Clinical Implementation of Second-generation Minimally Invasive Image-guided Cochlear Implantation Surgery

OBJECTIVE

Minimally invasive, image-guided cochlear implantation (CI) surgery consists of drilling a precise tunnel from the surface of the mastoid cortex through the facial recess to target the scala tympani. In the first set of clinical trials of this technique, heat-induced facial nerve paresis (House-Brackmann II/VI) occurred on a patient on the last day of the initial trial which was scheduled to be halted secondary to a change in the regulatory requirements dictated by the 2012 the Food and Drug Administration Safety and Innovation Act requiring Investigational Device Exemption approval for previously exempted customized medical device testing. To address this adverse event, extensive changes were made to the drilling protocol; additionally, a custom insertion tool was developed. To address the Food and Drug Administration Safety and Innovation Act, an Investigational Device Exemption was submitted and, subsequently approved. Herein is described our first clinical implementation of the modified technique.

PATIENT

Seventy-year-old with profound, postlingual sensorineural hearing loss who had previously undergone right CI via traditional approach in 2015.

INTERVENTION

Minimally invasive image-guided left CI.

MAIN OUTCOME MEASURE

Time of intervention, final location of CI electrode array within cochlea.

RESULTS

Surgery took 155 minutes of which the largest components (in descending order) were soft tissue work, closure, and drilling. Full scala tympani insertion with angular insertion depth of 557 degrees of the electrode array was achieved. There were no complications, and the patient had an uneventful recovery and activation.

CONCLUSIONS

Minimally invasive, image-guided CI surgery is achievable and reduces the mastoid depression associated with traditional CI surgery.

CLINICALTRIALSGOV INFORMATION

Study NCT03101917, Microtable Microstereotactic Frame and Drill Press and Associated Method for Cochlear Implantation.

LEVEL OF EVIDENCE

Case Report.

Anatomical texture patterns identify cerebellar distinctions between essential tremor and Parkinson's disease

Voxel-based morphometry is an established technique to study focal structural brain differences in neurologic disease. More recently, texture-based analysis methods have enabled a pattern-based assessment of group differences, at the patch level rather than at the voxel level, allowing a more sensitive localization of structural differences between patient populations. In this study, we propose a texture-based approach to identify structural differences between the cerebellum of patients with Parkinson's disease (n = 280) and essential tremor (n = 109). We analyzed anatomical differences of the cerebellum among patients using two features: T1-weighted MRI intensity, and a texture-based similarity feature. Our results show anatomical differences between groups that are localized to the inferior part of the cerebellar cortex. Both the T1-weighted intensity and texture showed differences in lobules VIII and IX, vermis VIII and IX, and middle peduncle, but the texture analysis revealed additional differences in the dentate nucleus, lobules VI and VII, vermis VI and VII. This comparison emphasizes how T1-weighted intensity and texture-based methods can provide a complementary anatomical structure analysis. While texture-based similarity shows high sensitivity for gray matter differences, T1-weighted intensity shows sensitivity for the detection of white matter differences.

Thalamic arousal network disturbances in temporal lobe epilepsy and improvement after surgery

OBJECTIVE

The effects of temporal lobe epilepsy (TLE) on subcortical arousal structures remain incompletely understood. Here, we evaluate thalamic arousal network functional connectivity in TLE and examine changes after epilepsy surgery.

METHODS

We examined 26 adult patients with TLE and 26 matched control participants and used resting-state functional MRI (fMRI) to measure functional connectivity between the thalamus (entire thalamus and 19 bilateral thalamic nuclei) and both neocortex and brainstem ascending reticular activating system (ARAS) nuclei. Postoperative imaging was completed for 19 patients >1 year after surgery and compared with preoperative baseline.

RESULTS

Before surgery, patients with TLE demonstrated abnormal thalamo-occipital functional connectivity, losing the normal negative fMRI correlation between the intralaminar central lateral (CL) nucleus and medial occipital lobe seen in controls (p < 0.001, paired t-test). Patients also had abnormal connectivity between ARAS and CL, lower ipsilateral intrathalamic connectivity, and smaller ipsilateral thalamic volume compared with controls (p < 0.05 for each, paired t-tests). Abnormal brainstem-thalamic connectivity was associated with impaired visuospatial attention (ρ = -0.50, p = 0.02, Spearman's rho) while lower intrathalamic connectivity and volume were related to higher frequency of consciousness-sparing seizures (p < 0.02, Spearman's rho). After epilepsy surgery, patients with improved seizures showed partial recovery of thalamo-occipital and brainstem-thalamic connectivity, with values more closely resembling controls (p < 0.01 for each, analysis of variance).

