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PubMed2004

[Comparison of high resolution whole-body MRI using parallel imaging and PET-CT. First experiences with a 32-channel MRI system].

PURPOSE: To compare the accuracy in the detection and staging of various malignant tumors with high resolution whole-body MRI using parallel imaging with whole-body dual-modality PET-CT. PATIENTS AND METHODS: Preliminary results of an interim analysis from a prospective, blinded study are presented, in which 20 patients (mean age 59 years, range 27-77 years) with different oncological diseases underwent whole-body dual modality FDG-PET-CT screening for tumor search or staging in case of confirmed or suspected metastatic disease. All patients also underwent whole-body MRI imaging with the use of parallel imaging (iPAT). High-resolution coronal T1w- and STIR-sequences of 5 body levels with 512 x 512 matrix, axial fast T2w imaging of lung and abdomen (HASTE), contrast-enhanced dynamic and static T1w-sequences of liver, brain, abdomen, and pelvis were performed. Using a 32-channel whole-body MRI scanner (Magnetom Avanto, Siemens Medical Solutions) with a total field of view of 205 cm and free table movement, all patients could be covered from head to toe within one examination. With this technique, high spatial resolution and acceptable scanning times could be obtained. Two experienced radiologists read the MRI-scans, one radiologist and one nuclear scientist read PET-CT scans, each in consensus in a clinical setting. Delineation of the primary tumor (T-stage) or recurrent tumor, pathologic lymph node involvement, as well as degree and localization of metastatic disease, was assessed using PET-CT as standard of reference. RESULTS: Metastases from gastrointestinal tumor (25%) and breast cancer (25%), genitourinary tumor (15%) and malignant melanoma (15%) were detected. In 4/20 patients the primary tumor was identified, 2/20 patients showed recurrent tumor. Of 140 malignant lesions detected by PET-CT, 124 lesions were detected with MRI, resulting in a sensitivity of 89% at a specificity of 86%. In malignant lymph node detection, sensitivity of MRI was 83% and specificity 85%. CONCLUSION: Whole-body MRI is a promising technique in the detection of primary tumor and metastatic disease. Sensitivity in the assessment of lymph node metastases seems to be limited. With the use of parallel imaging (iPAT), dedicated high-resolution whole-body MRI is possible within acceptable scanning times.

Adult↗

PubMed2004

Image resolution and magnification using a cone beam densitometer: optimizing data acquisition for hip morphometric analysis.

Bone mineral density (BMD) is a primary determinant of hip fracture risk. However, other factors, notably the femoral geometry, can influence hip fracture risk. The purpose of this study was to evaluate the potential of a new cone beam densitometer, the DMS Lexxos, in order to visualise femoral morphometry. Resolution, magnification and distortion were assessed in vitro using a line pair test pattern and a matrix test object. Results were given in comparison with currently available systems: the Hologic Discovery A and the Lunar Prodigy densitometers. The DMS Lexxos image resolution was the same in the longitudinal and transversal directions evaluated between 1.4 and 0.5 line pairs/mm (lps/mm) for an attenuation varying from 25 to 325 mm of Perplex. The longitudinal resolution was evaluated between 0.9 and 0.5 lps/mm with the Hologic Discovery densitometer, and inferior to 0.5 with the Lunar Prodigy; as for transversal resolution, it varied from 0.63 to 0.5 lps/mm and from 0.6 to inferior 0.5 lps/mm, respectively. The image was isotropic without magnification with the GE-Lunar Prodigy, whereas there was only a transversal magnification with the Hologic Discovery device. The magnification was about 1.17% cm(-1 )in the two directions, while increasing the distance of the phantom above the examination table with the Lexxos. This magnification was isotropic without distortion. The magnification could be evaluated from two images taken before and after translation of the C-arm, and a magnification correction could be applied. This method was applied to a phantom and to a human cadaver femoral bone.

Absorptiometry, Photon↗

PubMed2005

Feasibility of in vivo structural analysis of high-resolution magnetic resonance images of the proximal femur.

