Sequential Acquisitions Research Articles

A Review on Recent Developments in Positron Emission Tomography Technology

Recent years have witnessed an increased use of in vivo imaging techniques in both medicine and biomedical research. Positron Emission Tomography (PET) is a functional in vivo imaging technique relying on the metabolism of molecules which are injected into the blood stream and have a high chemical affinity to a specific biological substrate. These substances are labelled with positron emitters resulting in gamma rays which can be recorded outside the body using coincidence detection. Radiation can be detected using a variety of solutions, in terms of transducers and system geometries, and combined to produce images using suitable image reconstruction algorithms. The applications of PET imaging (e.g. cancer, brain function, cardiac function and gene expression) are currently benefiting from technological developments leading to the proposal of new PET systems. These are more sensitive, dedicated to imaging specific organs or research animals and are becoming multimodal. These advances are fostering new uses of PET, for example in biology labs to study the onset, progression and therapy of disease or to image the female breast. New materials and equipment are also, being developed allowing combining PET with other imaging techniques, such as MRI. The purpose of this review is to illustrate the most important inventions patented in PET since 2006 in areas concerning data acquisition and processing for human and research use of PET. Finally the manuscript will present the authors views about forthcoming developments in PET. Keywords: PET, PET developments, multimodality imaging, time of flight PET, dedicated PET systems, Positron Emission Tomography, in vivo imaging techniques, chemical affinity, Time-of-Flight PET, MRI, radiopharmaceuticals, PET-CT systems, sequential acquisitions, simultaneous acquisitions, scintillation light, split-magnet solution

Fast whole-body magnetic resonance angiography in mice

High-throughput magnetic resonance imaging (MRI) tools are required for the longitudinal investigation of vascular diseases in mouse models. Angiographic data from various anatomic regions may be needed in a single experiment. This study involves a three-dimensional (3D) time-of-flight (TOF) magnetic resonance angiography (MRA) method using sequential acquisitions of four data sets corresponding to the head, the thorax, the abdomen, and the hind limbs of a mouse. After repositioning the animal, each anatomic region was acquired in 2 min, and the TOF effect was provided by the spatial selectivity of the radio frequency (RF) resonator. No slab selection was needed and whole-body MRA was performed in a total experiment time of 10 min. The voxel size was equal to or greater than 131 × 195 × 188 μm(3). To suppress the signal arising from stationary tissues, both inversion recovery and interspersed saturation, used as magnetization preparations, were compared from a theoretical and an experimental perspective. The arterial tree (carotid, aortic, iliac, renal, and smaller arteries) was well visualized by this method, both in control healthy mice and in mice with common carotid artery ligation. The potential interest of this method for evaluating arterial diseases is discussed.

Test-Retest Variability of Volumetric Right Ventricular Measurements Using Real-Time Three-Dimensional Echocardiography

Substantial variability in sequential echocardiographic right ventricular (RV) quantification may exist. Interobserver and intraobserver values are well known, but acquisition (test-retest) variability has been rarely assessed. The objective of this study was to determine the test-retest variability of sequential RV volume and ejection fraction (EF) measurements by real-time three-dimensional echocardiography in patients with congenital heart disease and healthy controls. Twenty-eight participants (21 patients with congenital heart disease, seven healthy controls; mean age, 30 ± 14 years; 43% men) underwent a series of three echocardiographic studies. To obtain interobserver and intraobserver test-retest variability, two sonographers acquired sequential RV data sets in each participant during one outpatient visit. RV volumetric quantification was done using semiautomated three-dimensional border detection. The variability data were analyzed using correlation coefficients, Bland-Altman analysis, and coefficients of variation. Absolute mean differences for sequential intraobserver acquisitions were 12 ± 12 mL for end-diastolic volume, 7 ± 6 mL for end-systolic volume, and 4 ± 3% for EF. Interobserver and intraobserver test-retest variability, respectively, were 7% and 7% for RV end-diastolic volume, 14% and 7% for end-systolic volume, and 8% and 6% for EF. Good test-retest variability, besides the practical nature of real-time three-dimensional echocardiography for RV volume and EF assessment, makes it a valuable technique for serial follow-up. Although it may be challenging to diminish all factors that can influence echocardiographic examination for serial follow-up, standardization of RV size and functional measurements should be a goal to produce more interchangeable data.

