Asko Ristolainen | Tallinn University of Technology (original) (raw)

Papers by Asko Ristolainen

Journal of Atmospheric and Oceanic Technology, Aug 1, 2020

Monitoring vessel traffic in coastal regions is a key element of maritime security. For this reas... more Monitoring vessel traffic in coastal regions is a key element of maritime security. For this reason, additional ways of detecting moving vessels are explored by using the unique structure of their wake waves based on pressure measurements at the seabed. The experiments are performed at a distance of about 2 km from the sailing line using novel multisensor devices called ''hydromasts'' that track both pressure and near-bed water flow current velocities. The main tool for the analysis is a windowed Fourier transform that produces a spectrogram of the wake structure. It is shown that time series from the pressure sensors, measured at a frequency of 100 Hz, 0.2 m above the seabed are a valid source of input data for the spectrogram technique. This technique portrays the properties of both divergent and transverse waves with an accuracy and resolution that is sufficient for the evaluation of the speed and distance of the detected vessels from the measurement device. All the detected passings are matched with vessels using automatic identification system (AIS) data. The use of several time series from synchronized multisensor systems substantially suppresses noise and improves the quality of the outcome compared to one-point measurements. Additional information about variations in the water flow in wakes provides a simple and reasonably accurate tool for rapid detection of ship passages.

IEEE Sensors Journal, Dec 15, 2019

The near-bed velocity is a key physical parameter in hydrological, ecological and geomorphologica... more The near-bed velocity is a key physical parameter in hydrological, ecological and geomorphological studies. Considering climate change, measurement methods capable of providing continuous observations are needed to assess and predict the effects of increasing uncertainty. Therefore, a technology gap remains for continuous near-bed measurements. To address this gap, we have developed a multimodal flow measurement device, the hydromast. The hydromast uses a combination of pressure and inertial sensing to measure the nearbed (< 30 cm) velocity. We have previously shown that the device can be used to classify river hydromorphological units. Encouraged by these results, we now show that the same device is also capable of continuously measuring the near-bed velocity in rivers. Ten hydromast prototypes were built and calibrated over the range of 0.01-2 m/s in a large-scale laboratory tow tank and validated under natural conditions (0.35-1.2 m/s) using 118 turbulent flow measurements in a river. It was found that the streamwise, time-averaged velocity mean estimation error from the hydromast in continuous tests with different methods was 0.095 m/s, as compared with a state-of-the-art acoustic Doppler velocimeter. The contribution of this study is a new method for continuous near-bed velocity measurements, verified with turbulent field data from a river.

IEEE Transactions on Geoscience and Remote Sensing, 2018

Classification of river morphology is often based on hydromorphological units (HMUs) identified f... more Classification of river morphology is often based on hydromorphological units (HMUs) identified from field measurements. Established survey methods rely on expert judgment or collection of field point measurements. When used for HMU classification, these methods can suffer from high errors due to the variations in the sampling environment, causing low repeatability. In order to expedite field data collection and increase HMU classification accuracy, we propose a multisensory device, the hydromast. Each hydromast provides a new source of data to classify HMUs. The modules are inexpensive and highly portable, consisting of a synchronous array of commodity pressure and inertial sensors. Rapid, local changes in the flow field are recorded with absolute and differential pressure sensors. At the same time, slower depth-integrated flow signals are obtained from a small damped cylindrical mast, driven by vortexinduced vibrations. In contrast to existing passive flow measurement technologies, the hydromast uses fluid-body interactions to provide flow measurements. This allows for minimal signal processing and simple feature extraction. An array of three hydromasts was used to collect ten samples in three river HMUs with shallow depths and highly turbulent flows with smooth and rough beds. We investigated classification accuracy using single, dual, and triple hydromast arrays with pressure, inertial, and combined features using linear regression, a genetic algorithm, and a neural network. Although limited in scope, the set of spot measurements covering three HMUs showed that a single multimodal sensor could deliver an overall classification accuracy of 89% of the HMUs, and an increase of up to 99% was achieved using a multimodal triple hydromast array. These preliminary results show promise in using hydromasts for rapid and robust HMU classification, providing a new way to collect and assess river survey data.

