Cristian Tejos

Pontifical Catholic University of Chile, CiudadSantiago, Santiago, Chile

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Publications (32)76.81 Total impact

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    ABSTRACT: In the last years, there has been extensive research on non-invasive MRI techniques that provide quantitative fat-fraction (FF) measurements of the liver. A traditional method acquires images at multiples echoes, and multiple slices are obtained during several breath holds to cover the entire liver. Unfortunately, the number and length of the breath holds limits the achievable spatial resolution, number of echoes acquired and coverage. Moreover, these sequences may show imprecise alignment of the acquired slices, so a 3D liver FF map obtained from this approach may be inaccurate. Therefore, alternative motion correction strategies are necessary for fat quantification in the entire liver. Respiratory bellows and navigator beams1,2,3, can be used to correct respiratory motion, but they imply either using an external device or adding extra RF pulses, which might increase scan time and interfere with the imaging process4,5. To address these limitations, we propose to integrate a realtime respiratory self-gating approach to a 3D 3-point Dixon imaging sequence for total liver fat quantification.
    International Society of Magnetic Resonance in Medicine, Milán, Italia; 05/2014
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    ABSTRACT: INTRODUCTION: The increase in the liver blood flow resistance is one of the common consequences of chronic liver injury like fatty, alcoholic,and autoimmune liver disease among others, and leads to the main complication of this disease: ascites, encephalopathy to the liver perfusion in healthy volunteers and cirrhotic patients.require a subtraction step (TIR-ASL)2,3. In this work we sought to study the utility of this technique to indirectly quantify the portal vein contributionagent. We have previously demonstrated the feasibility to selectively visualized the intrahepatic portal vein using a new ASL technique that does notmainly with invasive procedures. MRI techniques have been also proposed, however most of the techniques require the use of intravascular contrasthepatic artery (the other source of liver perfusion) which is a “high pressure” system. Many techniques have been proposed to detect these changes,decrease in the portal vein contribution to the liver perfusion. This is mainly due because the portal vein is a “low pressure” system compare with thebleeding1. The increase in the intra-hepatic vascular resistance induces many hemodynamics changes. One of the earliest hemodynamic changes is theand esophageal varices
    International Society of Magnetic Resonance in Medicine, Milán, Italia; 05/2014
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    ABSTRACT: Due to the special nature of the acquisition domain, the quantization process of magnetic resonance imaging (MRI) data presents challenges that are not present in other medical imaging techniques. In this article, we demonstrate that the quantization error in MRI cannot be assumed to be a random variable with uniform distribution across the entire acquisition domain. Furthermore, the introduced error is not statistically independent of the input signal. On the contrary, we show that this error is correlated with the object that is being scanned, producing perceptually unpleasant artifacts in the image domain. Although the quantization error is not generally a critical issue in two-dimensional (2D) MRI acquisition, it could be in the case of 3D acquisitions and in noise estimation measurements. © 2014 Wiley Periodicals, Inc. Concepts Magn Reson Part A 43A: 79–89, 2014.
    Concepts in Magnetic Resonance Part A 05/2014; 43A(3). · 1.24 Impact Factor
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    ABSTRACT: To measure both transversal relaxation time (T2 ) and diffusion coefficients within a single scan using a multi-shot approach. Both measurements have drawn interest in many applications, especially in skeletal muscle studies, which have short T2 values. Multiple echo single-shot schemes have been proposed to obtain those variables simultaneously within a single scan, resulting in a reduction of the scanning time. However, one problem with those approaches is the associated long echo read-out. Consequently, the minimum achievable echo time tends to be long, limiting the application of these sequences to tissues with relatively long T2 . To address this problem, we propose to extend the multi-echo sequences using a multi-shot approach, so that to allow shorter echo times. A multi-shot dual-echo EPI sequence with diffusion gradients and echo navigators was modified to include independent diffusion gradients in any of the two echoes. The multi-shot approach allows us to drastically reduce echo times. Results showed a good agreement for the T2 and mean diffusivity measurements with gold standard sequences in phantoms and in vivo data of calf muscles from healthy volunteers. A fast and accurate method is proposed to measure T2 and diffusion coefficients simultaneously, tested in vitro and in healthy volunteers. J. Magn. Reson. Imaging 2013. © 2013 Wiley Periodicals, Inc.
