Medical Imaging

Anthony Brinton Wolbarst, PhD, a physicist formerly at Harvard Medical School, the National Cancer Institute, and the U.S. Environmental Protection Agency, is currently an Associate Professor at the University of Kentucky College of Health Sciences, Division of Radiation Sciences and College of Medicine, Department of Diagnostic Radiology in Lexington, Kentucky, USA. Patrizio Capasso, MD, is Professor and Division Chief of Vascular & Interventional Radiology in the Departments of Diagnostic Radiology and Surgery at the University of Kentucky Chandler Medical Center Lexington, Kentucky, USA. Andrew R. Wyant, MD, is Assistant Professor for Physician Assistant Studies at the University of Kentucky Chandler Medical Center Lexington, Kentucky, USA. Among many other courses that he teaches is a popular clinical skills seminar in in Radiographic Interpretation.

Preface x

Acknowledgments xiii

Introduction: Dr. Doe’s Headaches: An Imaging Case Study xiv

Computed tomography xiv

Picture archiving and communication system xv

T1, T2, and FLAIR MRI xvi

MR spectroscopy and a virtual biopsy xvii

Functional MRI xviii

Diffusion tensor MR imaging xviii

MR guided biopsy xx

Pathology xxi

Positron emission tomography? xxi

Treatment and follow-up xxii

1 Sketches of the Standard Imaging Modalities: Different Ways of Creating Visible Contrast Among Tissues 1

“Roentgen has surely gone crazy!” 2

Different imaging probes interact with different tissues in different ways and yield different kinds of medical information 4

Twentieth-century (analog) radiography and fluoroscopy: contrast from differential attenuation of X-rays by tissues 7

Twenty-first century (digital) images and digital planar imaging: computer-based images and solid-state image receptors 16

Computed tomography: three-dimensional mapping of X-ray attenuation by tissues 17

Nuclear medicine, including SPECT and PET: contrast from the differential uptake of a radiopharmaceutical by tissues 20

Diagnostic ultrasound: contrast from differences in tissue elasticity or density 26

Magnetic resonance imaging: mapping the spatial distribution of spin-relaxation times of hydrogen nuclei in tissue water and lipids 28

Appendix: selection of imaging modalities to assist in medical diagnosis 30

References 36

2 Image Quality and Dose: What Constitutes a “Good” Medical Image? 37

A brief history of magnetism 37

About those probes and their interactions with matter . . . 39

The image quality quartet: contrast, resolution, stochastic (random) noise, artifacts – and always dose 47

Quality assurance 57

Known medical benefits versus potential radiation risks 61

3 Creating Subject Contrast in the Primary X-ray Image: Projection Maps of the Body from Differential Attenuation of X-rays by Tissues 67

Creating a (nearly) uniform beam of penetrating X-rays 69

Interaction of X-ray and gamma-ray photons with tissues or an image receptor 75

What a body does to the beam: subject contrast in the pattern of X-rays emerging from the patient 83

What the beam does to a body: dose and risk 87

4 Twentieth-century (Analog) Radiography and Fluoroscopy: Capturing the X-ray Shadow with a Film Cassette or an Image Intensifier Tube plus Electronic Optical Camera Combination 91

Recording the X-ray pattern emerging from the patient with a screen-film image receptor 92

Prime determinants/measures of image quality: contrast, resolution, random noise, artifacts, . . . and, always, patient dose 98

Special requirements for mammography 114

Image intensifier-tube fluoroscopy: viewing in real time 122

Conclusion: bringing radiography and fluoroscopy into the twenty-first century with solid-state digital X-ray image receptors 125

Reference 126

5 Radiation Dose and Radiogenic Risk: Ionization-Induced Damage to DNA can cause Stochastic, Deterministic, and Teratogenic Health Effects – And How To Protect Against Them 127