CONCLUSIONS

Overall, patients with TLE demonstrate impaired connectivity in thalamic arousal networks that may be involved in visuospatial attention, but these disturbances may partially recover after successful epilepsy surgery. Thalamic arousal network dysfunction may contribute to morbidity in TLE.

Musical Sound Quality as a Function of the Number of Channels in Modern Cochlear Implant Recipients

Objectives

This study examined musical sound quality (SQ) in adult cochlear implant (CI) recipients. The study goals were to determine: the number of channels needed for high levels of musical SQ overall and by musical genre; the impact of device and patient factors on musical SQ ratings; and the relationship between musical SQ, speech recognition, and speech SQ to relate these findings to measures frequently used in clinical protocols.

Methods

Twenty-one post-lingually deafened adult CI recipients participated in this study. Electrode placement, including scalar location, average electrode-to-modiolus distance (M¯), and angular insertion depth were determined by CT imaging using validated CI position analysis algorithms (e.g., Noble et al., 2013; Zhao et al., 2018, 2019). CI programs were created using 4–22 electrodes with equal spatial distribution of active electrodes across the array. Speech recognition, speech SQ, music perception via a frequency discrimination task, and musical SQ were acutely assessed for all electrode conditions. Musical SQ was assessed using pre-selected musical excerpts from a variety of musical genres.

Results

CI recipients demonstrated continuous improvement in qualitative judgments of musical SQ with up to 10 active electrodes. Participants with straight electrodes placed in scala tympani (ST) and pre-curved electrodes with higher M¯ variance reported higher levels of musical SQ; however, this relationship is believed to be driven by levels of musical experience as well as the potential for preoperative bias in device selection. Participants reported significant increases in musical SQ beyond four channels for all musical genres examined in the current study except for Hip Hop/Rap. After musical experience outliers were removed, there was no relationship between musical experience or frequency discrimination ability and musical SQ ratings. There was a weak, but significant correlation between qualitative ratings for speech stimuli presented in quiet and in noise and musical SQ.

Conclusion

Modern CI recipients may need more channels for musical SQ than even required for asymptotic speech recognition or speech SQ. These findings may be used to provide clinical guidance for personalized expectations management of music appreciation depending on individual device and patient factors.

Cortical asymmetry in Parkinson's disease: early susceptibility of the left hemisphere

BACKGROUND AND PURPOSE

Clinically, Parkinson's disease (PD) presents with asymmetric motor symptoms. The left nigrostriatal system appears more susceptible to early degeneration than the right, and a left-lateralized pattern of early neuropathological changes is also described in several neurodegenerative conditions, including Alzheimer's disease, frontotemporal dementia, and Huntington's disease. In this study, we evaluated hemispheric differences in estimated rates of atrophy in a large, well-characterized cohort of PD patients.

METHODS

Our cohort included 205 PD patients who underwent clinical assessments and T1-weighted brain MRI's. Patients were classified into Early (n = 109) and Late stage (n = 96) based on disease duration, defined as greater than or less than 10 years of motor symptoms. Cortical thickness was determined using FreeSurfer, and a bootstrapped linear regression model was used to estimate differences in rates of atrophy between Early and Late patients.

RESULTS

Our results show that patients classified as Early stage exhibit a greater estimated rate of cortical atrophy in left frontal regions, especially the left insula and olfactory sulcus. This pattern was replicated in left-handed patients, and was not influenced by the degree of motor symptom asymmetry (i.e., left-sided predominant motor symptoms). Patients classified as Late stage exhibited greater atrophy in the bilateral occipital, and right hemisphere-predominant cortical areas.

CONCLUSIONS

We show that cortical degeneration in PD differs between cerebral hemispheres, and findings suggest a pattern of early left, and late right hemisphere with posterior cortical atrophy. Further investigation is warranted to elucidate the underlying mechanisms of this asymmetry and pathologic implications.

