Robust Quantitative Assessment of Trabecular Microarchitecture in Extremity Cone-Beam CT Using Optimized Segmentation Algorithms

Purpose

In-vivo evaluation of bone microarchitecture remains challenging because of limited resolution of conventional orthopaedic imaging modalities. We investigate the performance of flat-panel detector extremity Cone-Beam CT (CBCT) in quantitative analysis of trabecular bone. To enable accurate morphometry of fine trabecular bone architecture, advanced CBCT pre-processing and segmentation algorithms are developed.

Methods

The study involved 35 transilliac bone biopsy samples imaged on extremity CBCT (voxel size 75 μm, imaging dose ~13 mGy) and gold standard μCT (voxel size 7.67 μm). CBCT image segmentation was performed using (i) global Otsu's thresholding, (ii) Bernsen's local thresholding, (iii) Bernsen's local thresholding with additional histogram-based global pre-thresholding, and (iv) the same as (iii) but combined with contrast enhancement using a Laplacian Pyramid. Correlations between extremity CBCT with the different segmentation algorithms and gold standard μCT were investigated for measurements of Bone Volume over Total Volume (BV/TV), Trabecular Thickness (Tb.Th), Trabecular Spacing (Tb.Sp), and Trabecular Number (Tb.N).

Results

The combination of local thresholding with global pre-thresholding and Laplacian contrast enhancement outperformed other CBCT segmentation methods. Using this optimal segmentation scheme, strong correlation between extremity CBCT and μCT was achieved, with Pearson coefficients of 0.93 for BV/TV, 0.89 for Tb.Th, 0.91 for Tb.Sp, and 0.88 for Tb.N (all results statistically significant). Compared to a simple global CBCT segmentation using Otsu's algorithm, the advanced segmentation method achieved ~20% improvement in the correlation coefficient for Tb.Th and ~50% improvement for Tb.Sp.

Conclusions

Extremity CBCT combined with advanced image pre-processing and segmentation achieves high correlation with gold standard μCT in measurements of trabecular microstructure. This motivates ongoing development of clinical applications of extremity CBCT in in-vivo evaluation of bone health e.g. in early osteoarthritis and osteoporosis.

Planning, guidance, and quality assurance of pelvic screw placement using deformable image registration

Percutaneous pelvic screw placement is challenging due to narrow bone corridors surrounded by vulnerable structures and difficult visual interpretation of complex anatomical shapes in 2D x-ray projection images. To address these challenges, a system for planning, guidance, and quality assurance (QA) is presented, providing functionality analogous to surgical navigation, but based on robust 3D-2D image registration techniques using fluoroscopy images already acquired in routine workflow. Two novel aspects of the system are investigated: automatic planning of pelvic screw trajectories and the ability to account for deformation of surgical devices (K-wire deflection). Atlas-based registration is used to calculate a patient-specific plan of screw trajectories in preoperative CT. 3D-2D registration aligns the patient to CT within the projective geometry of intraoperative fluoroscopy. Deformable known-component registration (dKC-Reg) localizes the surgical device, and the combination of plan and device location is used to provide guidance and QA. A leave-one-out analysis evaluated the accuracy of automatic planning, and a cadaver experiment compared the accuracy of dKC-Reg to rigid approaches (e.g. optical tracking). Surgical plans conformed within the bone cortex by 3-4 mm for the narrowest corridor (superior pubic ramus) and  >5 mm for the widest corridor (tear drop). The dKC-Reg algorithm localized the K-wire tip within 1.1 mm and 1.4° and was consistently more accurate than rigid-body tracking (errors up to 9 mm). The system was shown to automatically compute reliable screw trajectories and accurately localize deformed surgical devices (K-wires). Such capability could improve guidance and QA in orthopaedic surgery, where workflow is impeded by manual planning, conventional tool trackers add complexity and cost, rigid tool assumptions are often inaccurate, and qualitative interpretation of complex anatomy from 2D projections is prone to trial-and-error with extended fluoroscopy time.

Deformable 3D-2D Registration for Guiding K-Wire Placement in Pelvic Trauma Surgery

Pelvic Kirschner wire (K-wire) insertion is a challenging surgical task requiring interpretation of complex 3D anatomical shape from 2D projections (fluoroscopy) and delivery of device trajectories within fairly narrow bone corridors in proximity to adjacent nerves and vessels. Over long trajectories (~10-25 cm), K-wires tend to curve (deform), making conventional rigid navigation inaccurate at the tip location. A system is presented that provides accurate 3D localization and guidance of rigid or deformable surgical devices ("components" - e.g., K-wires) based on 3D-2D registration. The patient is registered to a preoperative CT image by virtually projecting digitally reconstructed radiographs (DRRs) and matching to two or more intraoperative x-ray projections. The K-wire is localized using an analogous procedure matching DRRs of a deformably parametrized model for the device component (deformable known-component registration, or dKC-Reg). A cadaver study was performed in which a K-wire trajectory was delivered in the pelvis. The system demonstrated target registration error (TRE) of 2.1 ± 0.3 mm in location of the K-wire tip (median ± interquartile range, IQR) and 0.8 ± 1.4° in orientation at the tip (median ± IQR), providing functionality analogous to surgical tracking/navigation using imaging systems already in the surgical arsenal without reliance on a surgical tracker. The method offers quantitative 3D guidance using images (e.g., inlet/outlet views) already acquired in the standard of care, potentially extending the advantages of navigation to broader utilization in trauma surgery to improve surgical precision and safety.

