker arrangement.

A Comparative Study of Fiducial-Based and Fiducial-Less Registration Utilizing the O-Arm.

BACKGROUND: Frameless stereotactic surgery utilizing fiducial-based (FB) registration is an established tool in the armamentarium of deep brain stimulation (DBS) surgeons. Fiducial-less (FL) registration via intraoperative CT, such as the O-arm, has been routinely used in spine surgery, but its accuracy for DBS surgery has not been studied in a clinical setting. OBJECTIVE: We undertook a study to analyze the accuracy of the FL technique in DBS surgery and compare it to the FB method. METHODS: In this prospective cohort study, 97 patients underwent DBS surgery using the NexFrame and the O-arm registration stereotactic system. Patients underwent FB (n = 50) registration from 2015 to 2016 and FL (n = 47) O-arm registration from 2016 to 2017. RESULTS: The radial errors (RE) and vector/euclidean errors of FB and FL registration were not significantly different. There was no difference in additional passes between methods, but there was an increase in the number of RE ≥2.5 mm in the FL method. CONCLUSION: Although there was no statistically significant difference in RE or the need for additional passes, the increased number of errors ≥2.5 mm with the FL method (17 vs. 4% in FB) indicates the need for further study. We concluded that O-arm images of the implants should be utilized to assess and correct for this error.

Current accuracy of surface matching compared to adhesive markers in patient-to-image registration.

OBJECT: In the past, the accuracy of surface matching has been shown to be disappointing. We aimed to determine whether this had improved over the years by assessing application accuracy of current navigation systems, using either surface matching or point-pair matching. METHODS: Eleven patients, scheduled for intracranial surgery, were included in this study after a power analysis had shown this small number to be sufficient. Prior to surgery, one additional fiducial marker was placed on the scalp, the "target marker," where the entry point of surgery was to be expected. Using one of three different navigation systems, two patient-to-image registration procedures were performed: one based on surface matching and one based on point-pair matching. Each registration procedure was followed by the digitization of the target marker's location, allowing calculation of the target registration error. If the system offered surface matching improvement, this was always used; and for the two systems that routinely offer an estimate of neuronavigation accuracy, this was also recorded. RESULTS: The error in localizing the target marker using point-pair matching or surface matching was respectively 2.49 mm and 5.35 mm, on average (p < 0.001). In those four cases where an attempt was made to improve the surface matching, the error increased to 6.35 mm, on average. For the seven cases where the system estimated accuracy, this estimate did not correlate with target registration error (R2 = 0.04, p = 0.67). CONCLUSION: The accuracy of navigation systems has not improved over the last decade, with surface matching consistently yielding errors that are twice as large as when point-pair matching with adhesive markers is used. These errors are not reliably reflected by the systems own prediction, when offered. These results are important to make an informed choice between image-to-patient registration strategies, depending on the type of surgery at hand.

Convex Probe EBUS-guided Fiducial Placement for Malignant Central Lung Lesions.

BACKGROUND: Stereotactic body radiotherapy (SBRT) had become a therapeutic modality in patients with primary tumors, locally recurrent as well as oligometastasis involving the lung. Some modalities of SBRT require fiducial marker (FM) for dynamic tumor tracking. Previous studies have focused on evaluating bronchoscopic-guided FM placement for peripheral lung nodules. We describe the safety and feasibility of placing FM using real-time convex probe endobronchial ultrasound (CP-EBUS) for SBRT in patients with centrally located hilar/mediastinal masses or lymph nodes. METHODS: This is a retrospective review of patients who were referred to Beth Israel Deaconess Medical Center's multidisciplinary thoracic oncology program for FM placement to pursue SBRT. RESULTS: Thirty-seven patients who underwent real-time CP-EBUS were included. Patients had a median age of 71 years [interquartile range (IQR), 59.5 to 80.5]. The median size of the lesion was 2.2 cm (IQR, 1.4 to 3.3 cm). The median distance from the central airway was 2.4 cm (IQR, 0 to 3.4 cm). A total of 51 FMs (median of 1 per patient) were deployed in 37 patients. At the time of SBRT planning, 46 (90.2%) were confirmed radiologically in 32 patients. Patients with unsuccessful fiducial deployment (n=5) underwent a second procedure using the same technique. Of those, 3 patients had a successful fiducial placement via bronchoscopy, 1 patient required FM placement by percutaneous computed tomography-guided approach and 1 patient required FM placement through EUS by gastroenterology. CONCLUSION: CP-EBUS-guided FM placement for patients with malignant lymph nodes and central parenchymal lung lesions appears to be safe and feasible.