    Journal of Magnetic Resonance Imaging 10/2013; · 2.57 Impact Factor
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    ABSTRACT: BACKGROUND: Calcium (Ca2+) propagates within tissues serving as an important information carrier. In particular, cilia beat frequency in oviduct cells is partially regulated by Ca2+ changes. Thus, measuring the calcium density and characterizing the traveling wave plays a key role in understanding biological phenomena. However, current methods to measure propagation velocities and other wave characteristics involve several manual or time-consuming procedures. This limits the amount of information that can be extracted, and the statistical quality of the analysis. RESULTS: Our work provides a framework based on image processing procedures that enables a fast, automatic and robust characterization of data from two-filter fluorescence Ca2+ experiments. We calculate the mean velocity of the wave-front, and use theoretical models to extract meaningful parameters like wave amplitude, decay rate and time of excitation. CONCLUSIONS: Measurements done by different operators showed a high degree of reproducibility. This framework is also extended to a single filter fluorescence experiments, allowing higher sampling rates, and thus an increased accuracy in velocity measurements.
    BMC Bioinformatics 05/2013; 14(1):162. · 3.02 Impact Factor
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    ABSTRACT: Purpose:To validate the use of particle traces derived from four-dimensional (4D) flow magnetic resonance (MR) imaging to quantify in vivo the caval flow contribution to the pulmonary arteries (PAs) in patients who had been treated with the Fontan procedure.Materials and Methods:The institutional review boards approved this study, and informed consent was obtained. Twelve healthy volunteers and 10 patients with Fontan circulation were evaluated. The particle trace method consists of creating a region of interest (ROI) on a blood vessel, which is used to emit particles with a temporal resolution of approximately 40 msec. The flow distribution, as a percentage, is then estimated by counting the particles arriving to different ROIs. To validate this method, two independent observers used particle traces to calculate the flow contribution of the PA to its branches in volunteers and compared it with the contribution estimated by measuring net forward flow volume (reference method). After the method was validated, caval flow contributions were quantified in patients. Statistical analysis was performed with nonparametric tests and Bland-Altman plots. P < .05 was considered to indicate a significant difference.Results:Estimation of flow contributions by using particle traces was equivalent to estimation by using the reference method. Mean flow contribution of the PA to the right PA in volunteers was 54% ± 3 (standard deviation) with the reference method versus 54% ± 3 with the particle trace method for observer 1 (P = .4) and 54% ± 4 versus 54% ± 4 for observer 2 (P = .6). In patients with Fontan circulation, 87% ± 13 of the superior vena cava blood flowed to the right PA (range, 63%-100%), whereas 55% ± 19 of the inferior vena cava blood flowed to the left PA (range, 22%-82%).Conclusion:Particle traces derived from 4D flow MR imaging enable in vivo quantification of the caval flow distribution to the PAs in patients with Fontan circulation. This method might allow the identification of patients at risk of developing complications secondary to uneven flow distribution.© RSNA, 2013Supplemental material: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12120778/-/DC1.
    Radiology 01/2013; · 6.34 Impact Factor
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    ABSTRACT: Background: Visceral fat accumulation is associated with the development of metabolic diseases. Anthropometry is one of the methods used to quantify it. aim: to evaluate the relationship between visceral adipose tissue volume (VAT), measured with magnetic resonance imaging (MRI), and anthropometric indexes, such as body mass index (BMI) and waist circumference (WC), in type 2 diabetic patients (DM2). Patients and Methods: Twenty four type 2 diabetic patients aged 55 to 78 years (15 females) and weighting 61.5 to 97 kg, were included. The patients underwent MRI examination on a Philips Intera® 1.5T MR scanner. The MRI protocol included a spectral excitation sequence centered at the fat peak. The field of view included from L4-L5 to the diaphragmatic border. VAT was measured using the software Image J®. Weight, height, BMI, WC and body fat percentage (BF%), derived from the measurement offour skinfolds with the equation of Durnin and Womersley, were also measured. The association between MRIVAT measurement and anthropometry was evaluated using the Pearson's correlation coefficient. Results: Mean VAT was 2478 ± 758 ml, mean BMI29.5 ± 4.7 kg/m², and mean WC was 100 ± 9.7 cm. There was a poor correlation between VAT, BMI (r = 0.18) and WC (r = 0.56). Conclusions: BMI and WC are inaccurate predictors of VAT volume in type 2 diabetic patients.