Overcoming Nonlinear Partial Volume Effects in Known-Component Reconstruction of Cochlear Implants

Nonlinear partial volume (NLPV) effects can be significant for objects with large attenuation differences and fine detail structures near the spatial resolution limits of a tomographic system. This is particularly true for small metal devices like cochlear implants. While traditional model-based approaches might alleviate these artifacts through very fine sampling of the image volume and subsampling of rays to each detector element, such solutions can be extremely burdensome in terms of memory and computational requirements. The work presented in this paper leverages the model-based approach called "known-component reconstruction" (KCR) where prior knowledge of a surgical device is integrated into the estimation. In KCR, the parameterization of the object separates the volume into an unknown background anatomy and a known component with unknown registration. Thus, one can model projections of an implant at very high spatial resolution while limiting the spatial resolution of the anatomy - in effect, modeling NLPV effects where they are most significant. We present modifications of the KCR approach that can be used to largely eliminate NLPV artifacts, and demonstrate the efficacy of the modified technique (with improved image quality and accurate implant position estimates) for the cochlear implant imaging scenario.

Deep brain stimulation in early stage Parkinson's disease: operative experience from a prospective randomised clinical trial

BACKGROUND

Recent evidence suggests that deep brain stimulation of the subthalamic nucleus (STN-DBS) may have a disease modifying effect in early Parkinson's disease (PD). A randomised, prospective study is underway to determine whether STN-DBS in early PD is safe and tolerable.

OBJECTIVES/METHODS

15 of 30 early PD patients were randomised to receive STN-DBS implants in an institutional review board approved protocol. Operative technique, location of DBS leads and perioperative adverse events are reported. Active contact used for stimulation in these patients was compared with 47 advanced PD patients undergoing an identical procedure by the same surgeon.

RESULTS

14 of the 15 patients did not sustain any long term (>3 months) complications from the surgery. One subject suffered a stroke resulting in mild cognitive changes and slight right arm and face weakness. The average optimal contact used in symptomatic treatment of early PD patients was: anterior -1.1±1.7 mm, lateral 10.7±1.7 mm and superior -3.3±2.5 mm (anterior and posterior commissure coordinates). This location is statistically no different (0.77 mm, p>0.05) than the optimal contact used in the treatment of 47 advanced PD patients.

CONCLUSIONS

The perioperative adverse events in this trial of subjects with early stage PD are comparable with those reported for STN-DBS in advanced PD. The active contact position used in early PD is not significantly different from that used in late stage disease. This is the first report of the operative experience from a randomised, surgical versus best medical therapy trial for the early treatment of PD.

Cadaveric Percutaneous CI with AB, Cochlear, and Medel Electrodes

Objective: 1) Describe advances made in percutaneous cochlear implantation (PCI) that involves a minimally invasive approach to the cochlea via a single, image-guided drill pass from the lateral cranium through the facial recess. 2) Demonstrate implementation of PCI in cadavers using Advanced Bionics (AB), Cochlear, and Medel electrodes.

Method: PCI was performed on 7 cadaveric temporal bones with 2 AB, 2 Cochlear, and 3 Medel electrodes. This included preoperative CT scanning and PCI trajectory planning, placement of bone-implanted fiducial markers, intraoperative CT, CT registration, creation and attachment of customized microstereotactic frame, drilling from cortex to cochlea, and CI insertion.

Results: PCI was successfully performed on all 7 specimens using the AB insertion tool for AB electrodes and via manually threading and endaural advancement for Cochlear and Medel electrodes. Postoperative electrode position was assessed by CT scanning and histopathology. CT correctly predicted 6 of the 7 electrodes to be in scala tympani and 1 in scala vestibuli. Histopathology revealed minimal insertion trauma with preservation of the osseous spiral lamina and basilar membrane and complete scala tympani insertion in 6 of 7 specimens. One specimen, due to violation of the basilar membrane at the round window cochleostomy, had complete scala vestibuli insertion.

Conclusion: This is the first description of complete PCI using AB, Cochlear, and Medel electrodes. Using various insertion techniques, atraumatic electrode insertion was achieved in the majority of specimens, similar to results clinically reported for traditional CI surgery. Building on this, we anticipate clinical implementation of PCI in the near future.