024 Validation with thallium 201 of a new cadmium-zinc-telluride (CZT) cardiac camera

Novel dedicated ultrafast cardiac cameras had recently be introduced to improve myocardial perfusion imaging. We report the first validation study with thallium-201 and the new GE Discovery NM 530c CZT cardiac camera. We prospectively studied with thallium-201 153 consecutive patients referred for myocardial perfusion imaging at exercise (111 to 148 MBq) then redistribution (with a 37 MBq reinjection). We performed sequential acquisitions, first with conventional dual head tomographic Anger camera (CC) in 11 to 16 mn, then with CZT camera (CZT) in 5 mn, in prone then in supine position. – we excluded 7 patients: 1 for mispositionning, 1 for camera failure, 1 for too late acquisition after exercise, 1 for non acceptation of redistribution by the patient, 1 for motion of the patient. – acquisition was more comfortable with CZT for all patients. – counts rate was 3 times more with CZT than with CC (3.5 to 5 KCts/s vs 1 to 1.5). – myocardial counts rate was 6 to 8 times more with CZT than with CC. – comparison between CZT and CC: quality of CZT images was considered as better in 38%, equal in 59% and worse in 3% of cases; diagnostic conclusions were the same in 137 of 146 cases (94%); discordances were 2 artifacts with CC, 1 artifact with CZT, 4 early redistributions and 2 discordances about reversibility of thallium defect. – comparison between prone and supine with CZT: quality of prone images was considered as better in 41%, equal in 34% and worse in 25%; there were 2 times more artifacts in supine (14%) than in prone (7%) (p = 0.05). With thallium-201, the GE Discovery NM 530c CZT cardiac camera allows 5 minutes acquisitions with an increased image quality and a reliable diagnosis quality, both in prone and in supine positions.

Persistent performance and interaction effects in sequential cross-border mergers and acquisitions

This study examines the short-term wealth impact of cross-border mergers and acquisitions (M&As) by acquiring companies in 70 countries between the years of 1978 and 2008. We find persistent stock performance in sequential cross-border M&As; that is, the acquiring firms that gained positive (negative) abnormal returns in previous cross-border acquisitions are more likely to experience positive (negative) abnormal returns in subsequent cross-border acquisitions. The persistency of the stock performance is stronger in situations when the elapsed time between the sequential acquisitions is shorter. We find that the persistent stock performance is affected by investor sentiment as well as the choice of cash payments in the sequential cross-border M&As. We do not find that the acquiring firm's operating performance affects the persistency of the stock performance.

Robust Super-Resolution Volume Reconstruction From Slice Acquisitions: Application to Fetal Brain MRI

Fast magnetic resonance imaging slice acquisition techniques such as single shot fast spin echo are routinely used in the presence of uncontrollable motion. These techniques are widely used for fetal magnetic resonance imaging (MRI) and MRI of moving subjects and organs. Although high-quality slices are frequently acquired by these techniques, inter-slice motion leads to severe motion artifacts that are apparent in out-of-plane views. Slice sequential acquisitions do not enable 3-D volume representation. In this study, we have developed a novel technique based on a slice acquisition model, which enables the reconstruction of a volumetric image from multiple-scan slice acquisitions. The super-resolution volume reconstruction is formulated as an inverse problem of finding the underlying structure generating the acquired slices. We have developed a robust M-estimation solution which minimizes a robust error norm function between the model-generated slices and the acquired slices. The accuracy and robustness of this novel technique has been quantitatively assessed through simulations with digital brain phantom images as well as high-resolution newborn images. We also report here successful application of our new technique for the reconstruction of volumetric fetal brain MRI from clinically acquired data.