Previously, high resolution MRI to assess bone structure of deep-seated regions of the skeleton such as the proximal femur was substantially limited by signal-to-noise ratio (SNR). With the advent of new optimized pulse sequences in MRI at 1.5 T and 3 T, it may now be possible to depict and quantify the trabecular microarchitecture in the proximal femur. The purpose of this study was to investigate the feasibility of assessing trabecular microstructure of the human proximal femur in vivo with MR imaging at 1.5 T and 3 T. MR images of six young, healthy male and female subjects were acquired using standard clinical 1.5-T and high-field 3-T whole-body MR scanners. Using a T2/T1-weighted 3D FIESTA sequence (and a 3D FIESTA-C sequence at 3 T to avoid susceptibility artifacts) a resolution of 0.234 x 0.234 x 1.5 mm(3) was achieved in vivo. Structural parameters analogous to standard bone histomorphometry were determined in femoral head and trochanter regions of interest. Bone mineral density (BMD) measurements were also obtained using dual-energy X-ray absorptiometry (DXA) for the femoral trochanter in the same subjects. The bone structure of the proximal femur is substantially better depicted at 3 T than at 1.5 T. Correlation between the structural parameters obtained at both field strengths was up to R =0.86 for both the femoral head and the trochanteric region. However, the resolution of the images limits the application of 3D structural analysis, making the assessment more akin to 2D textural measures, which may be correlated to histomorphometric but are not identical measures. This feasibility study establishes the potential of MRI as a means of imaging proximal femur structure, and improvements in technique and resolution enhancements are warranted.

Adult↗

PubMed1998

Image-based assessment of spinal trabecular bone structure from high-resolution CT images.

The goal of this study was to assess whether a high-resolution CT measure of trabecular bone structure can enhance the discrimination between subjects with or without a vertebral fracture and having overall low hip or spine bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA). Sixty-one women with low BMD by DXA (T-score < -2.5 at hip or spine) were examined. Twenty women had sustained a vertebral fracture. Quantitative CT (QCT) BMD and high-resolution CT spinal scans were performed on a whole-body CT scanner. For the high-resolution images (0.31 mm pixel, 1.5 mm thick slice), trabecular bone was segmented from marrow using an adaptive threshold, region growth and skeletonization step. From the processed image we measured the apparent trabecular bone fraction (BV/TV), apparent trabecular thickness (I.Th) and apparent trabecular spacing (I.Sp). We also assessed the connectivity of the marrow space using region growing to derive a mean (HA) and maximum (HM) hole size. Despite the fact that the study population was preselected to have a low BMD by DXA, QCT BMD was highly associated with (p < 0.005) with fracture status. All structural parameters were correlated (r approximately 0.64 to 0.79) with BMD with p < 0.003 and showed significant differences between the fracture and non-fracture group. However, except for HA, this difference did not remain significant after adjustment for BMD. When BMD and then HA was entered into a paired linear regression model to predict fracture outcome, HA contributed with p = 0.03 and BMD with p = 0.86. ROC analysis was applied and showed that HA, BMD, I.Th and I.Sp discriminated the two groups with areas of 0.76, 0.75, 0.71 and 0.68, respectively. These findings suggest that an assessment of vertebral trabecular structure from high-resolution CT images is useful in discriminating subjects with vertebral fractures and potentially useful for predicting future fractures.

Aged↗

PubMed1999

High-resolution computed tomography for architectural characterization of human lumbar cancellous bone: relationships with histomorphometry and biomechanics.