    Revista medica de Chile 12/2012; 140(12):1535-43. · 0.36 Impact Factor
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    Revista medica de Chile 12/2012; 140(12):1535-1543. · 0.36 Impact Factor
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    ABSTRACT: Purpose – In medical applications, it is crucial to evaluate the geometric accuracy of rapid prototyping (RP) models. Current research on evaluating geometric accuracy has focused on identifying two or more specific anatomical landmarks on the original structure and the RP model, and comparing their corresponding linear distances. Such kind of accuracy metrics is ambiguous and may induce misrepresentations of the actual errors. The purpose of this paper is to propose an alternative method and metrics to measure the accuracy of RP models. Design/methodology/approach – The authors propose an accuracy metric composed of two different approaches: a global accuracy evaluation using volumetric intersection indexes calculated over segmented Computed Tomography scans of the original object and the RP model. Second, a local error metric that is computed from the surfaces of the original object and the RP model. This local error is rendered in a 3D surface using a color code, that allow differentiating regions where the model is overestimated, underestimated, or correctly estimated. Global and local error measurements are performed after rigid body registration, segmentation and triangulation. Findings – The results show that the method can be applied to different objects without any modification, and provide simple, meaningful and precise quantitative indexes to measure the geometric accuracy of RP models. Originality/value – The paper presents a new approach to characterize the geometric errors in RP models using global indexes and a local surface distribution of the errors. It requires minimum human intervention and it can be applied without any modification to any kind of object.
    Rapid Prototyping Journal 09/2012; 18(6):431-442. · 1.00 Impact Factor
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    ABSTRACT: OBJECTIVE: The purpose of this study was to analyze flow patterns in the pulmonary circulation of healthy volunteers by using 4D flow magnetic resonance imaging. MATERIALS AND METHODS: The study was approved by the local ethics committee and all subjects gave written informed consent. Eighteen volunteers underwent a 4D flow scan of the whole-heart. Two patients with congenital heart disease were also included to detect possible patterns of flow abnormalities (Patient 1: corrected transposition of great arteries (TGA); Patient 2: partial anomalous pulmonary venous return and atrial septal defect). To analyze flow patterns, 2D planes were placed on the main pulmonary artery (PA), left and right PA. Flow patterns were assessed manually by two independent viewers using vector fields, streamlines and particle traces, and semi-automatically by vorticity quantification. RESULTS: Two counter-rotating helices were found in the main PA of volunteers. Right-handed helical flow was detected in the right PA of 15 volunteers. Analysis of the helical flow by particles traces revealed that both helices contributed mainly to the flow in the right PA. In the patient with corrected TGA helical flow was not detected. Abnormal vortical flow was visualized in the main PA of patient 2, suggesting elevated mean PA pressure. CONCLUSIONS: Helical flow is normally present in the main PA and right PA. 4D flow is an excellent tool to evaluate noninvasively complex blood flow patterns in the pulmonary circulation. Knowledge of normal and abnormal flow patterns might help to evaluate patients with congenital heart disease adding functional information undetectable with other imaging modalities.
    Magnetic Resonance Imaging 08/2012; · 2.06 Impact Factor
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    CardioVascular and Interventional Radiology 06/2012; · 2.09 Impact Factor
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    ABSTRACT: PURPOSE: We present our experience with embolization of incompetent pelvic veins (IPV) in women with recurrence of varicose veins (VV) in lower limbs, as well as symptoms of pelvic congestion syndrome (PCS), after first surgery. In addition, we evaluated the effects of embolization in decreasing the symptoms of VV before surgery as well as its effects on PCS symptoms. MATERIALS AND METHODS: We included 10 women who had consulted a vascular surgeon because of recurrent VV in lower limbs after surgery. All of these patients were included in the study because they also had symptoms of PCS, probably due to IPV. In patients who had confirmed IPV, we performed embolization before a second surgery. VV and PCS were assessed before and at 3 months after embolization (before the second surgery) using a venous clinical severity score (VCSS) and a visual analog pain scale (VAS), respectively. Patients were controlled between 3 and 6 months after embolization. Paired Student t test analysis was used for comparing data before and after embolization. RESULTS: Fifteen vein segments in 10 women were suitable for embolization. There was a significant (p < 0.001) decrease of VCSS after embolization, and recurrence of VV was not detected within a period of 6 months. There was also significant (p < 0.01) relief of chronic pelvic pain related to PCS evaluated using VAS at 3 months after embolization. CONCLUSION: Embolization decreases the risk of VV recurrence after surgery and also improves PCS symptoms in women with VV in lower limbs and IPV.