Evaluation of dynamic contrast‐enhanced MRI in detecting renal scarring in a rat injury model

To create a reliable rat model with small renal cortical scars and evaluate the accuracy and sensitivity of dynamic contrast-enhanced MRI in detecting the kinds of lesions that are associated with reflux nephropathy. In 16 rats, three unilateral renal cortical lesions were created using either electrocautery or pure alcohol with the contralateral kidney serving as control. MRI on a 1.5 Tesla GE Signa was performed 10-14 days after surgery. After bolus injection of 0.2 mM/Kg Gd-DTPA, sequential MRI acquisitions were performed using a 4-inch quadrature birdcage coil. Renal and scar volumes and pathology were compared after scanning and killing. Of the 48 points of injury, 40 (83%) in the 16 rats were detected grossly. Under microscopy, 36 injuries (75%) were detected on mid-kidney cross-sections. The average lesion was 4.2 mm(3) corresponding to 0.5% of the kidney volume. Using pathological findings as the gold standard, the sensitivity and specificity of scar detection using MRI was 69% and 93%, respectively. A rat model was created to demonstrate the sensitivity of dynamic contrast-enhanced MRI for detecting renal scars. Alcohol and electrocautery created reliable renal scars that were confirmed pathologically. MRI detected these lesions that averaged 4.2 mm(3) (0.5% total renal volume) with sensitivity and specificity of 69% and 93%, respectively.

Semi‐quantitative tracking of intra‐airway fluids by computed tomography

Airway secretions are a source of complications for patients with acute and chronic lung diseases, yet lack of techniques to quantitatively track secretions hampers research into clinical measures to reduce their pathologic consequences. In a preserved swine lung model, we tracked a contrasted mucus simulant (CMS) using sequential computed tomography (CT). Known drivers of secretion movement - gravity and ventilation - were tested. Ten millilitres of CMS were unilaterally introduced (1 ml min(-1)) into the airways of 12 lung sets. After instillation, six lung sets were maintained prone and six were rotated 180 degrees . Subsequently, all were mechanically ventilated for 10 min. CTs were obtained before infusion, after infusion and after ventilation +/- rotation. For CT analysis, the lungs were partitioned into eight sub-cuboids using anatomic landmarks. The volumes of two CT number ranges representing CMS and poor aeration/collapse were computed in every sub-cuboid for each CT acquisition. Volume differences between study time points were used to quantify changes. CMS and poor aeration/collapse volume change distributed gravitationally after infusion. After ventilation without rotation, the CMS and poor aeration/collapse volumes remained within the originally injected sub-cuboid, although the poor aeration/collapse volume expanded (27.3 +/- 6.1-->50.5 +/- 7.4 ml, P<0.05). After ventilation + rotation, there was a reduction in the CMS and poor/aeration collapse volumes in the originally injected sub-cuboid (14.4 +/- 1.7-->4.4 +/- 0.6 ml, P<0.05 and 18.3 +/- 3.8-->11.9 +/- 2.7 ml, P<0.05, respectively) accompanied by increases in the gravitationally opposite sub-cuboid (1.7 +/- 0.2-->11.1 +/- 1.1 ml, P<0.05 and 0.8 +/- 0.5-->40.6 +/- 3.5 ml, P<0.05, respectively). Movement of fluids within the bronchial tree can be semi-quantitatively tracked with analysis of sequential CT acquisitions. In this isolated swine lung model, gravity had an important and brisk effect on movement of a viscous fluid, whereas ventilation tended to embed it peripherally.

A real options perspective on sequential acquisitions in China

We examine how firms in an emerging market address their information disadvantage in takeovers. One strategy is to complete the acquisition sequentially rather than as a one-time deal. In contrast to firms in developed economies that enjoy relatively easy access to information, emerging-market firms face differential access to information on target firms, owing to institutional weaknesses and constraints in those markets. Sequential acquisitions are conceptualized as a real-option-based strategy, whereby the sequential acquirer resolves valuation uncertainty through information gathering and learning after making a toehold purchase. The value of a sequential acquisition strategy increases for firms that are at an information disadvantage. Statistical analysis, based on 272 completed acquisitions of Chinese listed firms between 1995 and 2003, shows that the sequential method is more likely to be chosen over a single-transaction mode when the acquirer is a private firm, or when the acquirer is diversifying into a new business area. Thus we offer a new way of viewing real option strategies in emerging economies, by treating sequential acquisitions as information and learning mechanisms as opposed to tools for hedging.

