Maximum-likelihood parameter estimation in terahertz time-domain spectroscopy.

We present a maximum-likelihood method for parameter estimation in terahertz time-domain spectroscopy. We derive the likelihood function for a parameterized frequency response function, given a pair of time-domain waveforms with known time-dependent noise amplitudes. The method provides parameter estimates that are superior to other commonly used methods and provides a reliable measure of the goodness of fit. We also develop a simple noise model that is parameterized by three dominant sources and derive the likelihood function for their amplitudes in terms of a set of repeated waveform measurements. We demonstrate the method with applications to material characterization.

Simultaneous composition and thickness measurement of paper using terahertz time-domain spectroscopy.

We present a noncontact method for quantitative composition and thickness monitoring of flat sheet products using terahertz time-domain spectroscopy. We apply the method to obtain simultaneous measurement of thickness and moisture content of paper sheets. The paper is modeled as an effective medium of water mixed with fibers, and model parameters are estimated from fits to the measured transmission amplitude. We demonstrate the method on two different paper samples and obtain uncertainties that are comparable with existing sensor technology. Monte Carlo simulations indicate that these uncertainties can be reduced further by at least an order of magnitude.

Volumetric quantification of cement leakage following percutaneous vertebroplasty in metastatic and osteoporotic vertebrae.

OBJECT: The goal of this study was to quantify volumetrically cement fill and leakage in patients with osteoporotic and metastatic vertebral lesions undergoing percutaneous vertebroplasty and to establish whether these factors have any clinical significance at follow up. METHODS: Digital computerized tomography data were retrospectively collected from all cases at the authors' institution in which percutaneous vertebroplasty was performed for osteoporosis or metastatic disease. Patient selection was based on the consensus of a multidisciplinary team consisting of an orthopedic surgeon, an oncologist, and a neuroradiologist. A semiautomated thresholding technique was used to measure vertebral body volume, the volume of cement injected directly into the vertebra, and the volume of cement leakage. Pain-related scores were collected at four early stages of treatment, and all clinical complications were recorded. Cement leakage was found in 87.9% of vertebrae treated with percutaneous vertebroplasty. In osteoporotic vertebrae it occurred mainly in the disc, whereas in metastatic lesions, it was found in multiple areas. Irrespective of leakage, both patients with osteoporotic and metastatic disease experienced significant immediate pain relief postoperatively. CONCLUSIONS: Although there was no correlation between cement fill or cement leakage and pain relief, there exists a risk of serious complications due to cement leakage.

The effect of pre-vertebroplasty tumor ablation using laser-induced thermotherapy on biomechanical stability and cement fill in the metastatic spine.

A biomechanical study comparing simulated lytic vertebral metastases treated with laser-induced thermotherapy (LITT) and vertebroplasty versus vertebroplasty alone. To investigate the effect of tumor ablation using LITT prior to vertebroplasty on biomechanical stability and cement fill patterns in a standardized model of spinal metastatic disease. Vertebroplasty in the metastatic spine is aimed at reducing pain, but is associated with risk of cement extravasation in up to 10%. Six pairs of fresh-frozen cadaveric thoracolumbar spinal motion segments were tested in axial compression intact, with simulated metastases and following percutaneous vertebroplasty with or without LITT. Canal narrowing under load, pattern of cement fill, load to failure, and LITT temperature and pressure generation were collected. In all LITT specimens, cement filled the defect without extravasation. The canal extravasation rate was 33% in specimens treated without LITT. LITT and vertebroplasty yielded a trend toward improved posterior wall stability (P = 0.095) as compared to vertebroplasty alone. Moderate rises in temperature and minimal pressure generation was seen during LITT. In this model, elimination of tumor by LITT, facilitates cement fill, enhances biomechanical stability and reduces the risk of cement extravasation.

Stability of the metastatic spine pre and post vertebroplasty.

