Tomographic Methods in Nuclear Medicine

Physical Principles, Instruments, and Clinical Applications

Bhagwat D. Ahluwalia

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000141863
• Edition: 1
• Topic: Medicine
• Subtopic: Radiology, Radiotherapy & Nuclear Medicine

This publication is a compendium of physical principles, system descriptions, instrument quality assurance, and clinical applications of extant tomographic methods in nuclear medicine. Written by an expert in this pertinent field, each chapter deals with the topics in a comprehensive fashion to provide a ready reference of all the work done on the subject and an estimate of the future utilization. Descriptions of methods available to nuclear medicine for tomographic viewing include positron emission, single photon emission, and planar tomography. This is an excellent resource volume of general applicability for nuclear medicine physicians, nuclear medicine scientists, and researchers in organ imaging and processing techniques.

Chapter 1

PROMISING IMAGING MODALITIES

B. Ahluwalia

For a nuclear medicine imaging modality, the major requirements are to provide three-dimensional information, minimize the effect of overlying and underlying activity and structure, good resolution, and regional quantitation. Until about 10 years ago, the nuclear medicine imaging modality was limited to planar imaging. Cross-sectional imaging of the brain was undertaken by Kuhl and Edwards in 1963.¹ Progress in transverse section imaging has been quite slow due to the developments in instrumentation, radiopharmaceuticals, and computer systems. In the early 1970s, the positron imaging modality was utilized to provide transverse sections of organs with biological radionuclides. Nuclear medicine imaging equipment, radiopharmaceuticals, and computer systems have since undergone developments. Imaging modalities such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) offer great promise in the future. PET developments have displayed various frontiers for development of SPECT.

In this chapter, the future of imaging modalities is reviewed. Pertinent instrumentation and clinical applications of SPECT and PET are discussed in individual chapters in the book. For brain imaging, biological requirements for a radiopharmaceutical are ability to cross blood-brain-barrier (BBB) and to distribute in the brain, proportional to the blood flow. Once in the brain, the tracer should stay fixed for a time sufficient for SPECT data acquisition. To cross the BBB the complex has to be lipophilic, the molecules small in size, and possessing no charge. Iodine-123-labeled p-iodo-N-isopropylamphetamine (¹²³I-IMP) has been widely used for brain imaging and regional blood flow. ¹²³I-IMP has been shown to be quite promising for SPECT imaging.³

The growth and utility of SPECT for central nervous system disease has centered around the use of iodinated amphetamines and amphetamine derivatives. The accumulation of ¹²³I-IMP is proportional to blood flow. These amphetamine compounds cross the BBB and are retained, to a large extent, by the cerebral cells. The exact mechanism of uptake is not clear.^{3, 4} The IMP compounds are cleared from circulation in the first few minutes. This compound is useful for the study of cerebral infarctions, transient ischemia, and epilepsy. Other problems which will control the success of this imaging agent are availability and cost.

Routine myocardial perfusion imaging is performed with ²⁰¹T1 for noninvasive evaluation of coronary artery disease. Due to the low energy of radiation, ²⁰¹T1 is not a suitable agent. Furthermore, ²⁰¹T1 has a long half-life and serial imaging cannot be performed. A new technetium-labeled radiopharmaceutical, ^99mTc-carbomethoxyisopropyl isonitrile (^99mTc-CPI), has favorable characteristics. It accumulates in the myocardium and rapidly clears from lung and liver.^{5 99m}Tc-CPI can be prepared on site and can be applied for evaluation of acute myocardial infarction. This agent has great promise for SPECT imaging in evaluation of global and regional ventricular function. It does not redistribute into zones of ischemia, at least for several hours.

It is anticipated that new receptor-based radiopharmaceuticals will play an important role in the study of pathophysiology and organ imaging. Advanced SPECT imaging systems, radiolabeled amines, and radiolabeled isonitrile have opened new avenues for cerebral and myocardial imaging. For brain imaging, some investigators claim that the lesion size, location, and correlation of the areas with a clinical picture is superior to X-ray transmission. It is yet to be seen how the information obtained with these imaging agents correlates with magnetic resonance imaging (MRI).

SPECT imaging will still be affected by modifications in attenuation corrections. Most methods assume uniform attenuation in the thorax region, however, the attenuation is non-uniform in this region. This attenuation correction plays an important role as far as net image quantitation is concerned. The major target of the imaging modalities is to provide quantitative information from the transverse sectional images. SPECT imaging needs modification for quantitation.

Attenuation correction is more complex in SPECT than in PET. Due to the higher energies of annihilated gamma, the scatter contribution is less than the 140 keV gamma from ^99mTc. Similarly, the attenuation correction factor is much higher for technetium imaging than PET. The attenuation correction for PET depends upon the total distance along the line of flight. Attenuation corrections are simple in PET. Although most of the workers for SPECT assume a uniform attenuation factor, a true attenuation factor must be measured for each pixel for adequate correction in SPECT. In PET, sequential transmission and emission data are accumulated which is only possible when working with short-lived radionuclides. Furthermore, the quantitation will always be limited to the organ size in both PET and SPECT.