    CardioVascular and Interventional Radiology 05/2012; · 2.09 Impact Factor
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    ABSTRACT: We report hemodynamic findings in two patients with pulmonary atresia and intact ventricular septum (PAIVS) after "one-and-a-half ventricle repair" and placement of a bidirectional Glenn shunt using four-dimensional (4D) flow magnetic resonance imaging. Quantification of flow and analysis of flow patterns revealed the hemodynamic "battle" between the right ventricle (RV) and the Glenn shunt. Moreover, with a novel approach we calculated during Glenn anastomosis the flow distribution from the superior vena cava (SVC) to the pulmonary arteries. Our results showed a highly asymmetric flow distribution, with most of the flow from the SVC toward the RV and not to the lungs. The evidence provided by 4D flow demonstrates poor efficiency of this system and suggests that both patients might benefit from adding an artificial pulmonary valve to avoid right heart failure.
    Pediatric Cardiology 03/2012; · 1.20 Impact Factor
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    ABSTRACT: Human tissues are usually studied using a series of two-dimensional visualizations of in vivo or cutout specimens. However, there is no precise anatomical description of some of the processes of human fetal development. The purpose of our study is to develop a quantitative description of the normal axial skeleton by means of high-resolution three-dimensional magnetic resonance (MR) images, collected from six normal 20-week-old human fetuses fixed in formaldehyde. Fetuses were collected after spontaneous abortion and subsequently fixed with formalin. They were imaged using a 1.5 T MR scanner with an isotropic spatial resolution of 200 µm. The correct tissue discrimination between ossified and cartilaginous bones was confirmed by comparing the images achieved by MR scans and computerized axial tomographies. The vertebral column was segmented out from each image using a specially developed semi-automatic algorithm. Vertebral body dimensions and inter-vertebral distances were larger in the lumbar region, in agreement with the beginning of the ossification process from the thoracolumbar region toward the sacral and cephalic ends. In this article, we demonstrate the feasibility of using MR images to study the ossification process in formalin-fixed fetal tissues. A quantitative description of the ossification centers of vertebral bodies and arches is presented.
    Prenatal Diagnosis 03/2012; 32(3):252-8. · 2.68 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 02/2012; 14 Suppl 1:W66. · 4.44 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 02/2012; 14 Suppl 1:W8. · 4.44 Impact Factor
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    ABSTRACT: Isolated left ventricular non-compaction (LVNC) is an uncommon disorder characterized by the presence of increased trabeculations and deep intertrabecular recesses. In adults, it has been found that ejection fraction (EF) decreases significantly as non-compaction severity increases. In children however, there are a few data describing the relation between anatomical characteristics of LVNC and ventricular function. We aimed to find correlations between morphological features and ventricular performance in children and young adolescents with LVNC using cardiovascular magnetic resonance (CMR). 15 children with LVNC (10 males, mean age 9.7 y.o., range 0.6-17 y.o.), underwent a CMR scan. Different morphological measures such as the compacted myocardial mass (CMM), non-compaction (NC) to the compaction (C) distance ratio, compacted myocardial area (CMA) and non-compacted myocardial area (NCMA), distribution of NC, and the assessment of ventricular wall motion abnormalities were performed to investigate correlations with ventricular performance. EF was considered normal over 53%. The distribution of non-compaction in children was similar to published adult data with a predilection for apical, mid-inferior and mid-lateral segments. Five patients had systolic dysfunction with decreased EF. The number of affected segments was the strongest predictor of systolic dysfunction, all five patients had greater than 9 affected segments. Basal segments were less commonly affected but they were affected only in these five severe cases. The segmental pattern of involvement of non-compaction in children is similar to that seen in adults. Systolic dysfunction in children is closely related to the number of affected segments.
