[Show description][Hide description] DESCRIPTION: In recent years, rapid prototyping (RP) models have emerged as a useful tool in implant
design and surgery planning for oral and maxillofacial applications. For these applications, the
accuracy of the RP model is fundamental. There has been some attempts to quantify the error in the
construction of RP models, but these methods are ambiguous and do not provide information about the
error distribution throughout the model, nor global metrics to compare different models. In this work,
we applied a new methodology to assess the geometric accuracy of RP models of in-vivo human jaws.
We built RP models from CT images of 8 patients with different pathologies. Then, we scanned the
models to compare them with the images of the patients using our methodology. We computed global
and local accuracy measurements and found that the RP models consistently overestimate the size of
the jaw. We also found that larger errors tend to be localized at regions with high curvature. The RP
models overestimate areas with indentations, such as interdental and interradicular spaces, and
underestimate sharp structures, such as the lingula of the mandible. Our findings suggest that surgeons
should keep in mind these geometric errors that occur in manufacturing biomodels with RP
technologies, especially when faced with surgical planning in tight spaces or when extreme precision is
[Show abstract][Hide abstract] ABSTRACT: Portal hypertension (PH) is a frequent syndrome in patients with chronic liver diseases and it is characterized by an increased liver resistance to blood flow. The increased resistance induces a rise in the portal pressure gradient (PPG), leading to hepatic hemodynamic changes: decreasing the relative contribution of portal vein and increasing the relative contribution of hepatic artery to liver perfusion. The relevance of portal hypertension derives from the frequency and severity of its complications, which represent the first cause of hospital admission, death and liver transplantation in patients with cirrhosis. It has been suggested that the measure of the severity of PH should be evaluated in all patients with chronic liver diseases as a surrogate marker of the liver chronic damage and the response to treatments. The currently favored method for determining portal venous pressure involves catheterization of the hepatic vein and measurement of the hepatic venous pressure gradient (HVPG). However this method is invasive, expensive, and probably not suitable to screen asymptomatic high-risk patients. In this work we propose to indirectly measure the severity of PH by estimating the portal vein blood volume that flows into the intrahepatic circulation (IHPVBV) in a certain number of heart cycles. The rationality of this idea comes from the concept that PPG (like the gradient pressure in any vascular system) is determined by the product of portal vein flow (Q) and the liver vascular resistance (R). Therefore, the measurement of the intrahepatic blood volume that flows in a certain number of heart cycles would be a good estimation of the P/R ratio and indirectly of the severity of PH. In order to quantify the IHPVBV in one and two cardiac cycles we use the technique called TIR-ASL, which is a flow dependent non-contrast technique, that does not require a subtraction step as classic arterial spin labelling (ASL) methods, and could be easily adapted to evaluate the IHPVBV in one or two heart cycles.
ISMRM Workshop Non-contrast Cardiovascular MRI, Long Beach, CA, USA; 03/2015
[Show abstract][Hide abstract] ABSTRACT: Additive manufacturing (AM) models are used in medical applications for surgical planning, prosthesis design and teaching. For these applications, the accuracy of the AM models is essential. Unfortunately, this accuracy is compromised due to errors introduced by each of the building steps: image acquisition, segmentation, triangulation, printing and infiltration. However, the contribution of each step to the final error remains unclear.We performed a sensitivity analysis comparing errors obtained from a reference with those obtained modifying parameters of each building step. Our analysis considered global indexes to evaluate the overall error, and local indexes to show how this error is distributed along the surface of the AM models.Our results show that the standard building process tends to overestimate the AM models, i.e. models are larger than the original structures. They also show that the triangulation resolution and the segmentation threshold are critical factors, and that the errors are concentrated at regions with high curvatures.Errors could be reduced choosing better triangulation and printing resolutions, but there is an important need for modifying some of the standard building processes, particularly the segmentation algorithms.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] ABSTRACT: Background: Approximately 5–10% of adults with Congenital Heart Disease (CHD) develop Pulmonary Arterial Hypertension (PAH), mainly due to systemic to pulmonary shunting. Chronically raised pulmonary blood flow causes abnormal endothelial shear stress and a progressive pulmonary vasculopathy (1). Quantification of Wall Shear Stress (WSS) in the pulmonary circulation might be helpful to identify patients with CHD at risk for developing PAH. Using 4D flow MRI, the quantification of WSS has been recently reported in the aorta (2). However, there are few reports that have studied this parameter in the Pulmonary Artery (PA). The objective of this work was to develop a reproducible method to calculate WSS using a Strain Rate Tensor based on cylindrical coordinates obtained from 4D flow data. The method was applied to calculate WSS values in the main, right and left PA (MPA, RPA and LPA) of healthy volunteers and patients with CHD. Method: 4D flow data of the Whole Heart (reconstructed spatial resolution = 2.5 mm 3 , temporal resolution = 38 ms) was acquired on 17 volunteers and 5 patients with Congenital Heart Diseases (CHD) (repaired Transposition of the great arteries, two after one and a half ventricle repair, and two with partial anomalous pulmonary venous return, one of them with Atrial Septal Defect). Using a homemade software, three slices were reformatted perpendicular to the MPA, RPA and LPA. Subsequently, we segmented the blood pool, and calculated Magnitude (WSS-M), Axial (WSS-A), and Circumferential (WSS-C) WSS using a Strain Rate Tensor based on cylindrical coordinates. For each slice, we generated three contiguous slices to include variations of the velocity along the direction of the vessel. Two independent observers processed the data to study the reproducibility of the proposed method with our software.
Proc. Intl. Soc. Mag. Reson. Med. 20 (2012); 05/2014
[Show abstract][Hide abstract] 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