Bronchoscopic Laser Interstitial Thermal Therapy: An Experimental Study in Normal Porcine Lung Parenchyma.

BACKGROUND: Population aging and lung cancer screening strategies may lead to an increase in detection of early-stage lung cancer in medical inoperable patients. Recent advances in peripheral bronchoscopy have made it a suitable platform for ablation of small peripheral tumors. METHODS: We investigated the tissue-ablative effect of a diode laser bronchoscopically applied by a laser delivery fiber (LDF) with wide aperture on porcine lung parenchyma. Laser was tested ex vivo and in vivo to identify the most effective power settings and LDF. Chest computed tomography (CT) were obtained immediately after ablation and after 3 days of observation. At day 3, necropsy was performed. RESULTS: On the basis of our ex vivo and in vivo experiments, we selected the round-tip LDF to be activated at 25 W for 20 seconds. Ten ablations were performed in 5 pigs. One ablation resulted in a pneumothorax requiring aspiration. All animals remained stable for 72 hours. CT findings at days 1 and 3 showed an area of cavitation surrounded by consolidation and ground glass. Median size of CT findings (long axis) was 26 mm (range, 24 to 38) at day 1, and 34 mm (range, 30 to 44) at day 3. Necropsy showed an area of central char measuring from 0.8×0.7×0.9 cm to 2.4×3.5×1.2 cm, surrounded by a gray-brown to dark red area. On histology, variable degrees of necrosis were evident around the charred areas. CONCLUSION: Bronchoscopic laser interstitial thermal therapy can achieve relatively large areas of ablation of normal lung parenchyma with a low rate of periprocedural complications.

Target registration error minimization for minimally invasive interventions involving deformable organs.

Precise positioning of the target point during minimally invasive procedures is a major challenge associated with the use of image-based navigation systems. No significant dependence between fiducial registration error (FRE) and target registration error (TRE) was found. However, this investigation demonstrated the utility of using thin plate splines (TPS) and marker observation to monitor FRE during respiration to estimate target position based on the deformation field for minimally invasive procedures in deformable regions. The proposed methodology was verified via experiments involving 21 patients diagnosed with liver tumors. This method has been developed for real-time use while performing operations.

A fiducial skull marker for precise MRI-based stereotaxic surgery in large animal models.

BACKGROUND: Stereotaxic neurosurgery in large animals is used widely in different sophisticated models, where precision is becoming more crucial as desired anatomical target regions are becoming smaller. Individually calculated coordinates are necessary in large animal models with cortical and subcortical anatomical differences. NEW METHOD: We present a convenient method to make an MRI-visible skull fiducial for 3D MRI-based stereotaxic procedures in larger experimental animals. Plastic screws were filled with either copper-sulfate solution or MRI-visible paste from a commercially available cranial head marker. The screw fiducials were inserted in the animal skulls and T1 weighted MRI was performed allowing identification of the inserted skull marker. RESULTS: Both types of fiducial markers were clearly visible on the MRÍs. This allows high precision in the stereotaxic space. COMPARISON WITH EXISTING METHOD: The use of skull bone based fiducial markers gives high precision for both targeting and evaluation of stereotaxic systems. There are no metal artifacts and the fiducial is easily removed after surgery. CONCLUSION: The fiducial marker can be used as a very precise reference point, either for direct targeting or in evaluation of other stereotaxic systems.

Metal artifact reduction in MRI-based cervical cancer intracavitary brachytherapy.

Magnetic resonance imaging (MRI) plays an increasingly important role in brachytherapy planning for cervical cancer. Yet, metal tandem, ovoid intracavitary applicators, and fiducial markers used in brachytherapy cause magnetic susceptibility artifacts in standard MRI. These artifacts may impact the accuracy of brachytherapy treatment and the evaluation of tumor response by misrepresenting the size and location of the metal implant, and distorting the surrounding anatomy and tissue. Metal artifact reduction sequences (MARS) with high bandwidth RF selective excitations and turbo spin-echo readouts were developed for MRI of orthopedic implants. In this study, metal artifact reduction was applied to brachytherapy of cervical cancer using the orthopedic metal artifact reduction (O-MAR) sequence. O-MAR combined MARS features with view angle tilting and slice encoding for metal artifact correction (SEMAC) to minimize in-plane and through-plane susceptibility artifacts. O-MAR improved visualization of the tandem tip on T2 and proton density weighted (PDW) imaging in phantoms and accurately represented the diameter of the tandem. In a pilot group of cervical cancer patients (N = 7), O-MAR significantly minimized the blooming artifact at the tip of the tandem in PDW MRI. There was no significant difference observed in artifact reduction between the weak (5 kHz, 7 z-phase encodes) and medium (10 kHz, 13 z-phase encodes) SEMAC settings. However, the weak setting allowed a significantly shorter acquisition time than the medium setting. O-MAR also reduced susceptibility artifacts associated with metal fiducial markers so that they appeared on MRI at their true dimensions.