The aim of the present study on human vertebral cancellous bone was to validate structural parameters measured with high-resolution (150 microm) computed tomography (HRCT) by referring to histomorphometry and to try to predict mechanical properties of bone using HRCT. Two adjacent vertical cores were removed from the central part of human L2 vertebral body taken after necropsy in 22 subjects aged 47-95 years (10 women, 12 men; mean age 79 +/- 14 years). The right core was used for structural analysis performed by both HRCT and histomorphometry. Two cancellous bone specimens were extracted from the left core: a cube for HRCT and a compression test, and a cylinder for a shear test. Significant correlations were found between HRCT and histomorphometric measurements (BV/TV, trabecular thickness, separation and number, and node-strut analysis), but with higher values for most of the tomographic parameters (BV/TV and trabecular thickness determined by HRCT were overestimated by a factor 3.5 and 2.5 respectively, as compared with histomorphometry). The maximum compressive strength and Young's modulus were highly correlated (rho = 0.99, p<0.0005). Significant correlation was obtained between bone mineral density (determined using dual-energy X-ray absorptiometry) and the maximum compressive strength (rho = 0. 64, p = 0.002). In addition the maximum compressive strength and architectural parameters determined by HRCT or histomorphometry showed significant correlations (e.g., for HRCT, BV/TV: rho = 0.88, p<0.0005, N.Nd/TV: rho = 0.73, p<0.001). The shear strength was significantly correlated with BV/TV (rho = 0.62, p = 0.002), Tb.Sp (rho = -0.58, p = 0.004) and TSL (rho = 0.55, p = 0.006) measured by HRCT. In conclusion, an HRCT system with 150 microm resolution is not sufficient to predict the true values of the structural parameters measured by histomorphometry, although high correlations were found between the two methods. However, we showed that a resolution of 150 microm allowed us to predict the mechanical properties of human cancellous bone. In vivo peripheral systems with such a resolution should be of interest and would deliver an acceptable radiation dose to the patient.

Aged↗

PubMed1995

High resolution depth profile analysis by elastic recoil detection with heavy ions.

Elastic recoil detection (ERD) with energetic heavy ions (e.g. 60-120 MeV(127)I) is a suitable method to measure depth profiles of light and medium heavy elements in thin films. The advantages of this method are reliable and quantitative results and elementally and isotopically resolved depth profiles. A relative energy resolution of 0.07% has been measured in real ERD-experiments using the Q3D magnetic spectrograph at the Munich tandem accelerator and a large solid angle of detection of 5 msr. The good energy resolution allows atomic depth resolution near to the surface which has been obtained at flat and smooth carbon samples. A large solid angle of detection is necessary to measure a depth profile with the desired accuracy before the sample is significantly altered by the ion beam. As an example carbon profiles of thin carbon layers, prepared by a laser plasma ablation deposition process, have been investigated revealing the high depth resolution and its power to resolve elemental profiles at gradiated interfaces.

Journal Article↗

PubMed2005

Compartmental bone morphometry in the mouse femur: reproducibility and resolution dependence of microtomographic measurements.

Microcomputed tomography (microCT) is widely used for nondestructive bone phenotyping in small animals, especially in the mouse. Here, we investigated the reproducibility and resolution dependence of microCT analysis of microstructural parameters in three different compartments in the mouse femur. Reproducibility was assessed with respect to precision error (PE%CV) and intraclass correlation coefficient (ICC). We examined 14 left femurs isolated postmortem from two strains of mice (seven per group). Measurements and analyses were repeated five times on different days. In a second step, analysis was repeated again five times for a single measurement. Resolution dependence was assessed by high-resolution measurements (10 microm) in one strain and subsequent image degrading. Reproducibility was better in full bone compartment and in cortical bone compartment in the diaphysis (PE%CV = 0.06-2.16%) than in trabecular compartment in the distal metaphysis (PE(%CV) = 0.59-5.24%). Nevertheless, ICC (0.92-1.00) showed a very high reliability of the assessed parameters in all regions, indicating very small variances within repeated measurements compared to the population variances. Morphometric indices computed from lower- and higher-resolution images displayed in general only weak dependence and were highly correlated with each other (R2 = 0.91-0.99). The results show that parameters in the full and cortical compartments were very reproducible, whereas precision in the trabecular compartment was somewhat lower. Nevertheless, all compartmental analysis methods were very robust, as shown by the high ICC values, demonstrating high suitability for application in inbred strains, where highest precision is needed due to small population variances.

Animals↗

PubMed2003

Navigated diffusion-weighted imaging with interpolated phase-correction for high-resolution imaging of stroke.