C-arm Positioning Using Virtual Fluoroscopy for Image-Guided Surgery

INTRODUCTION

Fluoroscopically guided procedures often involve repeated acquisitions for C-arm positioning at the cost of radiation exposure and time in the operating room. A virtual fluoroscopy system is reported with the potential of reducing dose and time spent in C-arm positioning, utilizing three key advances: robust 3D-2D registration to a preoperative CT; real-time forward projection on GPU; and a motorized mobile C-arm with encoder feedback on C-arm orientation.

METHOD

Geometric calibration of the C-arm was performed offline in two rotational directions (orbit α, orbit β). Patient registration was performed using image-based 3D-2D registration with an initially acquired radiograph of the patient. This approach for patient registration eliminated the requirement for external tracking devices inside the operating room, allowing virtual fluoroscopy using commonly available systems in fluoroscopically guided procedures within standard surgical workflow. Geometric accuracy was evaluated in terms of projection distance error (PDE) in anatomical fiducials. A pilot study was conducted to evaluate the utility of virtual fluoroscopy to aid C-arm positioning in image guided surgery, assessing potential improvements in time, dose, and agreement between the virtual and desired view.

RESULTS

The overall geometric accuracy of DRRs in comparison to the actual radiographs at various C-arm positions was PDE (mean ± std) = 1.6 ± 1.1 mm. The conventional approach required on average 8.0 ± 4.5 radiographs spent "fluoro hunting" to obtain the desired view. Positioning accuracy improved from 2.6° ± 2.3° (in α) and 4.1° ± 5.1° (in β) in the conventional approach to 1.5° ± 1.3° and 1.8° ± 1.7°, respectively, with the virtual fluoroscopy approach.

CONCLUSION

Virtual fluoroscopy could improve accuracy of C-arm positioning and save time and radiation dose in the operating room. Such a system could be valuable to training of fluoroscopy technicians as well as intraoperative use in fluoroscopically guided procedures.

Medical student education in emergency medicine: do students meet the national standards for clinical encounters of selected core conditions?

BACKGROUND

Establishing a core curriculum for undergraduate Emergency Medicine (EM) education is crucial to development of the specialty. The Clerkship Directors in Emergency Medicine (CDEM) National Curriculum Task Force recommended that all students in a 4(th)-year EM clerkship be exposed to 10 emergent clinical conditions.

OBJECTIVES

To evaluate the feasibility of encountering recommended core conditions in a clinical setting during a 4(th)-year EM clerkship.

METHODS

Students from three institutions participated in this ongoing, prospective observation study. Students' patient logs were collected during 4-week EM clerkships between July 2011 and June 2012. De-identified logs were reviewed and the number of patient encounters for each of the CDEM-identified emergent conditions was recorded. The percentage of students who saw each of the core complaints was calculated, as was the average number of core complaints seen by each.

RESULTS

Data from 130 students at three institutions were captured; 15.4% of students saw all 10 conditions during their rotation, and 76.9% saw at least eight. The average number of conditions seen per student was 8.4 (range of 7.0-8.6). The percentage of students who saw each condition varied, ranging from 100% (chest pain and abdominal pain) to 31% (cardiac arrest).

CONCLUSIONS

Most students do not encounter all 10 conditions during patient encounters throughout a 4-week EM rotation, although most have exposure to at least eight. Certain conditions are far less likely than others to be encountered, and may need to be taught in a nonclinical setting.

Distractibility in schizophrenia

This study examined the effects of neuroleptic medication on the allocation of attentional resources to distracting stimuli in patients with schizophrenia. Twenty-five patients were tested twice (medication-free and after medication stabilization) on the Identical Pairs versions of the Continuous Performance Test under both distraction and no-distraction conditions. Sixteen patients were chronically ill adults and nine patients were young neuroleptic-native patients in the early stages of illness. Results indicated that neuroleptic treatment did not improve distractibility for either group and that both groups were comparably distractible. These findings suggest that medication does not improve the misallocation of attention to distracting stimuli in patients with schizophrenia.

Complications of laser therapy in the gynecological patient: a review of four patients

Complications of gynecological laser treatment for perineal disease have been seen with increasing frequency. This may be the result of more women undergoing therapy with this method. Four women presenting with unstable perineal scarring are discussed.

Needle localization in occult breast lesions

A series of 211 women underwent 211 preoperative needle localizations of nonpalpable breast lesions. All mammographic, operative, and pathological data were reviewed. Carcinoma occurred in 10%. Seventeen (81%) were invasive, and four (19%) were noninvasive. Forty-three percent of the cancers were minimal carcinoma (in situ or an invasive cancer measuring less than 5 mm in size with negative axillary nodes), and 48% were less than 1 cm. Overall, metastasis to axillary lymph nodes occurred in 10%. The mammographic findings of microcalcifications occurred in 81% of the malignant group and in 33% of the benign group. The combination of microcalcifications in association with an irregular density was only found in the malignant group. We conclude that 1) needle localization enables the early detection of stage I breast carcinoma and 2) the mammographic findings of microcalcifications in association with an irregular density occur only with malignancy in this series.