Automatic determination of optimal linear drilling trajectories for cochlear access accounting for drill-positioning error

BACKGROUND

Cochlear implantation is a surgical procedure in which an electrode array is permanently implanted into the cochlea to stimulate the auditory nerve and allow deaf people to hear. Percutaneous cochlear access, a new minimally invasive implantation approach, requires drilling a single linear channel from the skull surface to the cochlea. The focus of this paper addresses a major challenge with this approach, which is the ability to determine, in a pre-operative CT, a safe and effective drilling trajectory.

METHODS

A measure of the safety and effectiveness of a given trajectory relative to sensitive structures is derived using a Monte Carlo approach. The drilling trajectory that maximizes this measure is found using an optimization algorithm.

RESULTS

In tests on 13 ears, the technique was shown to find approximately twice as many acceptable trajectories as those found manually by an experienced surgeon.

CONCLUSIONS

Using this method, safe trajectories can be automatically determined quickly and consistently.

Anatomic verification of a novel method for precise intrascalar localization of cochlear implant electrodes in adult temporal bones using clinically available computed tomography

OBJECTIVES/HYPOTHESIS

We have previously described a novel, automated, nonrigid, model-based method for determining the intrascalar position of cochlear implant (CI) electrode arrays within human temporal bones using clinically available, flat-panel volume computed tomography (fpVCT). We sought to validate this method by correlating results with anatomic microdissection of CI arrays in cadaveric bones.

STUDY DESIGN

Basic science.

METHODS

Seven adult cadaveric temporal bones were imaged using fpVCT before and after electrode insertion. Using a statistical model of intracochlear anatomy, an active shape model optimization approach was employed to identify the scalae tympani and vestibuli on the preintervention fpVCT. The array position was estimated by identifying its midline on the postintervention scan and superimposing it onto the preintervention images using rigid registration. Specimens were then microdissected to demonstrate the actual array position.

RESULTS

Using microdissection as the standard for ascertaining electrode position, automatic identification of the basilar membrane coupled with postintervention fpVCT for electrode position identification accurately depicted the array location in all seven bones. In four specimens, the array remained within the scala tympani; in three, the basilar membrane was breached.

CONCLUSIONS

We have anatomically validated this automated method for predicting the intrascalar location of CI arrays using CT. Using this algorithm and pre- and postintervention CT, rapid feedback regarding implant location and expected audiologic outcomes could be obtained in clinical settings.

A method for assessing the microvasculature in a murine tumor model using contrast-enhanced ultrasonography

OBJECTIVE

The purpose of this study was to develop a method for assessing tumor vascularity in a preclinical model of breast cancer using contrast-enhanced ultrasonography.

METHODS

Eight mice were injected with 67NR breast cancer cells on their hind limbs and imaged with ultrasonography 8 days later. Mice were injected with an ultrasound contrast agent (UCA), and a sequence of images of the resultant backscattered echoes was recorded before and after high-power "destruction" pulses for each of multiple parallel planes. From these, data maps of the maximum contrast enhancement (within each time course) were constructed for each pixel, which enabled reconstruction of high-resolution coregistered sections into a 3-dimensional (3D) volume reflecting tumor vascularity. Additional studies were performed to determine the duration and repeatability of image enhancement, and images were correlated with conventional 3D power Doppler measurements.

RESULTS

The lifetime of the UCA in vivo was found to be 4.3 +/- 1.09 minutes (mean +/- SD). The 3D contrast-enhanced ultrasonographic technique produced images that correlated well with power Doppler images in specific regions but also depicted additional regions of flow surrounding the power Doppler signal. The mean correlation coefficient between voxel measurements of the central slice for each animal was 0.64 +/- 0.07 (P < .01). In addition, sequential studies in each animal were reproducible.

CONCLUSIONS

A method producing high-resolution volumetric assessments of tumor vascularity in a preclinical model of breast cancer is shown that correlates with other ultrasonographic measures of blood flow, which may provide greater sensitivity to the microvasculature.

Estimation of baseline dopamine D2 receptor occupancy in striatum and extrastriatal regions in humans with positron emission tomography with [18F] fallypride

BACKGROUND

This study examined whether positron emission tomography (PET) studies with [18F] fallypride performed before and after alpha-methyl-para-tyrosine (AMPT) administration can be used to estimate baseline dopamine (DA) D2 receptor occupancy in striatal and extrastriatal regions.