Stroke imaging was revolutionised with the introduction of diffusion-weighted MRI (DWI). The commonly used echoplanar DWI suffers from geometrical distortion near the skull base and the frontal regions and from reduced spatial resolution and fat suppression. To allow a voxel-by-voxel comparison between high-resolution spin-echo images, we implemented spin-echo-based DWI. Motion artefacts were eliminated by phase correction in hybrid frequency-Fourier domain using navigator echoes. In a novel approach, distorted navigator echoes which did not eliminate motion artefacts were replaced with interpolated navigator echoes, leading to restored image information. Navigated DWI yielded high-resolution images in 21 of 24 patients with brain ischaemia, allowing diagnosis of even small or diffuse zones of ischaemia. We determined the spatial distribution and mean of T(2)- and DWI signal intensity and apparent diffusion coefficients (ADC), using a multidimensional histogram-based analysis. Mean ADC were decreased in ischaemic areas less than 9 days old. The technique may also be useful for high-resolution DWI of tissue other than the brain.

Aged↗

PubMed2006

Resolution of cortical lesions on serial renal scans in children with acute pyelonephritis.

BACKGROUND: The ideal time for distinguishing a renal scar from acute inflammatory lesions by renal DMSA scintigraphy remains controversial. OBJECTIVE: To determine the time needed for resolution of lesions after acute pyelonephritis. MATERIALS AND METHODS: A total of 105 children with acute pyelonephritis underwent renal sonography, voiding cystourethrography and baseline DMSA scintigraphy. Two subsequent scans were performed during the 6th and 12th months in patients with abnormal findings on the previous scan. RESULTS: The baseline DMSA scintigraphy revealed cortical lesions in 37 patients. At 6 months, 13 patients (38.2%) were found to have cortical lesions. At 12 months, 6 patients (17.6%) were found to have persistent renal cortical lesions. The resolution rates for lesions detected on the first scan were 61.8% and 82.4% on the 6- and 12-month scans, respectively. Vesicoureteric reflux, and bilaterality or multifocality were not relevant for resolution of lesions. Female gender seemed to be associated with a higher persistence rate. CONCLUSIONS: The renal cortical defects present at 6 months have a high rate of resolution later during follow-up. DMSA scintigraphy performed 12 months after the infection provides more reliable data regarding persistence of renal cortical lesions.

Acute Disease↗

PubMed2006

High-resolution MR imaging of the proximal zone of the lunotriquetral ligament with a microscopy coil.

OBJECTIVE: To evaluate high-resolution MRI of the proximal zone of the lunotriquetral ligament (LTL) using a microscopy surface coil with a 1.5 T scanner. DESIGN AND SUBJECTS: The proximal zone of the LTL was reviewed in 90 subjects (23 asymptomatic normal volunteers and 67 patients with suspicion of triangular fibrocartilage complex injury) with high-resolution MRI using a 47-mm microscopy surface coil. High-resolution MR images were obtained with gradient recalled echo (GRE) T2*-weighted sequence and short tau inversion recovery imaging, with a 1- to 1.5-mm slice thickness, a 50-mm field of view, an imaging matrix of 140-224x512 using zero fill interpolation, and 3-4 excitations. As a qualitative analysis, the LTL was classified in shape and signal intensity. RESULTS: The triangle-shaped low-signal-intensity LTL was identified in 77 of 90 subjects (85.6%) on GRE images. The triangle was classified as regular (41.1%), broad-based (20.0%), narrow-based (6.7%), or asymmetrical (17.8%). The bar-shaped ligament was seen in one patient, and unclassified ligaments were seen in 12 patients. All volunteers showed triangle-shaped LTL. The MR signal intensity of the proximal zone in the LTL was characterized as homogeneously low intensity (type 1; 33.8%), linear intermediate or high signal intensity traversing the distal surface of the LTL (type 2; 45.5%), and linear intermediate or high intensity traversing both distal and proximal surfaces of LTL (type 3; 20.8%). CONCLUSION: The proximal zone of the LTL showed a broad spectrum of normal variations in shape and signal intensity on high-resolution MR images with a microscopy coil.

Adolescent↗

PubMed2002

Occupancy of dopamine D2 receptors in the mouse brain measured using ultra-high-resolution single-photon emission tomography and [123]IBF.