Investigation of the optimum location of external markers for patient setup accuracy enhancement at external beam radiotherapy.

In external beam radiotherapy, one of the most common and reliable methods for patient geometrical setup and/or predicting the tumor location is use of external markers. In this study, the main challenging issue is increasing the accuracy of patient setup by investigating external markers location. Since the location of each external marker may yield different patient setup accuracy, it is important to assess different locations of external markers using appropriate selective algorithms. To do this, two commercially available algorithms entitled a) canonical correlation analysis (CCA) and b) principal component analysis (PCA) were proposed as input selection algorithms. They work on the basis of maximum correlation coefficient and minimum variance between given datasets. The proposed input selection algorithms work in combination with an adaptive neuro-fuzzy inference system (ANFIS) as a correlation model to give patient positioning information as output. Our proposed algorithms provide input file of ANFIS correlation model accurately. The required dataset for this study was prepared by means of a NURBS-based 4D XCAT anthropomorphic phantom that can model the shape and structure of complex organs in human body along with motion information of dynamic organs. Moreover, a database of four real patients undergoing radiation therapy for lung cancers was utilized in this study for validation of proposed strategy. Final analyzed results demonstrate that input selection algorithms can reasonably select specific external markers from those areas of the thorax region where root mean square error (RMSE) of ANFIS model has minimum values at that given area. It is also found that the selected marker locations lie closely in those areas where surface point motion has a large amplitude and a high correlation.

Side marker creep: have radiographers changed their side marker habits?

Has side marker use changed with the introduction of digital radiography? This editorial explores this question.

A clinical audit of anatomical side marker use in a paediatric medical imaging department.

INTRODUCTION: The gold standard in general radiography is to place a radiopaque anatomical side marker in the field of view for each radiographic image prior to exposure. The advent of digital radiography has allowed for anatomical side markers to be digitally added to films as part of post-processing. The aim of this audit was to identify whether general X-ray images performed in a tertiary Women's and Children's Hospital were being appropriately annotated with a definitive side marker, and to identify factors that may contribute to inappropriately labelled images. METHODS: Four hundred images from 201 patients' examinations occurring within a randomly selected time period were assessed to ascertain whether radiographic anatomical side markers were visible when images were viewed via the hospitals main viewing platform. The audit occurred in January 2014. The scope included both mobile and in-department general X-ray examinations, with the patient age range extending from 1 day to 18 years. RESULTS: Of the 400 images evaluated, 88 (22%) were found to have a lead marker that matched the anatomy being imaged within the primary beam; 289 (72.3%) images contained a correct digital marker inserted as part of the post-processing of the image. In total, 377 (94.2%) images were appropriately marked. Of the 23 (5.8%) images not marked correctly, 22 images had no marker and 1 was incorrectly marked with a digital marker. There was a noticeable relationship between absent anatomical markers and chest X-rays performed outside of the medical imaging department. CONCLUSIONS: While it is encouraging that the majority of the images assessed were correctly annotated, with only a small number of missing markers, there are opportunities for further improvement. The audit findings suggest that reduced access to lead markers influences marker use. Strategies that may improve compliance at an individual level include distribution of personalised anatomical side markers, and targeted staff education sessions. At a department level, regular audits and monitoring should be encouraged.

Prostate Dose-painting Radiotherapy and Radiobiological Guided Optimisation Enhances the Therapeutic Ratio.

AIMS: To describe the treatment of 11 patients with radiobiologically guided dose-painting radiotherapy and report on toxicity. MATERIALS AND METHODS: Boost volumes were identified with functional magnetic resonance imaging scans in 11 patients with high-risk prostate cancer. Patients were treated using a dose-painting approach; the boost dose was limited to 86 Gy in 37 fractions, while keeping the rectal normal tissue complication probability to 5-6%. Rotational intensity-modulated radiotherapy was used with daily image guidance and fiducial markers. RESULTS: The median dose to the prostate (outside the boost volume) and urethra was 75.4 Gy/37 fractions (range 75.1-75.8 Gy), whereas the median boost dose was 83.4 Gy (range 79.0-87.4 Gy). The tumour control probability (TCP) (Marsden model) increased from 71% for the standard plans to 83.6% [76.6-86.8%] for the dose-painting boost plans. The mean (Lyman-Kutcher-Burman) normal tissue complication probability for rectal bleeding was 5.2% (range 3.3-6.2%) and 5.2% for faecal incontinence (range 3.6-7.8%). All patients tolerated the treatment well, with a low acute toxicity profile. At a median follow-up of 36 months (range 24-50) there was no grade 3 late toxicity. Two patients had grade 2 late urinary toxicity (urethral stricture, urinary frequency and urgency), one patient had grade 1 and one grade 2 late rectal toxicity. The mean prostate-specific antigen at follow-up was 0.81 ng/ml after stopping hormone therapy; one patient relapsed biochemically at 32 months (2.70 ng/ml). CONCLUSIONS: The toxicity for this radiobiological guided dose-painting protocol was low, but we have only treated a small cohort with limited follow-up time. The advantages of this treatment approach should be established in a clinical trial.

