Healy, John J.
Healy, John J.
Healy, John J.
Now showing 1 - 10 of 15
- PublicationAutomated Filter Selection for Suppression of Gibbs Ringing Artefacts in MRI(Elsevier, 2022-11)
;Gibbs ringing creates artefacts in magnetic resonance images that can mislead clinicians. Reconstruction algorithms attempt to suppress Gibbs ringing, or an additional ringing suppression algorithm may be applied post reconstruction. Novel reconstruction algorithms are often compared with filtered Fourier reconstruction, but the choices of filters and filter parameters can be arbitrary and sub-optimal. Evaluation of different reconstruction and post-processing algorithms is difficult to automate or subjective: many metrics have been used in the literature. In this paper, we evaluate twelve of those metrics and demonstrate that none of them are fit for purpose. We propose a novel metric and demonstrate its efficacy in 1D and 2D simulations. We use our new metric to optimise and compare 17 smoothing filters for suppression of Gibbs artefacts. We examine the transfer functions of the optimised filters, with counter-intuitive results regarding the highest-performing filters. Our results will simplify and improve the comparison of novel MRI reconstruction and post-processing algorithms, and lead to the automation of ringing suppression in MRI. They also apply more generally to other applications in which data is captured in the Fourier domain. 9
- PublicationWigner cross-terms in sampled and other periodic signals(Optical Society of America, 2009-10-11)
; ;If we sample a scalar wave field, it becomes periodic in frequency. We examine the cross-terms which occur between these periodic replicas in the Wigner-Ville distribution function of such a signal. We present analytic results for Gaussian signals. The results also have implications for physical systems which contain periodic gratings. 290
- Publication2D Non-separable Linear Canonical Transform (2D-NS-LCT) based cryptography(2017-04-27)
; ; ; ; ;The 2D non-separable linear canonical transform (2D-NS-LCT) can describe a variety of paraxial optical systems. Digital algorithms to numerically evaluate the 2D-NS-LCTs are not only important in modeling the light field propagations but also of interest in various signal processing based applications, for instance optical encryption. Therefore, in this paper, for the first time, a 2D-NS-LCT based optical Double-random-Phase-Encryption (DRPE) system is proposed which offers encrypting information in multiple degrees of freedom. Compared with the traditional systems, i.e. (i) Fourier transform (FT); (ii) Fresnel transform (FST); (iii) Fractional Fourier transform (FRT); and (iv) Linear Canonical transform (LCT), based DRPE systems, the proposed system is more secure and robust as it encrypts the data with more degrees of freedom with an augmented key-space. 417
- PublicationInclusive Teaching & Learning Case Studies in Engineering, Architecture & Affiliated Disciplines(UCD Access & Lifelong Learning, 2021-10-14)
; ; ; ; ; ; ; ; ; ; ;Diversity and inclusion are core to UCD values. We seek to attract students from a wide range of social and economic backgrounds and students who reflect the true diversity of the country. And as a global university, UCD attracts international students from over 100 countries. This diversity enriches our campus, and the experience of our students. The University's strategy 2020-2024 'Rising to the Future' also recognises the importance of inclusion and diversity, in seeking to "provide an inclusive educational experience that defines international best practice and prepares our graduates to thrive in present and future societies." However, an inclusive educational experience will not be achieved by simply creating diversity in the student body. It requires that we adjust our approach in everything we do to support and encourage our students’ success. We have clearly articulated in our strategy, and further emphasised in our Education and Student Success strategy, that our goal is to "equip all our educators with the tools and resources required to embed Universal Design for Learning on an institution-wide basis". 34
- PublicationReevaluation of the direct method of calculating Fresnel and other linear canonical transforms(Optical Society of America, 2010-04-01)
;The linear canonical transform may be used to simulate the effect of paraxial optical systems on wave fields. Using a recent definition of the discrete linear canonical transform, phase space diagram analyses of the sampling requirements of the direct method of calculating the Fresnel and other linear canonical transforms are more favorable than previously thought. Thus the direct method of calculating these Transforms may be used with fewer samples than previously reported simply by making use of an appropriate reconstruction filter on the samples output by the algorithm. 375Scopus© Citations 39
- PublicationDigital computation of the complex linear canonical transform(Optical Society of America, 2011-07-01)
; ; ; ; ;An efficient algorithm for the accurate computation of the linear canonical transform with complex transform parameters and with complex output variable is presented. Sampling issues are discussed and the requirements for different cases given. Simulations are provided to validate the results. 506Scopus© Citations 11
- PublicationCross terms of the Wigner distribution function and aliasing in numerical simulations of paraxial optical systems(Optical Society of America, 2010-04-15)
; ;Sampling a function periodically replicates its spectrum. As a bilinear function of the signal, the associated Wigner distribution function contains cross terms between the replicas. Often neglected, these cross terms affect numerical simulations of paraxial optical systems. We develop expressions for these cross terms and show their effect on an example calculation 452Scopus© Citations 11
- PublicationThe choice of optical system is critical for the security of double random phase encryption systems(Society of Photo-optical Instrumentation Engineers (SPIE), 2017-06-14)
; ; ; ; ; ; ;The linear canonical transform (LCT) is used in modeling a coherent light field propagation through first-order optical systems. Recently, a generic optical system, known as the Quadratic Phase Encoding System (QPES), for encrypting a two-dimensional (2D) image has been reported. In such systems, two random phase keys and the individual LCT parameters (, , ) serve as secret keys of the cryptosystem. It is important that such encryption systems also satisfies some dynamic security properties. In this work, we therefore examine such systems using two cryptographic evaluation methods, the avalanche effect and bit independence criterion, which indicate the degree of security of the cryptographic algorithms using QPES. We compared our simulation results with the conventional Fourier and the Fresnel transform based DRPE systems. The results show that the LCT based DRPE has an excellent avalanche and bit independence characteristics compared to the conventional Fourier and Fresnel based encryption systems.Keywords: Quadratic Phase Encoding system, linear canonical transform, Double Random Phase Encryption, Avalanche effect and bit independence criterion. 478Scopus© Citations 2
- PublicationCases where the linear canonical transform of a signal has compact support or is band-limited(Optical Society of America, 2008-02-01)
;A signal may have compact support, be band-limited (i.e., its Fourier transform has compact support), or neither (“unbounded”). We determine conditions for the linear canonical transform of a signal having these properties. We examine the significance of these conditions for special cases of the linear canonical transform and consider the physical significance of our results 366Scopus© Citations 52
- PublicationSparsity based Terahertz reflective off-axis digital holography(Society of Photo-optical Instrumentation Engineers (SPIE), 2017-04-24)
; ; ; ; ; ; ; ;Terahertz radiation lies between the microwave and infrared regions in the electromagnetic spectrum. Emitted frequencies range from 0.1 to 10 THz with corresponding wavelengths ranging from 30 m to 3 mm. In this paper, a continuous-wave Terahertz off-axis digital holographic system is described. A Gaussian fitting method and image normalisation techniques were employed on the recorded hologram to improve the image resolution. A synthesised contrast enhanced hologram is then digitally constructed. Numerical reconstruction is achieved using the angular spectrum method of the filtered off-axis hologram. A sparsity based compression technique is introduced before numerical data reconstruction in order to reduce the dataset required for hologram reconstruction. Results prove that a tiny amount of sparse dataset is sufficient in order to reconstruct the hologram with good image quality. 379Scopus© Citations 5