Acta Radiologica Short Reports, 2014

Lecture Notes in Computer Science, 2016

Fish have developed advanced hydrodynamic sensing capabilities using neuromasts, a series of coll... more Fish have developed advanced hydrodynamic sensing capabilities using neuromasts, a series of collocated inertial sensors distributed over their body. We have developed the hydromast, an upscaled version of this sensing modality in order to facilitate near bed sensing for aquatic systems. Here we introduce the concept behind this bioinspired flow sensing device as well as the first results from laboratory investigations.

International Journal of Advanced Robotic Systems, 2015

AASRI Procedia, 2014

New technological methods to assist percutaneous cryoablation procedures are here presented, name... more New technological methods to assist percutaneous cryoablation procedures are here presented, namely a planning software and a simulation algorithm. The first has the role to calculate a feasible displacement of the tools to ensure an effective ablation of the lesion, satisfying well-specified procedural constraints. Starting from intra-operative CT scans of the patient, a virtual model of the anatomical site is obtained and uploaded. The displacement of the cryoprobes is computed in order to cover the whole volume of the tumour with the developed iceball, but minimizing the damage to surrounding healthy renal tissue. On the other hand, the simulation algorithm is a graphical tool useful to assess the temperature distribution throughout the evolution of the procedure. A discrete iterative function calculates the heat transfer from the probes to the surrounding tissue within a specified three-dimensional grid: the isolation of significant isotherms can help to assess whether the whole tumour will be frozen or not. By using a real intra-operative dataset of a successful percutaneous cryoablation, the volume of the real iceball has been matched with that generated from the simulator, showing a good accuracy in terms of dimension and shape. Even though been designed to be integrated within a robotic system, this method is usable and extensible for different purposes and adapted to simulate other scenarios or procedures.

International Journal of Advanced Robotic Systems, 2015

Alternatives to laboratory animals: ATLA

The use of animals for testing and validating new medical devices and surgical techniques has rai... more The use of animals for testing and validating new medical devices and surgical techniques has raised ethical issues for a long time. Following the introduction of the Three Rs principle, significant efforts have been made to achieve a reduction in the numbers of animals used in testing. Nevertheless, the number of large animals used for testing purposes is still too high. This article describes a potential alternative to the use of large animals in the early phase of the development of surgical equipment - a high-definition phantom pig abdomen. The phantom pig abdomen was developed from computed tomography scans by using affordable materials, and it was used with two different robotic platforms. It permitted the testing of minimally-invasive robotic pancreatic enucleation, with or without intraoperative ultrasound guidance. The phantom pig abdomen has proven to be a realistic tool, with the potential to reduce the cost and time-frame of the experiments.

Alternatives to laboratory animals : ATLA, 2013

The use of animals for testing and validating new medical devices and surgical techniques has rai... more The use of animals for testing and validating new medical devices and surgical techniques has raised ethical issues for a long time. Following the introduction of the Three Rs principle, significant efforts have been made to achieve a reduction in the numbers of animals used in testing. Nevertheless, the number of large animals used for testing purposes is still too high. This article describes a potential alternative to the use of large animals in the early phase of the development of surgical equipment -- a high-definition phantom pig abdomen. The phantom pig abdomen was developed from computed tomography scans by using affordable materials, and it was used with two different robotic platforms. It permitted the testing of minimally-invasive robotic pancreatic enucleation, with or without intraoperative ultrasound guidance. The phantom pig abdomen has proven to be a realistic tool, with the potential to reduce the cost and time-frame of the experiments.

2012 4th IEEE RAS & EMBS International Conference on Biomedical Robotics and Biomechatronics (BioRob), 2012

ABSTRACT

2011 15th International Conference on Advanced Robotics (ICAR), 2011

Abstract This paper presents a bio-mimetic approach to the design and control of a fish-like robo... more Abstract This paper presents a bio-mimetic approach to the design and control of a fish-like robot with compliant parts. One of the key contributions of this work is the use of continuous structures instead of discrete assemblies. In this framework, the motion of the robot is ...