METHODS

Six normal subjects underwent PET with [18 F] fallypride before and after administration of AMPT. The DA D2 receptor binding potentials (bp) were calculated with the reference region method. Percent changes in bp in striatal and extrastriatal regions were calculated with both region-of-interest analysis and on a voxel by voxel basis with parametric images of DA D2 receptor levels.

RESULTS

The results of the current study indicate that AMPT treatment significantly increased the bp in the caudate, putamen, ventral striatum, and substantia nigra. A trend level increase was seen in the medial thalamus.

CONCLUSIONS

This study demonstrates that PET with [18F] fallypride can be used to estimate baseline DA D2 receptor occupancy in striatal and extrastriatal regions.

Occupancy of striatal and extrastriatal dopamine D2 receptors by clozapine and quetiapine

Clozapine and quetiapine have a low incidence of extrapyramidal side effects at clinically effective doses, which appears to be related to their significantly lower occupancy of striatal dopamine D2 receptors (DA D2r) compared to typical antipsychotic drugs (APDs). Animal studies have indicated that clozapine and quetiapine produce selective effects on cortical and limbic regions of the brain and in particular on dopaminergic neurotransmission in these regions. Previous PET and SPECT studies have reported conflicting results regarding whether clozapine produces preferential occupancy of cortical DA D2r. To examine whether clozapine and/or quetiapine produce preferential occupancy of DA D2r in cortex and limbic regions, we studied the occupancy of putamenal, ventral striatal, thalamic, amygdala, substantia nigra, and temporal cortical DA D2r using PET with [18F]fallypride in six schizophrenic subjects receiving clozapine monotherapy and in seven schizophrenic subjects receiving quetiapine monotherapy. Doses were chosen clinically to minimize psychopathology at tolerable levels of side effects such as drowsiness. All had minimal positive symptoms at the time of the study. Regional receptor occupancies were estimated using mean regional DA D2r levels calculated for 10 off-medication schizophrenic subjects. Both clozapine and quetiapine produced lower levels of putamenal DA D2r occupancy than those reported for typical APDs, 47.8 and 33.5%, respectively. Clozapine produced preferential occupancy of temporal cortical vs putamenal DA D2r, 59.8% (p=0.05, corrected for multiple comparisons), and significantly lower levels of occupancy in the substantia nigra, 18.4% (p=0.0015, corrected for multiple comparisons). Quetiapine also produced preferential occupancy of temporal cortical DA D2r, 46.9% (p=0.03, corrected for multiple comparisons), but did not spare occupancy of substantia nigra DA D2r. The therapeutic effects of clozapine and quetiapine appear to be achieved at less than the 65% threshold for occupancy seen with typical APDs, consistent with the involvement of non-DA D2r mechanisms in at least partially mediating the therapeutic effects of these drugs. Preferential occupancy of cortical DA D2r, sparing occupancy of substantia nigra receptors, and non-DA D2r-mediated actions may contribute to the antipsychotic actions of these and other atypical APDs.

Sex differences in amphetamine-induced displacement of [(18)F]fallypride in striatal and extrastriatal regions: a PET study

OBJECTIVE

The authors examined gender differences in d-amphetamine-induced displacements of [(18)F]fallypride in the striatal and extrastriatal brain regions and the correlations of these displacements with cognition and sensation seeking.

METHOD

Six women and seven men underwent positron emission tomography (PET) with [(18)F]fallypride before and after an oral dose of d-amphetamine. Percent displacements were calculated using regions of interest and parametric images of dopamine 2 (D(2)) receptor binding potential.

RESULTS

Parametric images of dopamine release suggest that the female subjects had greater dopamine release than the male subjects in the right globus pallidus and right inferior frontal gyrus. Gender differences were observed in correlations of changes in cognition and sensation seeking with regional dopamine release.

CONCLUSION

Findings revealed a greater dopamine release in women as well as gender differences in the relationship between regional dopamine release and sensation seeking and cognition.