Functional Role of HSP90 Complexes with Endothelial Nitric-oxide Synthase (eNOS) and Calpain on Nitric Oxide Generation in Endothelial Cells

Although several reports have indicated that eNOS is a highly sensitive calpain substrate, the occurrence of a concomitant Ca(2+)-dependent activation of the synthase and of the protease has never been analyzed in specific direct experiments. In this study, we have explored in vivo how eNOS can undergo Ca(2+)-dependent translocation and activation, protected against degradation by activated calpain. Here we demonstrate that following a brief exposure to Ca(2+)-loading, the cytosolic eNOS-HSP90 complex recruits calpain in a form in which the chaperone and the synthase are almost completely resistant to digestion by the protease. Furthermore, in the presence of the HSP90 inhibitor geldanamycin, a significant decrease in NO production and an extensive degradation of eNOS protein occurs, indicating that dissociation from membranes and association with the chaperone is correlated to the protection of the synthase. Experiments with isolated membrane preparations confirm the primary role of HSP90 in dissociation of eNOS from caveolae. Prolonged exposure of cells to Ca(2+)-loading resulted in an extensive degradation of both eNOS and HSP90, accompanied by a large suppression of NO production. We propose that the protective effect exerted by HSP90 on eNOS degradation mediated by calpain represents a novel and critical mechanism that assures the reversibility of the intracellular trafficking and activation of the synthase.

Comparing Statistical and Neural Network Methods Applied to Very High Resolution Satellite Images Showing Changes in Man-Made Structures at Rocky Flats

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Parametric and nonparametric approaches to evaluate land-cover change detection using very high resolution (VHR) satellite imagery are applied to the analysis of the demolition of the Rocky Flats nuclear weapons facility located near Denver, CO. Both maximum-likelihood and neural network classifiers are used to validate a new parallel architecture which improves the accuracy when applied to VHR satellite imagery for the study of land-cover change between sequential satellite acquisitions. An enhancement of about 14% was found between the single-step classification and the new parallel architecture, confirming the advantage and the robust improvement obtained with this architecture regardless of the classification algorithm used. In this paper, we demonstrate and document the demolition and removal of hundreds of buildings taken down to bare soil between 2003 and 2005 at the Rocky Flats site. </para>

Differential Evanescence Nanometry: Live-Cell Fluorescence Measurements with 10-nm Axial Resolution on the Plasma Membrane

We present a method to resolve components within a diffraction-limited object by tracking simultaneously the average axial positions of two different sets of fluorescent molecules within it. The axial positions are then subtracted from each other to determine the separation of the two sets of fluorophores. This method follows the dynamic changes in the separation of the two sets of fluorophores with freely rotating dipoles using sequential acquisitions with total internal reflection and wide-field illumination, and it can be used to measure the formation of small structures on living cells. We have verified that we can achieve a resolution of 10 nm, and we have used the method to follow the location of clathrin and its adaptor AP-2 as they are recruited to a diffraction-limited coated pit during its assembly at the plasma membrane. We find a gradually increasing axial separation between the centroids of clathrin and AP-2 distribution, up to a final value of 30 nm just before coated-pit pinching and formation of the coated vesicle.

Optimizing the number of equivalent iterations of 3D OSEM in SPECT reconstruction of I-131 focal activities