European Journal of Radiology, 2013

This paper describes manufacturing of economically affordable renal biopsy phantoms for radiology... more This paper describes manufacturing of economically affordable renal biopsy phantoms for radiology residents and practicing radiologists. We reconstructed a realistic 3-dimensional patient-specific kidney model from CT data, manufactured an organ mould and casted the kidney phantoms. Using gelatin gel materials with calibrated parameters allowed making phantoms with realistic mechanical, ultrasound and CT properties including various pathologies. The organ phantoms with cysts included were further casted into gelatin gel medium. They were validated by radiology residents in biopsy training and compared against self-made phantoms traditionally used in the curriculum of interventional radiology. The realism, durability, price and suitability for training were evaluated. The results showed that our phantoms are more realistic and easier to use than the traditional ones. Our proposed technology allows creating a low-cost (50$/kg) alternative to the pricy commercial training phantoms available today.

Acta Radiologica Short Reports, 2014

Background: Trends in interventional radiology and urology training are orientated towards reduci... more Background: Trends in interventional radiology and urology training are orientated towards reducing costs and increasing efficiency. In order to comply with the trends, we propose training on inexpensive patient-specific kidney phantoms. Purpose: To develop a new kidney phantom for puncture and drainage training in interventional urology and radiology, and to evaluate their anatomical correctness and suitability for training compared to the traditional way of training on home-made phantoms. Material and Methods: A case study for validation of kidney phantoms was conducted with nine radiology students divided into two groups: one trained on standard home-made training phantom (n ¼ 4) and the other on our kidney phantoms (n ¼ 5). Another test phantom was used to evaluate the effectiveness of the training of the two groups. The tests were video recorded and analyzed. Duration of the procedure was used as the primary indicator of procedure's quality. Comparison tests were also conducted with professional radiologists. Anatomical correctness of the kidney phantom was evaluated by comparing the post mortem kidney scans with reconstructed models from CT scans. Subjective feedback was also collected from the participants. Wider use of kidney phantoms was analyzed. Results: The average volumetric difference between post mortem kidney scans and reconstructed CT kidney models was 4.70 AE 3.25%. All five students practicing on the kidney phantom improved their performance and the results were almost equal to the results of the professional radiologist while in the other group two students out of four trained on standard home-made training phantoms failed to improve their performance. However, the small number of test subjects prevents us from drawing general conclusions about the efficiency of the new practice. The kidney phantoms were found usable also for nephrostomy catheter placement training under fluoroscopy. Conclusion: The feedback from radiologists showed that the anatomically correct features of the phantom is an added value for the training and has a potential to increase the quality of minimally invasive procedures in general.

Journal of Atmospheric and Oceanic Technology, Aug 1, 2020

Monitoring vessel traffic in coastal regions is a key element of maritime security. For this reas... more Monitoring vessel traffic in coastal regions is a key element of maritime security. For this reason, additional ways of detecting moving vessels are explored by using the unique structure of their wake waves based on pressure measurements at the seabed. The experiments are performed at a distance of about 2 km from the sailing line using novel multisensor devices called ''hydromasts'' that track both pressure and near-bed water flow current velocities. The main tool for the analysis is a windowed Fourier transform that produces a spectrogram of the wake structure. It is shown that time series from the pressure sensors, measured at a frequency of 100 Hz, 0.2 m above the seabed are a valid source of input data for the spectrogram technique. This technique portrays the properties of both divergent and transverse waves with an accuracy and resolution that is sufficient for the evaluation of the speed and distance of the detected vessels from the measurement device. All the detected passings are matched with vessels using automatic identification system (AIS) data. The use of several time series from synchronized multisensor systems substantially suppresses noise and improves the quality of the outcome compared to one-point measurements. Additional information about variations in the water flow in wakes provides a simple and reasonably accurate tool for rapid detection of ship passages.

IEEE Sensors Journal, Dec 15, 2019

The near-bed velocity is a key physical parameter in hydrological, ecological and geomorphologica... more The near-bed velocity is a key physical parameter in hydrological, ecological and geomorphological studies. Considering climate change, measurement methods capable of providing continuous observations are needed to assess and predict the effects of increasing uncertainty. Therefore, a technology gap remains for continuous near-bed measurements. To address this gap, we have developed a multimodal flow measurement device, the hydromast. The hydromast uses a combination of pressure and inertial sensing to measure the nearbed (< 30 cm) velocity. We have previously shown that the device can be used to classify river hydromorphological units. Encouraged by these results, we now show that the same device is also capable of continuously measuring the near-bed velocity in rivers. Ten hydromast prototypes were built and calibrated over the range of 0.01-2 m/s in a large-scale laboratory tow tank and validated under natural conditions (0.35-1.2 m/s) using 118 turbulent flow measurements in a river. It was found that the streamwise, time-averaged velocity mean estimation error from the hydromast in continuous tests with different methods was 0.095 m/s, as compared with a state-of-the-art acoustic Doppler velocimeter. The contribution of this study is a new method for continuous near-bed velocity measurements, verified with turbulent field data from a river.