One of the major limitations of SPECT imaging is that studies can be performed only at equilibrium and rapid time sequential studies cannot be performed. This is due to limitations of the equipment and the time involved in imaging. This drawback can be overcome by adopting multi-ring cameras or multi-planar cameras for sequential imaging. The cost of such equipment will be comparable with the cost of PET imaging devices.

It is evident that three important modalities, PET, SPECT, and MRI, are the imaging modalities of the future. PET imaging has undergone great advances in technology and radiopharmaceuticals, however, due to the complexities and instrumentation, it will be limited to the larger diagnostic centers. Advances of PET are being translated into SPECT with the availability of ¹²³I and ^99mTc radiopharmaceuticals. The growth utilization of SPECT is slow but forthcoming, and true organ quantitation is yet to be seen. Developments in instrumentation are also expected to improve the sensitivity of the SPECT system.

Chapters 2, 3, and 4 deal with the reconstruction theory and instrumentation of SPECT imaging. Chapter 5 covers the clinical applications of SPECT. Chapter 6 provides a brief review of PET instrumentation and organ imaging applications. Chapters 7, 8, 9, 10, and 11 deal with some of the other section imaging techniques used in nuclear medicine.

To summarize, PET and SPECT both have important roles to play in the future of nuclear medicine imaging. PET imaging offers more than SPECT, as a host of biological tracer-labeled radiopharmaceuticals are available and their pertinent kinetics are well understood. It is anticipated that studies of organ pathophysiology performed with positron emitters will provide guiding pathways for development of SPECT receptor radiopharmaceutical and organ imaging applications.

REFERENCES

1.  Kuhl, D. E. and Edwards, R. Q., Image separation radioisotope scanning, Radiology, 80, 653, 1963.

2.  Kuhl, D. E. and Edwards, R. Q., Cylindrical section radioisotope scanning of the liver and brain, Radiology, 83, 926, 1964.

3.  Lee, R. G., Hill, T. C., Holman, B. L., et al., N-Isopropyl-I-123-p-iodoamphetamine brain scans with single photon emission tomography. Discordance with transmission computed tomography, Radiology, 145, 795, 1982.

4.  Holman, B. L., Lee, R. G. L., Hill, T. C., et al., A comparison of two cerebral perfusion tracers, N-isopropyl-I-123-p-iodoamphetamine and I123-HIPDM in human, J. Nucl. Med., 25, 25, 1984.

5.  Holman, B. L., Sporn, V., Jones, A. G., et al., Myocardial imaging with Tc-99m-CPI. Initial experience in human, J. Nucl. Med., 28, 13, 1987.

Chapter 2

THEORY OF TOMOGRAPHIC RECONSTRUCTION

A. Todd-Pokropek

TABLE OF CONTENTS

I.	Introduction
II.	Mathematical Overview A. The Radon Transform (How the Data Are Sampled) B. Tools: Forward- and Backprojection C. The Inverse Radon Transform D. General Matrix Inverse E. Solutions to III-Posed Problems
III.	Iterative Algorithms A. ART B. ART2 and ART3 C. Simultaneous Iterative Reconstruction Technique (SIRT)
IV.	2-D Fourier Reconstruction A. Filtered Backprojection B. A Basic Reconstruction Using Filtered Backprojection
V.	Practical Considerations A. Filters B. Noise C. Interpolation D. Pre- and Postprocessing Filters
VI.	Other Reconstruction Methods A. Maximum Likelihood and Entropy B. Steepest Descent and Conjugate Gradient Algorithms C. Regularization
VII.	Attenuation Correction in ECT A. Preprocessing B. Attenuated Inverse Radon Transform C. Postprocessing Correction D. Iterative Methods E. Scatter Correction F. Models G. Inverse Monte Carlo Methods
VIII.	3-D Reconstruction
IX.	Summary and Conclusions
References

I. INTRODUCTION

Tomographic reconstruction techniques have been employed in a variety of medical applicati...

Table of contents

• Cover
• Title Page
• Copyright Page
• Foreword
• Preface
• The Editor
• Acknowledgments
• Dedication
• Contributors
• Table of Contents
• Chapter 1 Promising Imaging Modalities
• Chapter 2 Theories of Tomographic Reconstruction
• Chapter 3 Computer and Hardware Requirements for Single Photon Emission
• Chapter 4 Single Photon Emission Computed Tomography: Imaging Systems and Quality Control
• Chapter 5 Clinical Applications and Protocols of Single Photon Emission Computed Tomography
• Chapter 6 Positron Emission Tomography Imaging Systems and Applications
• Chapter 7 Focal Plane Dual Head Longitudinal Tomography
• Chapter 8 Limited Angle Slant Hole Tomography
• Chapter 9 Seven-Pinhole Tomography in Nuclear Medicine
• Chapter 10 Seven-Pinhole Tomographic Reconstruction Algorithm
• Chapter 11 Coded Aperture Tomography
• Index