Amphetamine-induced displacement of [18F] fallypride in striatum and extrastriatal regions in humans

This study examined D-amphetamine (D-AMPH)-induced displacements of [18F] fallypride in striatal and extrastriatal regions and the correlations of these displacements with cognition, affect, and sensation-seeking behavior. In all, 14 normal subjects, six females and eight males (ages 21-32, mean age 25.9 years), underwent positron emission tomography (PET) with [18F]fallypride before and 3 h after a 0.43 mg/kg oral dose of D-AMPH. Levels of dopamine (DA) D2 receptor density were calculated with the reference region method of Lammerstma. Percent displacements in striatal and extrastriatal regions were calculated for the caudate, putamen, ventral striatum, medial thalamus, amygdala, substantia nigra, and temporal cortex. Correlations of changes in cognition, affect, and sensation seeking with parametric images of D-AMPH-induced DA release were computed. Significant displacements were seen in the caudate, putamen, ventral striatum substantia nigra, and temporal cortex with a trend level change in the amygdala. Greatest displacements were seen in striatal subdivisions-5.6% in caudate, 11.2% in putamen, 7.2% in ventral striatum, and 6.6% in substantia nigra. Lesser decrements were seen in amygdala-4.4%, temporal cortex-3.7%, and thalamus-2.8%. Significant clusters of correlations of regional DA release with cognition and sensation-seeking behavior were observed. The current study demonstrates that [18F]fallypride PET studies using oral D-AMPH (0.43 mg/kg) can be used to study D-AMPH-induced DA release in the striatal and extrastriatal regions in humans, and their relationship with cognition and sensation-seeking behavior.

Occupancy of striatal and extrastriatal dopamine D2/D3 receptors by olanzapine and haloperidol

There have been conflicting reports as to whether olanzapine produces lower occupancy of striatal dopamine D(2)/D(3) receptor than typical antipsychotic drugs and preferential occupancy of extrastriatal dopamine D(2)/D(3) receptors. We performed [(18)F] fallypride PET studies in six schizophrenic subjects treated with olanzapine and six schizophrenic subjects treated with haloperidol to examine the occupancy of striatal and extrastriatal dopamine receptors by these antipsychotic drugs. [(18)F] setoperone PET studies were performed in seven olanzapine-treated subjects to determine 5-HT(2A) receptor occupancy. Occupancy of dopamine D(2)/D(3) receptors by olanzapine was not significantly different from that seen with haloperidol in the putamen, ventral striatum, medial thalamus, amygdala, or temporal cortex, that is, 67.5-78.2% occupancy; olanzapine produced no preferential occupancy of dopamine D(2)/D(3) receptors in the ventral striatum, medial thalamus, amygdala, or temporal cortex. There was, however, significantly lower occupancy of substantia nigra/VTA dopamine D(2)/D(3) receptors in olanzapine-treated compared to haloperidol-treated subjects, that is, 40.2 vs 59.3% (p=0.0014, corrected for multiple comparisons); in olanzapine-treated subjects, the substantia nigra/VTA was the only region with significantly lower dopamine D(2)/D(3) receptor occupancy than the putamen, that is, 40.2 vs 69.2% (p<0.001, corrected for multiple comparison). Occupancy of 5-HT(2A) receptors was 85-93% in the olanzapine- treated subjects. The results of this study demonstrated that olanzapine does not produce preferential occupancy of extrastriatal dopamine D(2)/D(3) receptors but does spare substantia nigra/VTA receptors. Sparing of substantia nigra/VTA dopamine D(2)/D(3) receptor occupancy may contribute to the low incidence of extrapyramidal side effects in olanzapine-treated patients.