IEEE Transactions on Geoscience and Remote Sensing, 2018

Classification of river morphology is often based on hydromorphological units (HMUs) identified f... more Classification of river morphology is often based on hydromorphological units (HMUs) identified from field measurements. Established survey methods rely on expert judgment or collection of field point measurements. When used for HMU classification, these methods can suffer from high errors due to the variations in the sampling environment, causing low repeatability. In order to expedite field data collection and increase HMU classification accuracy, we propose a multisensory device, the hydromast. Each hydromast provides a new source of data to classify HMUs. The modules are inexpensive and highly portable, consisting of a synchronous array of commodity pressure and inertial sensors. Rapid, local changes in the flow field are recorded with absolute and differential pressure sensors. At the same time, slower depth-integrated flow signals are obtained from a small damped cylindrical mast, driven by vortexinduced vibrations. In contrast to existing passive flow measurement technologies, the hydromast uses fluid-body interactions to provide flow measurements. This allows for minimal signal processing and simple feature extraction. An array of three hydromasts was used to collect ten samples in three river HMUs with shallow depths and highly turbulent flows with smooth and rough beds. We investigated classification accuracy using single, dual, and triple hydromast arrays with pressure, inertial, and combined features using linear regression, a genetic algorithm, and a neural network. Although limited in scope, the set of spot measurements covering three HMUs showed that a single multimodal sensor could deliver an overall classification accuracy of 89% of the HMUs, and an increase of up to 99% was achieved using a multimodal triple hydromast array. These preliminary results show promise in using hydromasts for rapid and robust HMU classification, providing a new way to collect and assess river survey data.

Acta Radiologica Short Reports, 2014

Lecture Notes in Computer Science, 2016

Fish have developed advanced hydrodynamic sensing capabilities using neuromasts, a series of coll... more Fish have developed advanced hydrodynamic sensing capabilities using neuromasts, a series of collocated inertial sensors distributed over their body. We have developed the hydromast, an upscaled version of this sensing modality in order to facilitate near bed sensing for aquatic systems. Here we introduce the concept behind this bioinspired flow sensing device as well as the first results from laboratory investigations.

International Journal of Advanced Robotic Systems, 2015

AASRI Procedia, 2014

New technological methods to assist percutaneous cryoablation procedures are here presented, name... more New technological methods to assist percutaneous cryoablation procedures are here presented, namely a planning software and a simulation algorithm. The first has the role to calculate a feasible displacement of the tools to ensure an effective ablation of the lesion, satisfying well-specified procedural constraints. Starting from intra-operative CT scans of the patient, a virtual model of the anatomical site is obtained and uploaded. The displacement of the cryoprobes is computed in order to cover the whole volume of the tumour with the developed iceball, but minimizing the damage to surrounding healthy renal tissue. On the other hand, the simulation algorithm is a graphical tool useful to assess the temperature distribution throughout the evolution of the procedure. A discrete iterative function calculates the heat transfer from the probes to the surrounding tissue within a specified three-dimensional grid: the isolation of significant isotherms can help to assess whether the whole tumour will be frozen or not. By using a real intra-operative dataset of a successful percutaneous cryoablation, the volume of the real iceball has been matched with that generated from the simulator, showing a good accuracy in terms of dimension and shape. Even though been designed to be integrated within a robotic system, this method is usable and extensible for different purposes and adapted to simulate other scenarios or procedures.

International Journal of Advanced Robotic Systems, 2015

Alternatives to laboratory animals: ATLA

The use of animals for testing and validating new medical devices and surgical techniques has rai... more The use of animals for testing and validating new medical devices and surgical techniques has raised ethical issues for a long time. Following the introduction of the Three Rs principle, significant efforts have been made to achieve a reduction in the numbers of animals used in testing. Nevertheless, the number of large animals used for testing purposes is still too high. This article describes a potential alternative to the use of large animals in the early phase of the development of surgical equipment - a high-definition phantom pig abdomen. The phantom pig abdomen was developed from computed tomography scans by using affordable materials, and it was used with two different robotic platforms. It permitted the testing of minimally-invasive robotic pancreatic enucleation, with or without intraoperative ultrasound guidance. The phantom pig abdomen has proven to be a realistic tool, with the potential to reduce the cost and time-frame of the experiments.