To externally estimate the radiation dose to a tumor during therapy with I-131 radiopharmaceuticals, and its distribution, one must accurately estimate activity, and its distribution, by means of SPECT imaging. Our objective is to characterize the quantification of the total activity in focal targets and in their uniform background, and of the activity distribution within the targets, after 3D Ordered Subsets Expectation Maximization (OSEM) reconstruction with attenuation and scatter correction and no post smoothing, in the good-counting-statistics case. A cylindrical phantom containing seven spheres simulating tumors, ranging in volume from 209 to 4.2 cm 3, and filled with an I-131 water solution containing background, was imaged. A Siemens Symbia SPECT/CT scanner was used to acquire 128×128 projection images, employing 60 angles over 360°. With dynamic SPECT, 10 sequential acquisitions of 15 min duration each were obtained and each was reconstructed with particular values of the number of subsets and the number of iterations. Let the product of the number of subsets times the number of iterations equal the equivalent number of iterations, EI. The counts-to-activity conversion factor was derived from the average ratio of total count divided by true activity for the largest sphere at the largest value of EI. Then, for the activity of each sphere at each of the values of EI, we evaluated (1) the fractional variance (variance in estimate over true activity), (2) the fractional bias (average estimate bias over true activity) and (3) the fractional error (the root mean square error (RMSE) in the estimate divided by the true activity). The fractional bias and fractional variance were smaller for the larger spheres compared to the smaller (the fractional bias decreased faster with an increase in the fractional variance for them as well). The RMSE was dominated by the bias. The fractional error decreased as EI increased for all sphere sizes. The minimum average value was 25.8% and occurred for EI=480 although that image had texture and was noisy. The average fractional error was already approaching that value by EI=80 at 30.7%, and that image was less noisy. For the water background surrounding the spheres, the fractional error in the activity estimate varied but little with EI; its smallest value was 13.1% at EI=480. The error in a measure of estimated sphere activity non-uniformity worsened as EI increased. (The OSEM initial guess of identical counts/voxel everywhere was perfect as far as uniformity for these uniform-activity targets.) Therefore, the EI to employ in reconstruction depends on the activity-estimation task of interest. Different dosimetric goals might best be met by employing different images generated at different values of EI.

Acute tumor vascular effects following fractionated radiotherapy in human lung cancer: In vivo whole tumor assessment using volumetric perfusion computed tomography

To quantitatively assess the in vivo acute vascular effects of fractionated radiotherapy for human non-small-cell lung cancer using volumetric perfusion computed tomography (CT). Sixteen patients with advanced non-small-cell lung cancer, undergoing palliative radiotherapy delivering 27 Gy in 6 fractions over 3 weeks, were scanned before treatment, and after the second (9 Gy), fourth (18 Gy), and sixth (27 Gy) radiation fraction. Using 16-detector CT, multiple sequential volumetric acquisitions were acquired after intravenous contrast agent injection. Measurements of vascular blood volume and permeability for the whole tumor volume were obtained. Vascular changes at the tumor periphery and center were also measured. At baseline, lung tumor vascularity was spatially heterogeneous with the tumor rim showing a higher vascular blood volume and permeability than the center. After the second, fourth, and sixth fractions of radiotherapy, vascular blood volume increased by 31.6% (paired t test, p = 0.10), 49.3% (p = 0.034), and 44.6% (p = 0.0012) respectively at the tumor rim, and 16.4% (p = 0.29), 19.9% (p = 0.029), and 4.0% (p = 0.0050) respectively at the center of the tumor. After the second, fourth, and sixth fractions of radiotherapy, vessel permeability increased by 18.4% (p = 0.022), 44.8% (p = 0.0048), and 20.5% (p = 0.25) at the tumor rim. The increase in permeability at the tumor center was not significant after radiotherapy. Fractionated radiotherapy increases tumor vascular blood volume and permeability in human non-small-cell lung cancer. We have established the spatial distribution of vascular changes after radiotherapy; greater vascular changes were demonstrated at the tumor rim compared with the center.

Quantitative Assessment of Lung Cancer Perfusion Using MDCT: Does Measurement Reproducibility Improve with Greater Tumor Volume Coverage?

To date, quantitative assessment of tumor vascularity using perfusion CT has been limited to a single tumor level, with the potential for measurement error in heterogeneous tumors. We aimed to determine if greater z-axis tumor coverage improves the reproducibility of perfusion CT measurements in lung cancer. Paired perfusion studies were performed on 10 patients who had histologically confirmed advanced non-small cell lung cancer. Using 16-MDCT, multiple sequential volumetric acquisitions encompassing the entire tumor were acquired after infusion of i.v. contrast material. Using Patlak analysis, median values of tumor permeability (mL/100 mL/min) and blood volume (mL/100 mL) were measured for 10-mm z-axis coverage, and for 40-mm z-axis coverage in each of the paired perfusion studies. Measurement reproducibility was evaluated using Bland-Altman statistics. Mean difference (95% limits of agreement) for tumor permeability was 1.4 (-4.0 to 6.8) for 10-mm coverage and 0.8 (-3.6 to 5.2) for 40-mm coverage. Mean difference (95% limits of agreement) for blood volume was 1.9 (-5.1 to 8.9) for 10-mm coverage and 1.4 (-3.7 to 6.6) for 40-mm coverage. The coefficient of variation for permeability was 18.7% for 10-mm coverage, improving to 11.9% for 40-mm coverage. The coefficient of variation for blood volume was 41.7% for 10-mm coverage, improving to 32.6% for 40-mm coverage. Our results show that an improvement in tumor perfusion measurement reproducibility may be achieved with greater z-axis coverage.