Our exposure to ionizing radiation has doubled over the past few decades 127

Radiation health effects are caused by damage to DNA 129

Stochastic health effects: cancer may arise from mutations in a single cell 132

Deterministic health effects at high doses: radiation killing of a large number of tissue cells 139

The Four Quartets of radiation safety 146

References 151

6 Twenty-first Century (Digital) Imaging: Computer-Based Representation, Acquisition, Processing, Storage, Transmission, and Analysis of Images 152

Digital computers 153

Digital acquisition and representation of an image 157

Digital image processing: enhancing tissue contrast, SNR, edge sharpness, etc. 166

Computer networks: PACS, RIS, and the Internet 168

Image analysis and interpretation: computer-assisted detection 170

Computer and computer-network security 172

Liquid crystal displays and other digital displays 173

The joy of digital 174

7 Digital Planar Imaging: Replacing Film and Image Intensifiers with Solid State, Electronic Image Receptors 176

Digital planar imaging modalities 176

Indirect detection with a fluorescent screen and a CCD 178

Computed radiography 178

Digital radiography with an active matrix flat panel imager 179

Digital mammography 184

Digital fluoroscopy and digital subtraction angiography 186

Digital tomosynthesis: planar imaging in three dimensions 189

References 190

8 Computed Tomography: Superior Contrast in Three-Dimensional X-Ray Attenuation Maps 191

Computed tomography maps out X-ray attenuation in two and three dimensions 192

Image reconstruction 198

Seven generations of CT scanners 204

Technology and image quality 208

Patient- and machine-caused artifacts 219

Dose and QA 221

Appendix: mathematical basis of filtered back-projection 229

References 233

9 Nuclear Medicine: Contrast from Differential Uptake of a Radiopharmaceutical by Tissues 234

Unstable atomic nuclei: radioactivity 235

Radiopharmaceuticals: gamma- or positron-emitting radionuclei attached to organ-specific agents 245

Imaging radiopharmaceutical concentration with a gamma camera 248

Static and dynamic studies 254

Tomographic nuclear imaging: SPECT and PET 260

Quality assurance and radiation safety 270

References 273

10 Diagnostic Ultrasound: Contrast from Differences in Tissue Elasticity or Density Across Boundaries 274

Medical ultrasound 274

The US beam: MHz compressional waves in tissues 277

Production of an ultrasound beam and detection of echoes with a transducer 280

Piezoelectric transducer elements 281

Transmission and attenuation of the beam within a homogeneous material 285

Reflection of the beam at an interface between materials with different acoustic impedances 288

Imaging in 1 and 1 × 1 dimensions: A- and M-modes 291

Imaging in two, three, and four dimensions: B-mode 294

Doppler imaging of blood flow 300

Elastography 302

Safety and QA 303

11 MRI in One Dimension and with No Relaxation: A Gentle Introduction to a Challenging Subject 307

Prologue to MRI 308

“Quantum” approach to proton nuclear magnetic resonance 310

Magnetic resonance imaging in one dimension 316

“Classical” approach to NMR 321

Free induction decay imaging (but without the decay) 331

Spin-echo imaging (still without T1 or T2 relaxation) 338

MRI instrumentation 343

Reference 351

12 Mapping T1 and T2 Relaxation in Three Dimensions 352

Longitudinal spin relaxation and T1 353

Transverse spin relaxation and T2-w images 364

T2∗ and the gradient-echo (G-E) pulse sequence 372

Into two and three dimensions 374

MR imaging of fluid movement/motion 382

13 Evolving and Experimental Modalities 387

Optical and near-infrared imaging 388

Molecular imaging and nanotechnology 390

Thermography 392

Terahertz (T-ray) imaging of epithelial tissues 393

Microwave and electron spin resonance imaging 393

Electroencephalography, magnetocardiography, and impedance imaging 394

Photo-acoustic imaging 396

Computer technology: the constant revolution 397

Imaging with a crystal ball 399

References 399

Suggested Further Reading 400

Index 403