    Journal of Cardiovascular Magnetic Resonance 01/2012; 14:9. · 4.44 Impact Factor
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    ABSTRACT: Objective: Hemodynamic parameters are critical to perform a proper diagnosis. However, due to the large number of variables that can be obtained, overall analysis may represent a complex task. To facilitate this, we propose to create a model for classifying different hemodynamic variables between those belonging to a healthy individual and to a pathological patient. For this purpose, we employed data mining techniques to identify relationships among various aortic hemodynamic parameters obtained through multi-dimensional (4D flow) MR imaging. Method: A 4D flow sequence of whole heart and great vessels was acquired using MRI in 19 healthy volunteers and 2 patients (one with aortic coarctation and one with repaired coarctation of the aorta). Retrospectively, data were reformatted along the aorta; three MRI acquisitions were performed for volunteers and 30 sequences for each patient. In each slice the aorta was segmented and various parameters were quantified: area, maximum velocity, minimum velocity, flow and volumen, with following values being calculated for last four parameters: maximum, average, standard deviation, kurtosis, skewness, proportion of time to reach the maximum value, among others. A total of 26 variables for each acquisition were obtained. In order to classify data, the CART Technique (Classification and Regression Trees) was applied. To validate the model, two extra projections were generated per each volunteer and 20 slice per each patient. Results: By using only 7 variables, the CART Technique allows discrimination between images performed either on volunteers or patients with an error rate of 14.1%, a sensitivity of 82.5%, and a specificity of 89.4%. Conclusions: 4D flow MR imaging provides a wealth of hemodynamic data that can be difficult to analyze. In this paper we demonstrate that by using data mining techniques it is possible to classify images from relevant hemodynamic parameters and their relationships in order to support the diagnosis of cardiovascular disorders.
    Revista Chilena de Radiologia 01/2012; 18(2):62-67.
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    ABSTRACT: Chemical species separation techniques in image space are prone to incorporate several distortions. Some of these are signal accentuation in borders and geometrical warping from field inhomogeneity. These errors come from neglecting intraecho time variations. In this work, we present a new approach for chemical species separation in MRI with simultaneous estimation of field map and T2* decay, formulated entirely in k-space. In this approach, the time map is used to model the phase accrual from off-resonance precession and also the amplitude decay due to T2*. Our technique fits the signal model directly in k-space with the acquired data minimizing the l(2)-norm with an interior-point algorithm. Standard two dimensional gradient echo sequences in the thighs and head were used for demonstrating the technique. With this approach, we were able to obtain excellent estimation for the species, the field inhomogeneity, and T2* decay images. The results do not suffer from geometric distortions derived from the chemical shift or the field inhomogeneity. Importantly, as the T2* map is well positioned, the species signal in borders is correctly estimated. Considering intraecho time variations in a complete signal model in k-space for separating species yields superior estimation of the variables of interest when compared to existing methods.
    Magnetic Resonance in Medicine 12/2011; 68(2):400-8. · 3.27 Impact Factor
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    ABSTRACT: The classic paradigm for MRI requires a homogeneous B(0) field in combination with linear encoding gradients. Distortions are produced when the B(0) is not homogeneous, and several postprocessing techniques have been developed to correct them. Field homogeneity is difficult to achieve, particularly for short-bore magnets and higher B(0) fields. Nonlinear magnetic components can also arise from concomitant fields, particularly in low-field imaging, or intentionally used for nonlinear encoding. In any of these situations, the second-order component is key, because it constitutes the first step to approximate higher-order fields. We propose to use the fractional Fourier transform for analyzing and reconstructing the object's magnetization under the presence of quadratic fields. The fractional fourier transform provides a precise theoretical framework for this. We show how it can be used for reconstruction and for gaining a better understanding of the quadratic field-induced distortions, including examples of reconstruction for simulated and in vivo data. The obtained images have improved quality compared with standard Fourier reconstructions. The fractional fourier transform opens a new paradigm for understanding the MR signal generated by an object under a quadratic main field or nonlinear encoding.
    Magnetic Resonance in Medicine 10/2011; 68(1):17-29. · 3.27 Impact Factor

Publication Stats

55 Citations
76.81 Total Impact Points

Institutions

  • 2008–2014
    • Pontifical Catholic University of Chile
      • Departamento de Ingeniería Eléctrica
      CiudadSantiago, Santiago, Chile
  • 2013
    • University of Santiago, Chile
      CiudadSantiago, Santiago, Chile
  • 2004
    • University of Cambridge
      • School of Clinical Medicine
      Cambridge, ENG, United Kingdom