PET statistical image variance with modified LOR acquisition times or efficiencies

It would be possible to design a positron emission tomography scanner to enhance the acquisition durations or efficiencies of certain lines of response (LORs) at the expense of others. In this report we develop the theory of statistical image variance when such weighting is applied, and work out the optimal duration distribution for sequential acquisitions. We further consider a simple example of LOR efficiency modification. The effect of these weighting schemes on image noise variance and relative signal-to-noise ratio (SNR) is evaluated for a clinical example. We find that although global SNR can usually be improved modestly, local SNR may decrease in some regions, and image noise may be less spatially uniform with the weighting than without.

Advances in the noninvasive evaluation of coronary artery disease with multislice computed tomography

Current noninvasive detection of coronary artery disease (CAD) is based on the demonstration of ischemia using stress-rest imaging, which is an indirect way of identifying CAD by demonstration of the hemodynamic consequences, rather than direct visualization of the obstructive lesions in the coronary arteries. Multislice computed tomography (MSCT) has recently emerged as an extremely rapidly developing noninvasive imaging modality, which allows anatomical imaging of the coronary arteries or noninvasive coronary angiography. In addition, total plaque burden, plaque morphology and (to some extent) plaque constitution can be assessed by MSCT. The technique also provides information on resting left ventricular systolic function, and possibly resting perfusion. Ideally, stress function and perfusion should also be evaluated, since this would allow detection of ischemia and would complete the picture of CAD. However, this is not routinely performed, since sequential acquisitions are associated with high radiation doses and thus pose a limitation for cardiovascular applications of MSCT. It is anticipated that, with a reduction in radiation, MSCT may become an important player in the diagnostic and prognostic work-up of patients with known or suspected CAD.

Evaluation of31P high-resolution magic angle spinning of intact tissue samples

The first detailed evaluation is presented of high-resolution (31)P MRS using magic angle spinning (MAS) of intact tissue samples and comparison with the conventional method of studying tissue extracts. The main motivation is that MAS leaves the sample intact at the end of the study for histopathological evaluation. While MAS of tissue samples has previously been demonstrated for (1)H MRS, (31)P MRS is better suited to study of the phospholipid metabolites of importance in cancer. Samples of rhabdomyosarcoma and RIF-1 experimental tumours were maintained at 4 degrees C, spun at 3 kHz and measured in 28-min acquisitions at 11.7 and 14 T. Metabolite stability was evaluated using four sequential 28-min acquisitions. High-resolution MRS was performed on extracts of the same tissue samples. (31)P HR-MAS yielded well-resolved high-resolution spectra, showing peaks from phosphoethanolamine (PE), phosphocholine (PC), inorganic phosphate, glycerophosphoethanolamine and glycerophosphocholine, with linewidths in the range 3-20 Hz. In tumour samples there was no significant change in peak areas over a 2-h period, while peaks sensitive to pH (inorganic phosphate, PE and PC) showed a small change in chemical shift, corresponding to a change of 0.13 +/- 0.06 pH units. Tissue metabolite concentrations showed good agreement with concentrations measured from extracts of the same pieces of tissue. For calculation of metabolite concentrations, the measurement of a reference compound in a separate measurement is more robust than using the signal from a reference compound in the rotor with the sample. Compared with performing tissue extracts, use of MAS of intact tissue samples requires less preparation, is quicker and permits the same sample to be used for subsequent histopathology. The methodology has particular application in studying phospholipid metabolism in cancer and in monitoring tumour response to treatment, where concentrations of phospholipid-related metabolites are found to alter following response to a wide range of anti-cancer therapies.

