Chapter 18: Image Perception and Assessment Slide set of 110 slides based on the chapter authored by I. Reiser of the IAEA publication (ISBN 978-92-0-131010-1): Diagnostic Radiology Physics: A Handbook for Teachers and Students Objective: To provide the student with an introduction to human visual perception and to task-based objective assessment of an imaging system. Slide set prepared by E. Berry (Leeds, UK and The Open University in London) IAEA International Atomic Energy Agency CHAPTER 18 TABLE OF CONTENTS 18.1. Introduction 18.2. The Human Visual System 18.3. Specifications of Observer Performance 18.4. Experimental Methodologies 18.5. Observer Models IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18. Slide 1 (02/110) 18.1 INTRODUCTION 18.1 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.1 Slide 1 (03/110) 18.1 INTRODUCTION 18.1 Introduction (1 of 2) The main purpose of a medical image is to provide information to a human reader, such as a radiologist, so that a diagnosis can be reached - rather than to display the beauty of the human internal workings It is important to understand how the human visual system affects the perception of contrast and spatial resolution of structures that are present in the image If the image is not properly displayed, or the environment is not appropriate, subtle clinical signs may go unnoticed, which can potentially lead to a misdiagnosis IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.1 Slide 2 (04/110) 18.1 INTRODUCTION 18.1 Introduction (2 of 2) This chapter provides an introduction to human visual perception and to task-based objective assessment of an imaging system Model for the contrast sensitivity of the human visual system • used to derive the gray-scale standard display function (GSDF) for medical displays Task-based assessment measures the quality of an imaging system • requires a measure of the ability of an observer to perform a well-defined task, based on a set of images • metrics for observer performance • experimental methodologies for the measurement of human performance • estimation of task performance based on mathematical observer models IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.1 Slide 3 (05/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2 Slide 1 (06/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2 The human visual system 18.2.1 The human eye 18.2.2 The Barten model 18.2.3 Perceptual linearization 18.2.4 Viewing conditions IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2 Slide 2 (07/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.1 THE HUMAN EYE IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.1 Slide 1 (08/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2 The human visual system 18.2.1 The human eye 18.2.2 The Barten model 18.2.3 Perceptual linearization 18.2.4 Viewing conditions IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.1 Slide 2 (09/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.1 The Human Eye Structure of the human eye The human eye is a complex organ that processes visual images and relates information to the brain (Dragoi) The retina is the light-sensitive part of the eye There are two types of photosensitive cells in the retina • rods and cones • light quanta are converted into chemical energy, giving rise to an impulse that is transferred via neurons to the optic nerve Each cone is connected to the optic nerve via a single neuron • spatial acuity of cones is higher than that for rods Several rods are merged into one neuron that connects to the optic nerve • light sensitivity is greater for rods than for cones IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.1 Slide 3 (10/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.1 The Human Eye Day and night vision The highest spatial acuity of the human visual field is in the fovea , a small region of the retina with a visual field of 2 degrees, where the density of cones is largest In scotopic (night) vision, luminance levels are low and only rods respond to light • rods are not colour sensitive; therefore objects appear grey at night In photopic (day) vision, both rods and cones are activated • photopic vision occurs at luminance levels between 1 and 10 6 cd/m2. IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.1 Slide 4 (11/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 THE BARTEN MODEL IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 1 (12/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2 The human visual system 18.2.1 The human eye 18.2.2 The Barten model 18.2.3 Perceptual linearization 18.2.4 Viewing conditions IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 2 (013/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Barten model A model for the contrast sensitivity of the human visual system has been developed by Barten Contrast sensitivity ( S) is inversely proportional to the threshold modulation ( mt) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 3 (14/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Threshold modulation mt is the ratio of the minimum luminance amplitude to the mean luminance amplitude luminance of a sinusoidal grating such that the grating has a 50% mean luminance probability to be detected IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 4 (15/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Barten model and threshold modulation Contrast sensitivity of the human visual system can be measured by presenting images of a sinusoidal grating to a human observer From repeated trials, the threshold modulation can be determined Barten’s model is based on the observation that in order to be detected, the threshold modulation of a grating needs to be larger than that of the internal noise mn by a factor k mt = kmn IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 5 (16/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Barten model and MTF As the image enters the eye, it is distorted and blurred by the pupil and optical lens This is described by the optical MTF ( Mopt ) and is a function of spatial frequency ( u) and incident luminance ( L) − (2 πσ (L)u)2 M opt (u,L) = e • the width of Mopt depends on pupil diameter which controls the 2 2 2 impact of lens aberrations ( Cab ), σ (L) = σ 0 + (Cabd(L)) • the dependence of pupil diameter ( d, in mm) on luminance ( L, in cd/m2) can be approximated by d(L) = 5 − 3tanh(0.4log L) IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 6 (17/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Barten model and photon noise When light quanta are detected by the retinal cells, photon noise ( Fph ) is incurred with a spectral density given by 1 Φ ph (L) = where ηpE(L) • η is the quantum detection efficiency of the eye • p is the luminous flux to photon conversion factor • E is the retinal illumination πd(L)2 E(L) = L{1− (d(L)/9.7)2 + (d(L)/12.4)4} 4 E(L) includes the Stiles-Crawford effect that accounts for variations in efficiency as light rays enter the pupil at different locations IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 7 (18/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Barten model – lateral inhibition and neural noise Lateral inhibition occurs in retinal cells surrounding an excited cell, and is described as 2 2 − (2 u / u0 ) M lat (u) =1− e Neural noise (F 0) further degrades the retinal signals These factors are included in threshold and noise modulation, to give the Barten model modulation IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 8 (19/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Barten model modulation (Φ (L) + Φ )M 2 (u) + Φ m M (u,L)M (u) = 2k ph ext lat 0 t opt lat XYT where • X, Y, T are the spatial and temporal dimensions of the object IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 9 (20/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Complete Barten model for contrast sensitivity of the human eye The complete Barten model is given by 1 M (u,L)/ k S(u,L) = = opt m (u,L) 2 1 1 u2 1 Φ t + + + 0 + Φ 2 2 2 2 ext T X 0 X max Nmax ηpE(L) M lat (u) where • X0 angular extent of the object • Xmax maximum integration angle of the eye • Nmax maximum number of cycles over which the eye can integrate IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 10 (21/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Contrast sensitivity as a function of luminance -3 -3 Parameters used were k=3, s0=8.33x10 deg, Cab =1.33x10 deg/mm, T=0.1 s, -8 2 Xmax =12 deg, Nmax =15 cycles, h=0.03, F0=3x10 sec/deg , u0=7 cycles/deg, 6 2 X0=2 deg, p=1x10 phot/sec/deg /Td The values for X0 and p correspond to the standard target used in DICOM14 IAEA Diagnostic Radiology Physics: A Handbook for Teachers and Students – 18.2.2 Slide 11 (22/110) 18.2 THE HUMAN VISUAL SYSTEM 18.2.2 The Barten Model Contrast sensitivity as a function of spatial frequency -3 -3 Parameters used were k=3, s0=8.33x10 deg, Cab =1.33x10 deg/mm, T=0.1 s, -8 2 Xmax =12 deg, Nmax =15 cycles, h=0.03, F0=3x10 sec/deg , u0=7 cycles/deg,