Thoracic spine localization using preoperative placement of fiducial markers and subsequent CT. A technical report.

STUDY DESIGN: A retrospective case series evaluating the use of fiducial markers with subsequent computed tomography (CT) or CT myelography for intraoperative localization. OBJECTIVE: To evaluate the safety and utility of preoperative fiducial placement, confirmed with CT myelography, for intraoperative localization of thoracic spinal levels. SUMMARY OF BACKGROUND DATA: Thoracic spine surgery is associated with serious complications, not the least of which is the potential for wrong-level surgery. Intraoperative fluoroscopy is often used but can be unreliable due to the patient's body habitus and anatomical variation. METHODS: Sixteen patients with thoracic spine pathology requiring surgical intervention underwent preoperative fiducial placement at the pedicle of the level of interest in the interventional radiology suite. CT or CT myelogram was then done to evaluate fiducial location relative to the level of pathology. Surgical treatment followed at a later date in all patients. RESULTS: All patients underwent preoperative fiducial placement and CT or CT myelography, which was done on an outpatient basis in 14 of the 16 patients. Intraoperatively, fiducial localization was easily and quickly done with intraoperative fluoroscopy leading to correct localization of spinal level in all cases. All patients had symptomatic improvement following surgery. There were no complications from preoperative localization or operative intervention. CONCLUSIONS: Preoperative placement of fiducial markers confirmed with a CT or CT myelogram allows for reliable and fast intraoperative localization of the spinal level of interest with minimal risks and potential complications to the patient. In most cases, a noncontrast CT should be sufficient. This should be an equally reliable means of localization while further decreasing potential for complications. CT myelography should be reserved for pathology that is not evident on noncontrast CT. Accuracy of localization is independent of variations in rib number or vertebral segmentation. The technique is a safe, reliable, and rapid means of localizing spinal level during surgery.

Inter- and intrafractional setup errors and baseline shifts of fiducial markers in patients with liver tumors receiving free-breathing postoperative radiation analyzed by cone-beam computed tomography.

This study was to evaluate the interfractional and intrafractional setup errors and baseline shifts of golden fiducial markers in patients receiving postoperative radio- therapy (RT) using cone-beam computed tomography (CBCT) in order to calculate PTV margins for patients with liver cancer. Twelve patients with liver tumors under- went postoperative RT. CBCT images were acquired before and after the treatment. Off-line vertebral body match and fiducial marker match were used, respectively. The results of vertebral body match represented the setup errors of the patients, while the results of fiducial marker match represented the absolute position errors of the target volume. Baseline shifts of the target volume were calculated as the absolute target position errors minus setup errors. A total of 12 patients with 214 acquisi- tions of CBCTs were analyzed. Both Σ and σ of setup errors and baseline shifts in left-right (L/R), superior-inferior (S/I), and anterior-posterior(A/P) directions were calculated, including interfractional and intrafractional uncertainties. Planning target volume (PTV) margins were calculated according to margin = 2.5Σ + 0.7σ. Margins of 1.8 mm, 3.8 mm, and 1.4 mm in L/R, S/I, and A/P directions are needed to compensate intrafractional errors when daily online CBCT correction is used. When CBCT correction with no action level (NAL) protocol is used, PTV margin should be 2.6 mm, 5.9 mm, and 2.6 mm in L/R, S/I, and A/P directions. Margins of 5.5 mm, 14.6 mm, and 7.2 mm were needed to compensate the baseline shifts when electronic portal imaging devices (EPID) or CBCT with bone match is used for online correction of setup error. 

X-ray image calibration and its application to clinical orthopedics.