Corneal flap thickness and topography changes induced by flap creation during laser in situ keratomileusis

To determine the corneal flap thickness profile produced by 3 microkeratomes and the topographic changes induced by flap creation in laser in situ keratomileusis (LASIK). Cornea and Refractive Surgery Unit, Instituto de Microcirugia Ocular de Barcelona, Autonoma University, Barcelona, Spain. In this prospective consecutive nonrandomized comparative study, patients were divided into 2 groups. In Group 1 (75 eyes), 3 microkeratomes were used: Moria LSX One, Moria M2, and Amadeus (AMO); 25 eyes per microkeratome. Pachymetry was measured with a DGH pachymeter in the center of the cornea and 3.0 mm from the center at 4 cardinal points (superior, inferior, nasal, and temporal; 3 measurements at each point) before and after the cut. The flap thickness in each sector was calculated by subtracting the mean post-flap corneal thickness from the mean pre-flap corneal thickness. In Group 2 (33 eyes), the M2 microkeratome with a 130 microm plate was used to create a superotemporal hinged flap (9 eyes) or a superonasal hinged flap (24 eyes). The topographic change induced by the microkeratome cut was evaluated using 4 sequential data acquisitions by the Keratron Scout topographic unit (Optikon) before and immediately after the cut (before laser ablation). Cardinal and oblique astigmatism and change in the axis were calculated by vectorial analysis of the simulated keratometry. Topographic Zernike analysis was performed in a subgroup. With the LSX One microkeratome, the mean flap thickness was 151.7 microm centrally, 161.9 microm superiorly, 151.4 microm inferiorly, 156.1 microm temporally, and 167.5 microm nasally. There was no statistically significant difference between the areas studied (P<.05). With the M2, the mean flap thickness was 131.7 microm centrally, 155.5 microm superiorly, 146.7 microm inferiorly, 143.7 microm temporally, and 160.5 microm nasally. There was a statistically significant difference between flap thickness centrally and in the other areas (P>.05). With the Amadeus microkeratome, the mean flap thickness was 140.0 microm centrally, 152.5 superiorly, 128.5 microm inferiorly, 145.0 microm temporally, and 147.0 microm nasally. Statistically significant differences (P>.05) were found in the 4 sectors of the flap. With vectorial analysis, there was no statistically significant difference between superonasal and superotemporal hinge placement in the cardinal and oblique components but there was a statistically significant difference in the axis change with both placements (P>.05). The LSX One microkeratome was the most predictable. A significant difference was noted in all sectors except superiorly with the M2 and in all sectors with the Amadeus. No differences between nasal and superior hinge placement were found with the M2. Topographic Zernike analysis demonstrated a difference in the orientation of the induced coma as a function of hinge position.

Magnetic resonance imaging of hepatocellular carcinoma

Hepatocellular carcinoma (HCC), the most common primary hepatic malignancy, usually develops in patients with cirrhosis, growing sequentially from low-grade dysplastic nodules to frank malignant HCC. Its recognition is critical because curative treatment and prognosis require early diagnosis. Survival in patients with HCC relates directly to the number, size, and extent of lesions at diagnosis. Imaging of HCC is complicated because the tumor has a varied imaging appearance and frequently coexists with other cirrhotic nodules. Magnetic resonance imaging (MRI), the best available diagnostic technique, offers good contrast resolution and diagnostic sensitivity ranging from 33% to 77%. The main difficulty is not in diagnosing large tumors, but rather small tumors (<2 cm), because of considerable overlap on imaging between benign (regenerative), borderline (dysplastic), and malignant nodules. Increasing degrees of histological malignancy are associated with increasing arterialization and loss of portal blood supply; therefore, recognition of HCC requires dynamic imaging with gadolinium-enhanced T1-weighted sequence. Typically, HCC is a focal lesion with high signal intensity on T2-weighted images, variable signal intensity on T1-weighted images, intense arterial phase enhancement after gadolinium injection, and isointensity or hypointensity at the portal venous phase. The sensitivity of MRI for detecting small lesions is low, and improvement is still needed. Newer contrast agents, higher field strength (3 Tesla) imaging, and perfusion and diffusion MRI techniques possibly will provide greater sensitivity and specificity for detecting small HCCs in the future.