Functional imaging of small animals, such as mice and rats, using ultra-high-resolution positron emission tomography (PET) and single-photon emission tomography (SPET) should be a valuable tool in studies of drug occupancy of cerebral binding sites. In this study we aimed to demonstrate the feasibility of using ultra-high-resolution SPET to measure the occupancy of dopamine D2 receptors by a competing drug, using the dopamine D2 receptor-specific radioligand iodine-123 5-iodo-7-N-[(1-ethyl-2-pyrrolidinyl) methyl] carboxamido-2,3-dihydrobenzofuran ([123I]IBF). Fourteen normal male mice (CD-1) were jugular vein-cannulated and a bolus infusion protocol was used to deliver 360 MBq [123I]IBF into the mouse (bolus-to-infusion ratio 1.8:1). The mice were scanned using an ultra-high-resolution triple-headed SPET system equipped with pinhole collimators. After sustained equilibrium had been achieved, varying doses of raclopride, a potent dopamine D2 receptor antagonist, were injected through the tail vein and the tracer was allowed to regain equilibrium. A simple equilibrium ratio of striatum to cerebellum provided a measure of D2 receptor binding both before and after injection of raclopride. Following raclopride administration, the system returned to equilibrium with lower specific binding in the striatum, while the counts in the cerebellum were unaffected. Receptor occupancy was 5.2% +/- 2.9% (control), 52.1% +/- 11.1% (0.3 mg/kg), 79.3% +/- 4.8% (1.0 mg/kg), and 94.7% +/- 2.2% (3.0 mg/kg), which gave an ED50=0.26 +/- 0.03 mg/kg using a single receptor site saturation model. This study has demonstrated clearly that ultra-high-resolution SPET of small animals is capable of measuring displacement and occupancy of dopamine D2 receptors by competing ligands.

Animals↗

PubMed2006

Resolution recovery in pinhole SPECT based on multi-ray projections: a phantom study.

PURPOSE: Low sensitivity can become a major problem when very small pinholes are used in SPECT imaging. Although a larger pinhole aperture will improve the sensitivity, this will be at the cost of the spatial resolution. With a view to improving the resolution-sensitivity trade-off, this paper explores an iterative reconstruction algorithm that models the pinhole aperture based on multi-ray projections. METHODS: This new implementation was validated using simulated data and phantom experiments. Two approaches were investigated. Firstly, the pinhole aperture was modelled in both the forward and the back projector. Secondly, the dual matrix implementation was investigated by modelling the pinhole aperture only in the forward projector. The systematic error, the full-width at half-maximum (FWHM) and the statistical error were quantified using the simulated data. Experimental phantom data were acquired for visual comparison with the reconstructions obtained from the simulated data. RESULTS: For a predefined number of iterations, the systematic error, the FWHM and the statistical error could be decreased when the pinhole aperture was modelled during iterative reconstruction. For a fixed, predefined statistical error of +/-10%, smaller systematic errors and smaller FWHM were obtained when modelling the pinhole opening. When the dual matrix implementation was used, equivalent results could be obtained as when modelling the pinhole opening in both the forward and the back projector. CONCLUSION: The multi-ray method to accomplish resolution recovery during the reconstruction of pinhole SPECT projection images offers a better trade-off between spatial resolution and noise compared with a reconstruction which does not model the pinhole aperture.

Algorithms↗

PubMed2007

The natural history and traditional management of appendicitis revisited: spontaneous resolution and predominance of prehospital perforations imply that a correct diagnosis is more important than an early diagnosis.