X-ray imaging is one of the most commonly used medical imaging modality. Albeit X-ray radiographs provide important clinical information for diagnosis, planning and post-operative follow-up, the challenging interpretation due to its 2D projection characteristics and the unknown magnification factor constrain the full benefit of X-ray imaging. In order to overcome these drawbacks, we proposed here an easy-to-use X-ray calibration object and developed an optimization method to robustly find correspondences between the 3D fiducials of the calibration object and their 2D projections. In this work we present all the details of this outlined concept. Moreover, we demonstrate the potential of using such a method to precisely extract information from calibrated X-ray radiographs for two different orthopedic applications: post-operative acetabular cup implant orientation measurement and 3D vertebral body displacement measurement during preoperative traction tests. In the first application, we have achieved a clinically acceptable accuracy of below 1° for both anteversion and inclination angles, where in the second application an average displacement of 8.06±3.71 mm was measured. The results of both applications indicate the importance of using X-ray calibration in the clinical routine.

Propagation of calibration errors in prospective motion correction using external tracking.

PURPOSE: Prospective motion correction of MRI scans using an external tracking device (such as a camera) is becoming increasingly popular, especially for imaging of the head. In order for external tracking data to be transformed into the MR scanner reference frame, the pose (i.e., position and orientation) of the camera relative to the scanner--or cross-calibration--must be accurate. In this study, we investigated how errors in cross-calibration affect the accuracy of motion correction feedback in MRI. THEORY AND METHODS: An operator equation is derived describing how calibration errors relate to errors in applied motion compensation. By taking advantage of spherical symmetry and performing a Taylor approximation for small rotation angles, a closed form expression and upper limit for the residual tracking error is provided. RESULTS: Experiments confirmed theoretical predictions of a bilinear dependence of the residual rotational component on the calibration error and the motion performed, modulated by a sinusoidal dependence on the angle between the calibration error axis and motion axis. The residual translation error is bounded by the sum of the rotation angle multiplied by the translational calibration error plus the linear head displacement multiplied by the calibration error angle. CONCLUSION: The results make it possible to calculate the required cross-calibration accuracy for external tracking devices for a range of motions. Scans with smaller expected movements require less accuracy in cross-calibration than scans involving larger movements. Typical clinical applications require that the calibration accuracy is substantially below 1 mm and 1°.

Commissioning of MRI-only based treatment planning procedure for external beam radiotherapy of prostate.

In radiotherapy, target tissues are defined best on MR images due to their superior soft tissue contrast. Computed tomography imaging is geometrically accurate and it is needed for dose calculation and generation of reference images for treatment localization. Co-registration errors between MR and computed tomography images can be eliminated using magnetic resonance imaging-only based treatment planning. Use of ionizing radiation can be avoided which is especially important in adaptive treatments requiring several re-scans. We commissioned magnetic resonance imaging-only based procedure for external radiotherapy, treatment planning of the prostate cancer. Geometrical issues relevant in radiotherapy, were investigated including quality assurance testing of the scanner, evaluation of the displacement of skin contour and radiosensitive rectum wall, and detection of intraprostatic fiducial gold seed markers used for treatment localization. Quantitative analysis was carried out for 30 randomly chosen patients. Systematic geometrical errors were within 2.2 mm. The gold seed markers were correctly identified for 29 out of the 30 patients. Positions of the seed midpoints were consistent within 1.3 mm in magnetic resonance imaging and computed tomography. Positional error of rectal anterior wall due to susceptibility effect was minimal. Geometrical accuracy of the investigated equipment and procedure was sufficient for magnetic resonance imaging-only based radiotherapy, treatment planning of the prostate cancer including treatment virtual simulation.

Monitoring image guidance system accuracy during spinal surgery with mini-screw fiducials: technical note.

STUDY DESIGN: A technical note. OBJECTIVE: To describe a technique for measuring accuracy of intraoperative image guidance systems in spine surgery. SUMMARY OF BACKGROUND DATA: Image guidance may be of use when performing complex procedures on the spine. However, as the operation progresses and, in particular, once any deformity has been corrected, the image guidance system may become unreliable. In practice, this often results in repeated image acquisitions thus increasing the radiation exposure to the patient. METHODS: Small titanium, cranio-facial screws were placed on the dorsal aspect of the spine intraoperatively, before the acquisition of images and used as fiducials. RESULTS: The authors were able to accurately discern the true precision of the image guidance system used with an intraoperative computed tomography scanner, throughout the procedure. CONCLUSIONS: By using intraoperatively placed mini-screw fiducials, the surgeon may check and quantify the underlying system accuracy both initially and throughout the surgery. In the future, "auto-adjust" functions may be integrated into the computer software to automatically recalibrate the system when a probe is placed into the fiducials without the need for rescanning.