OBJECTIVE: Vertebrae with lytic metastases have an elevated risk of burst fracture and resultant neurologic compromise. Prophylactic vertebroplasty has the potential to reduce pain and the risk of burst fracture in the metastatic spine. The purpose of this study was to quantify the ability of vertebroplasty to stabilize metastatically involved vertebrae against the risk of burst fracture initiation with a standardized model of vertebral metastases. METHODS: Metastases were simulated in eight fresh-frozen cadaveric thoracolumbar spinal motion segments by removing a central core of trabecular bone and filling the defect with tumor tissue. Specimens were tested under a physiologic level of axial compression, intact, with a simulated tumor and post-vertebroplasty, and ultimately tested to failure. Axial load induced canal narrowing (CN) was used as a measure of the risk of burst fracture initiation. Following testing, vertebrae were axially sectioned to visualize cement fill. RESULTS: Vertebrae with simulated metastases exhibited significantly higher CN than intact specimens (227%+/-109%; P<0.05). Post vertebroplasty, three vertebrae exhibited reduced CN compared with the simulated tumor configuration, whereas the other five had increased CN. Specimens with reduced CN were found to have cement posterior to the tumor, whereas specimens with an increase in CN had cement anterior and lateral to the tumor only. Percutaneous vertebroplasty is effective in decreasing CN if tumor is surrounded posteriorly with cement. However, injecting cement into the posterior third of the vertebral body is risky due to potential extravasation into the canal. CONCLUSION: Future work aimed at improving cement fill is necessary for safe and consistent stabilization of the metastatic spine with vertebroplasty.

Metastatic burst fracture risk prediction using biomechanically based equations.

Clinical guidelines are a useful adjunct to select patients with spinal metastases for prophylactic intervention. The objective of this study is to determine the ability of biomechanically based models to accurately predict metastatic burst fracture risk. Ninety-two vertebrae with osteolytic spinal metastases were examined retrospectively. Vertebrae were categorized as burst fractured, wedge fractured, or intact and analyzed using three predictive models: vertebral bulge (maximum radial displacement under load), vertebral axial displacement (maximum axial displacement under load), and a volumetric estimate of tumor size. The load-bearing capacity parameter (tumor volume, bone mineral density, disc quality, pedicle involvement) was determined from computed tomography while the load-bearing requirement parameter (pressure load, loading rate) was determined using computed tomography and patient records (retrieved for 37 patients [52%]). Fracture prediction was optimized using the vertebral bulge model considering only load-bearing capacity with a specificity, sensitivity, and confidence interval of 1 to yield a clear threshold for burst fracture risk. Fracture prediction in the other two models, vertebral axial displacement considering only load-bearing capacity and tumor size, also was strong with receiver-operator curve values of 0.992 and 0.988, respectively. The predictive power of these models can provide useful clinical information for prophylactic decision-making.

A biomechanical analysis of intravertebral pressures during vertebroplasty of cadaveric spines with and without simulated metastases.

STUDY DESIGN: A biomechanical cadaveric study of thoracic and lumbar vertebrae with simulated metastases quantifying intravertebral pressures during transpedicular vertebroplasty. OBJECTIVE: To compare intravertebral pressures during percutaneous vertebroplasty in vertebrae with and without simulated lytic metastases. SUMMARY OF BACKGROUND DATA: Percutaneous vertebroplasty is designed to provide stability to vertebrae weakened by osteoporosis or metastatic disease. The complication rate is higher when the procedure is used for the treatment of lytic vertebral lesions. The major complications reported are radiculopathy, spinal cord compression, and embolic phenomena. METHODS: Ten fresh-frozen cadaveric vertebrae were tested intact (7 lumbar, 3 thoracic) and 7 were tested with simulated lytic defects (4 lumbar, 3 thoracic). Defects were created by replacing a core of cancellous bone with soft tumor tissue in the center of the vertebral body. Simplex P (Howmedica Osteonics, Mahwah, NJ) cement was injected into each vertebra through a unipedicular approach at a constant rate of 3 mL per minute. Cement volume, injection force, and intravertebral pressures at the posterior vertebral body wall were recorded. Following the procedure, the vertebrae were sectioned to visualize cement and tumor disbursement. RESULTS: There was no significant difference between the two groups for age, size, trabecular density, and cement volume. Vertebrae with simulated metastases generated an average maximum pressure of 39.66 kPa during cement injection versus 6.83 kPa in intact vertebrae (P < 0.05). Higher pressures were also generated in smaller vertebrae based on a power relationship (r2 = 0.71 intact, r2 = 0.43 tumor). CONCLUSIONS: Percutaneous vertebroplasty produces higher intravertebral pressures in vertebrae containing a simulated lytic metastasis than in intact vertebrae. Pressures generated in the tumor specimens are sufficiently elevated to cause embolic phenomena.