Phantom validation of coregistration of PET and CT for image-guided radiotherapy

Radiotherapy treatment planning integrating positron emission tomography (PET) and computerized tomography (CT) is rapidly gaining acceptance in the clinical setting. Although hybrid systems are available, often the planning CT is acquired on a dedicated system separate from the PET scanner. A limiting factor to using PET data becomes the accuracy of the CT/PET registration. In this work, we use phantom and patient validation to demonstrate a general method for assessing the accuracy of CT/PET image registration and apply it to two multi-modality image registration programs. An IAEA (International Atomic Energy Association) brain phantom and an anthropomorphic head phantom were used. Internal volumes and externally mounted fiducial markers were filled with CT contrast and 18F-fluorodeoxyglucose (FDG). CT, PET emission, and PET transmission images were acquired and registered using two different image registration algorithms. CT/PET Fusion (GE Medical Systems, Milwaukee, WI) is commercially available and uses a semi-automated initial step followed by manual adjustment. Automatic Mutual Information-based Registration (AMIR), developed at our institution, is fully automated and exhibits no variation between repeated registrations. Registration was performed using distinct phantom structures; assessment of accuracy was determined from registration of the calculated centroids of a set of fiducial markers. By comparing structure-based registration with fiducial-based registration, target registration error (TRE) was computed at each point in a three-dimensional (3D) grid that spans the image volume. Identical methods were also applied to patient data to assess CT/PET registration accuracy. Accuracy was calculated as the mean with standard deviation of the TRE for every point in the 3D grid. Overall TRE values for the IAEA brain phantom are: CT/PET Fusion = 1.71 +/- 0.62 mm, AMIR = 1.13 +/- 0.53 mm; overall TRE values for the anthropomorphic head phantom are: CT/PET Fusion = 1.66 +/- 0.53 mm, AMIR = 1.15 +/- 0.48 mm. Precision (repeatability by a single user) measured for CT/PET Fusion: IAEA phantom = 1.59 +/- 0.67 mm and anthropomorphic head phantom = 1.63 +/- 0.52 mm. (AMIR has exact precision and so no measurements are necessary.) One sample patient demonstrated the following accuracy results: CT/PET Fusion = 3.89 +/- 1.61 mm, AMIR = 2.86 +/- 0.60 mm. Semi-automatic and automatic image registration methods may be used to facilitate incorporation of PET data into radiotherapy treatment planning in relatively rigid anatomic sites, such as head and neck. The overall accuracies in phantom and patient images are < 2 mm and < 4 mm, respectively, using either registration algorithm. Registration accuracy may decrease, however, as distance from the initial registration points (CT/PET fusion) or center of the image (AMIR) increases. Additional information provided by PET may improve dose coverage to active tumor subregions and hence tumor control. This study shows that the accuracy obtained by image registration with these two methods is well suited for image-guided radiotherapy.

Topological median filters

This paper describes the definition and testing of a new type of median filter for images. The topological median filter implements some existing ideas and some new ideas on fuzzy connectedness to improve, over a conventional median filter, the extraction of edges in noise. The concept of alpha-connectivity is defined and used to create an algorithm for computing the degree of connectedness of a pixel to all the other pixels in an arbitrary neighborhood. The resulting connectivity map of the neighborhood effectively disconnects peaks in the neighborhood that are separated from the center pixel by a valley in the brightness topology. The median of the connectivity map is an estimate of the median of the peak or plateau to which the center pixel belongs. Unlike the conventional median filter, the topological median is relatively unaffected by disconnected features in the neighborhood of the center pixel. Four topological median filters are defined. Qualitative and statistical analyses of the four filters are presented. It is demonstrated that edge detection can be more accurate on topologically median filtered images than on conventionally median filtered images.

Distribution of vascular resistance in terminal arteriolar networks of cat sartorius muscle

Morphometric information on the terminal arteriolar networks (n = 10) in cat sartorius muscle [Koller et al., Am. J. Physiol. 253 (Heart Circ. Physiol. 22): H154-H164, 1987] is utilized in the calculations of distribution of vascular hindrance throughout the networks. These networks have tree-type geometry, i.e., they do not contain closed loops. The results are discussed in terms of simulated flow distribution. The flow calculations are based on the exact geometry of the arteriolar networks (the control and dilated diameter and the length of each vascular segment) and on assumed values of postarteriolar resistances. Three cases of postarteriolar resistances are considered: zero, constant, and randomly distributed. With zero postarteriolar resistances, the distribution of flow in the terminal arteriolar segments would be highly heterogenous. The simulated flow in each terminal segment is determined primarily by the number of bifurcations on the pathway leading to the terminal segment, with a slight compensation for the length of the pathways. The coefficient of variation of flow in the control state, CV(Qc), would be close to the value in the dilated state, CV(Qd). When each of the terminal segments is connected to a constant postarteriolar resistance, the CV's in both states decrease. The coefficient of variation in the dilated state becomes significantly smaller than in the control state. When postarteriolar resistances are randomly distributed, both CV's increase, and their values become closer to each other. These results suggest that postarteriolar resistances may play a very important role in distribution of flow in the microvascular network. This study formulates a framework for the quantification of the effect of arteriolar dilation on flow redistribution in the network.