BACKGROUND: The principle of early exploration on wide indications in order to prevent perforation has been the guiding star for the management of patients with suspected appendicitis for over 100 years, dating back to a time when appendicitis was a significant cause of mortality. Since then there has been a dramatic decrease in mortality due to appendicitis. Emerging evidence calls for a new understanding of the natural history of untreated appendicitis. This motivates a reappraisal of the fundamental principles for the management of patients with suspected appendicitis. METHODS: Analysis of epidemiologic and clinical studies that elucidate the natural history of appendicitis, i.e. the possibility of spontaneous resolution or the risk of progression to perforation, the determinants of the proportion of perforations and mortality, and the consequence of in-hospital delay. RESULTS: The results presented in a number of studies suggest that spontaneous resolution of appendicitis is common, that perforation can seldom be prevented, that the risk of perforation has been exaggerated and that in-hospital delay is safe. An alternative understanding of the inverse relationship between the proportion of negative explorations and perforation and the increasing proportion of perforation with length of time is presented, mainly explaining these findings by selection due to spontaneous resolution. CONCLUSION: Evidence suggests that spontaneous resolution of untreated, non-perforated appendicitis is common and that perforation can rarely be prevented and is associated with a lower increase in mortality than was previously thought. This motivates a shift in focus from the prevention of perforation to the early detection and treatment of advanced appendicitis. In order to minimize mortality, morbidity and costs avoidance of negative appendectomies is more important then preventing perforation. In patients with an equivocal diagnosis where advanced appendicitis is deemed less likely a correct diagnosis is more important than a rapid diagnosis. These patients can safely be managed by active observation with an improved diagnostic work-up under observation, which has consistently shown a low proportion of negative appendectomies without an increase in the proportion of perforations or morbidity. A high proportion of perforations can be explained by selection due to undiagnosed resolving appendicitis. The proportion of perforation is therefore a questionable measure of the quality of the management of patients with suspected appendicitis and should be used with caution.

Appendectomy↗

PubMed2006

Ultra-high resolution flat-panel volume CT: fundamental principles, design architecture, and system characterization.

Digital flat-panel-based volume CT (VCT) represents a unique design capable of ultra-high spatial resolution, direct volumetric imaging, and dynamic CT scanning. This innovation, when fully developed, has the promise of opening a unique window on human anatomy and physiology. For example, the volumetric coverage offered by this technology enables us to observe the perfusion of an entire organ, such as the brain, liver, or kidney, tomographically (e.g., after a transplant or ischemic event). By virtue of its higher resolution, one can directly visualize the trabecular structure of bone. This paper describes the basic design architecture of VCT. Three key technical challenges, viz., scatter correction, dynamic range extension, and temporal resolution improvement, must be addressed for successful implementation of a VCT scanner. How these issues are solved in a VCT prototype and the modifications necessary to enable ultra-high resolution volumetric scanning are described. The fundamental principles of scatter correction and dose reduction are illustrated with the help of an actual prototype. The image quality metrics of this prototype are characterized and compared with a multi-detector CT (MDCT).

Algorithms↗

PubMed2001

Lymphocytic interstitial pneumonia in children with AIDS: high-resolution CT findings.

Pulmonary involvement in children with acquired immunodeficiency syndrome (AIDS) represents a wide spectrum of diseases. Among the non-infectious, non-neoplastic affections associated with AIDS, lymphocytic interstitial pneumonia (LIP) is now a well-recognized entity, but its radiological pattern studied with high-resolution computed tomography (HRCT) has rarely been described in children. The aim of this study was to illustrate the HRCT spectrum of pulmonary involvement in children with LIP and to evaluate its usefulness in the early diagnosis of this entity. Twelve children with AIDS, aged 3-9 years (mean age 5 years 7 months), underwent chest radiographs and HRCT. A control group of 7 healthy aged-matched children was also studied in the same conditions. Diagnosis of LIP was based on clinical data and HRCT findings. Eight children of 12 had a reticulonodular pattern on chest radiographs. Two children had normal chest films and two children showed peribronchiolar thickening. High-resolution CT displayed micronodules, 1-3 mm in diameter, with a perilymphatic distribution in all patients. High-resolution CT demonstrated also subpleural nodules in children without reticulonodular opacities on chest radiographs. High-resolution CT is able to define a more specific pattern of abnormalities than conventional chest radiographs in children with LIP, allows an earlier and more confident diagnosis and may be useful for the detection of other pathologies associated with AIDS, such as opportunistic infections or superimposed malignancies.

Acquired Immunodeficiency Syndrome↗

PubMed1998

Magnification survey and spot view mammography with a new microfocus X-ray unit: detail resolution and radiation exposure.