Alternatives to laboratory animals : ATLA, 2013

The use of animals for testing and validating new medical devices and surgical techniques has rai... more The use of animals for testing and validating new medical devices and surgical techniques has raised ethical issues for a long time. Following the introduction of the Three Rs principle, significant efforts have been made to achieve a reduction in the numbers of animals used in testing. Nevertheless, the number of large animals used for testing purposes is still too high. This article describes a potential alternative to the use of large animals in the early phase of the development of surgical equipment -- a high-definition phantom pig abdomen. The phantom pig abdomen was developed from computed tomography scans by using affordable materials, and it was used with two different robotic platforms. It permitted the testing of minimally-invasive robotic pancreatic enucleation, with or without intraoperative ultrasound guidance. The phantom pig abdomen has proven to be a realistic tool, with the potential to reduce the cost and time-frame of the experiments.

2012 4th IEEE RAS & EMBS International Conference on Biomedical Robotics and Biomechatronics (BioRob), 2012

ABSTRACT

2011 15th International Conference on Advanced Robotics (ICAR), 2011

Abstract This paper presents a bio-mimetic approach to the design and control of a fish-like robo... more Abstract This paper presents a bio-mimetic approach to the design and control of a fish-like robot with compliant parts. One of the key contributions of this work is the use of continuous structures instead of discrete assemblies. In this framework, the motion of the robot is ...

European Journal of Radiology, 2013

This paper describes manufacturing of economically affordable renal biopsy phantoms for radiology... more This paper describes manufacturing of economically affordable renal biopsy phantoms for radiology residents and practicing radiologists. We reconstructed a realistic 3-dimensional patient-specific kidney model from CT data, manufactured an organ mould and casted the kidney phantoms. Using gelatin gel materials with calibrated parameters allowed making phantoms with realistic mechanical, ultrasound and CT properties including various pathologies. The organ phantoms with cysts included were further casted into gelatin gel medium. They were validated by radiology residents in biopsy training and compared against self-made phantoms traditionally used in the curriculum of interventional radiology. The realism, durability, price and suitability for training were evaluated. The results showed that our phantoms are more realistic and easier to use than the traditional ones. Our proposed technology allows creating a low-cost (50$/kg) alternative to the pricy commercial training phantoms available today.

Acta Radiologica Short Reports, 2014

Background: Trends in interventional radiology and urology training are orientated towards reduci... more Background: Trends in interventional radiology and urology training are orientated towards reducing costs and increasing efficiency. In order to comply with the trends, we propose training on inexpensive patient-specific kidney phantoms. Purpose: To develop a new kidney phantom for puncture and drainage training in interventional urology and radiology, and to evaluate their anatomical correctness and suitability for training compared to the traditional way of training on home-made phantoms. Material and Methods: A case study for validation of kidney phantoms was conducted with nine radiology students divided into two groups: one trained on standard home-made training phantom (n ¼ 4) and the other on our kidney phantoms (n ¼ 5). Another test phantom was used to evaluate the effectiveness of the training of the two groups. The tests were video recorded and analyzed. Duration of the procedure was used as the primary indicator of procedure's quality. Comparison tests were also conducted with professional radiologists. Anatomical correctness of the kidney phantom was evaluated by comparing the post mortem kidney scans with reconstructed models from CT scans. Subjective feedback was also collected from the participants. Wider use of kidney phantoms was analyzed. Results: The average volumetric difference between post mortem kidney scans and reconstructed CT kidney models was 4.70 AE 3.25%. All five students practicing on the kidney phantom improved their performance and the results were almost equal to the results of the professional radiologist while in the other group two students out of four trained on standard home-made training phantoms failed to improve their performance. However, the small number of test subjects prevents us from drawing general conclusions about the efficiency of the new practice. The kidney phantoms were found usable also for nephrostomy catheter placement training under fluoroscopy. Conclusion: The feedback from radiologists showed that the anatomically correct features of the phantom is an added value for the training and has a potential to increase the quality of minimally invasive procedures in general.