Analysis of vascular pattern and dimensions in arteriolar networks of the retractor muscle in young hamsters

A quantitative analysis of the distribution of microvascular blood flow and oxygen delivery requires a detailed description of the vascular network geometry. The distributions of lengths and diameters were determined in terminal arteriolar networks of the cheek pouch retractor muscle of young (34 +/- 2 days) hamsters. We compared the Strahler centripetal vessel ordering scheme, which assigns lowest order to the capillaries and proceeds upstream toward the larger vessels, with the centrifugal ordering scheme, which begins with the input arteriole and proceeds downstream toward the capillaries. The terminal networks of the retractor muscle typically contain 2 to 4 Strahler orders and 2 to 6 centrifugal orders. The coefficients of variation of diameter and length are smaller for Strahler ordering than for centrifugal ordering. In addition, for Strahler ordering, we found that the sequence of number of vessels obeyed Horton's law. We have compared three different methods of calculating the bifurcation, diameter, and length ratios. As an alternative method for analyzing network topology, we also studied the distribution of the number of segments on each pathway from the inlet of a network to a capillary. The information obtained from this analysis is useful for the mathematical modeling of flow in the microvascular network.

Quantitative analysis of arteriolar network architecture in cat sartorius muscle

The geometry of the arteriolar network is one of the major determinants of blood flow distribution within a tissue. The purpose of this study was to describe the distribution of geometrical variables (lengths, diameters) as well as the pattern of branching in the nonarcading portion of the arteriolar network in skeletal muscle. The exteriorized cat sartorius muscle was used as the experimental model. The intravascular fluorescence of fluorescein isothiocyanate (FITC)-labeled Dextran 150 was observed with a low-light-level video camera, and the vascular networks were mapped. Arteriolar lengths and diameters were measured, and vessel position in the network was characterized by Strahler's method of ordering, in which the first-order arterioles give rise to most of the capillaries. Typically, the nonarcading, terminal networks contain three or four arteriolar orders. The sequences of the number of vessels, mean diameter, and mean length for each order are accurately described by geometric progressions (Horton's law). The distribution of diameters within each order was rather narrow: typically two-thirds of the vessels fell within 20% of the mean value. The spread was reduced by half when vessels within a single network were considered. During vasodilation to a standard stimulus the relative dispersion of diameters increased modestly. The distribution of vessel lengths was broader than for diameters. Two-thirds of vessels of a single order fell within 50-75% of the mean. The spread was less within individual networks. The variability of vessel geometry and branching patterns was substantially less within a single network than for a population drawn from a group of networks.

Effect of dispersion of vessel diameters and lengths in stochastic networks. II. Modeling of microvascular hematocrit distribution

A microvascular network model described in the preceding paper (B. Dawant, M. Levin, and A. S. Popel, 1986, Microvasc. Res. 31, 203-222) featuring random distribution of vessel diameters and lengths, is extended to calculate the distributions of red blood cell flux and discharge hematocrit throughout the network. A relationship between red blood cell fluxes and blood flow rates at vascular bifurcations is incorporated into the model, and the effect of the form of this relationship on RBC distribution is investigated. The mean capillary discharge hematocrit is sensitive to the variation of parameters describing this relationship; thus detailed experimental information is required for bifurcations of different sizes and types.

Effect of dispersion of vessel diameters and lengths in stochastic networks. I. Modeling of microcirculatory flow

A microvascular network model is proposed with random arrangement and random dimensions of vessels. In addition to stochasticity of the topological characteristics of the model networks, as previously introduced by Fenton and Zweifach (1981, Ann. Biomed. Eng., 9, 303-321), the vessel diameters and lengths are treated as random variables following certain probability distributions for each vascular order. Flow and pressure distributions are calculated for each network configuration assuming a linear relationship between the blood flow rate and pressure drop for each vascular segment. The mean, coefficient of variation, skewness, kurtosis, and histograms of the hemodynamic variables are computed using an ensemble of random networks. The results indicate that dispersion of vessel diameters and lengths may significantly affect the distributions of microvascular variables such as capillary flow and pressure, and the flow distribution at bifurcations. It is shown that the dispersion of vessel diameters causes a decrease of total flow whereas the dispersion of lengths causes its increase.