The aim of our study was to evaluate a mammography unit capable of magnification of up to fourfold at an equivalent or lower dose than with current systems. A prototype mammography tube with an electron-beam-focusing technology resulting in a focal spot size of 40-120 micro m was combined with a highly intensifying screen-film system. To evaluate contrast-detail resolution, phantom radiographs were performed with the prototype magnification mammography system using a magnification factor of 1.7 for survey views and a magnification factor of 4.0 for spot views. They were compared with unmagnified survey views and magnification spot views (magnification factor 1.9) of a state-of-the-art mammography system. The radiation exposure was measured and mean glandular doses were calculated. The contrast-detail resolution with both prototype (m = 1.7) and conventional (m = 1.1) survey views was equivalent while the entrance dose and the mean glandular dose were approximately 50 % lower with the prototype. For spot views, the contrast-detail resolution was substantially higher for the prototype than for conventional magnification while the dose was equivalent. Dose reduction and improved detail resolution are possible with this new technology.

Acrylic Resins↗

PubMed1999

In vivo imaging of the human zonular apparatus with high-resolution ultrasound biomicroscopy.

BACKGROUND: To investigate the potential of high-resolution ultrasound biomicroscopy (UBM) for studying the zonular apparatus of human beings in vivo. METHODS: Using transducer frequencies of 34 MHz and 50 MHz, criteria were developed to identify transcorneal and transscleral sections that allowed reproducible identification of the different fiber groups of the zonular architecture. For that purpose, 10 volunteers between the ages of 14 and 41 years underwent high-resolution ultrasound biomicroscopy under conditions of consensual far- and near-accommodation. The online video recordings of the respective UBM investigations were afterwards analyzed image by image. Good visibility of zonular fibers was obtained when the ultrasound wave propagation comprised an angle close to 90 degrees with the fiber orientation and when the oscillations of the UBM scan had a strict radial orientation towards the limbus and avoided, simultaneously, the ciliary processes. RESULTS: In all the volunteers, high-resolution ultrasound biomicroscopy imaged the zonular fiber groups known from histology. In addition, it detected fibers that do not follow the course of the inner ciliary body surface but take a direct route from the ora serrata to the lens. It also demonstrated that fibers that seem to change direction at crossings with other fibers. Under conditions of near-accommodation, the zonular fibers showed signs of relaxation. CONCLUSIONS: High-resolution ultrasound biomicroscopy seems well suited for in vivo investigations of the zonular apparatus and of accommodation in man. The results support the fundamental features of the Helmholtz theory on accommodation.

Accommodation, Ocular↗

PubMed2000

Resolution-improved dual-beam and standard optical coherence tomography: a comparison.

BACKGROUND: The purpose of the study was to demonstrate the improved axial resolution and longitudinal stability of dual-beam optical coherence tomography (OCT) in comparison to conventional OCT setups used in commercially available OCT instruments. METHODS: The conventional OCT technique is based on an interferometric setup that is rather sensitive to axial eye motions. We have developed a special dual-beam OCT technique which eliminates the influence of axial eye motions. This is achieved by using the anterior corneal surface as the reference surface for the interferometric ranging. To improve the signal quality, the different wavefront curvatures of beams reflected at cornea and retina are matched by a diffractive optical element. To improve the axial resolution, a broadband synthesized light source with an effective bandwidth of 50 nm is used, and the group dispersion of the ocular media is compensated. Tomographic images were recorded in the fovea and the optic nerve head of healthy volunteers. For comparison purposes, approximately the same locations in the same eyes were imaged by a commercially available OCT instrument. RESULTS: Compared to the standard OCT technique, the dual-beam OCT images show considerably improved axial resolution. Especially in tomograms recorded at the fovea, dual-beam OCT resolves microstructural details that are not visible in the standard OCT images. Furthermore, the axial stability of dual-beam OCT enables the recording of exact geometrical contours of fundus layers. CONCLUSIONS: Dual-beam OCT is able to provide structural information on the ocular fundus that is not obtained with standard OCT. The long recording times of our instrument limit the transverse resolution to 100-150 microm at present.

Fovea Centralis↗