Index of content:
Volume 33, Issue 6, June 2006
- Imaging Continuing Education Course: Valencia B
- CE: Computed Technology Physics and Technology ‐ III
33(2006); http://dx.doi.org/10.1118/1.2241666View Description Hide Description
Over the past decade, x‐ray computed tomography has experienced tremendously technological advancements: the introduction of helical/spiral and multi‐slice/volumetric acquisition. These advancements not only allow improved image quality and enable new clinical applications, but also significantly increase the technical challenges associated with image reconstruction.
The first part of this lecture will cover the fundamentals of image reconstruction. For the ease of understanding, we start with an explanation of the central slice theorem (Fourier slice theorem). Both theoretical and intuitive approaches are used to illustrate the concept. The reconstruction algorithm is then extended to fan beam geometry by mathematical derivation and graphic description.
Using the central slice theorem as the foundation, reconstruction algorithms for helical acquisition are discussed in the second part of the lecture. We analyze, for single slice, the major difference between helical and step‐andshoot acquisitions. Implications of different reconstruction approaches on image quality and computational complexity are also discussed.
Cone beam reconstruction discussion will start with one of the most popular algorithms: FDK algorithm. The derivation of the algorithm from the fan‐beam case is first described and its extension to helical/spiral acquisition is then presented. The lecture ends with a discussion on some of the most recent advances in cone beam reconstruction, including both approximate and exact methods.
Jiang Hsieh is an employee of GE Healthcare Technologies.
1. Learn the fundamentals of x‐ray CTreconstruction.
2. Understand recent advancements in reconstruction algorithms.
- CE: Computed Tomography Physics and Technology ‐ I
33(2006); http://dx.doi.org/10.1118/1.2241390View Description Hide Description
In CT scanning,image quality has many components and is influenced by many technical parameters. While image quality has always been a concern for the physics community, clinically acceptable image quality has become even more of an issue as strategies to reduce radiation dose — to all patients, but especially to pediatric patients— has become a focus in many radiology practices.
The purpose of this presentation will be to first describe several of the components of CTimage quality — noise, slice thickness (Z‐axis resolution), low contrast resolution and high contrast resolution— as well as radiation dose and to describe how each of these may be affected by technical parameter selection. This presentation will pay particular attention to the tradeoffs that exist between different aspects of image quality, especially when the reduction of radiation dose is one of the objectives.
The presentation will then explore several mechanisms that can be used to reduce radiation dose in CT exams and the implications for the diagnostic image quality of the exam. Specifically, the implications of varying the tube current*time product (mAs), pitch or tablespeed (or for axial imaging, the table increment), slice thickness, beam energy (kVp), patient (or phantom) size and dose reduction options (such as tube current modulation) will be described for both radiation dose and diagnostic image quality. Finally, this presentation will emphasize that the tradeoffs between radiation dose and image quality are clinical‐task dependent; that is, the goals of the clinically indicated exam dictate what aspect of image quality may be emphasized for that exam (low contrast resolution or high contrastspatial resolution, etc.) and this will have implications for the amount of radiation dose reduction that is acceptable. This will be illustrated with examples from selected diagnostic imaging exams.
1. Understand key components of image quality in CT scanning as well as reinforce CT radiation dose concepts.
2. Understand the impact that technical parameter selection has on the various aspects of image quality and radiation dose.
3. Examine the tradeoffs between various aspects of image quality and radiation dose.
4. Examine the impact of these tradeoffs on a few clinical imaging protocols and illustrate the task‐dependence of image quality requirements.
- CE: Computed Tomography Physics and Technology ‐ II
33(2006); http://dx.doi.org/10.1118/1.2241482View Description Hide Description
This presentation will focus on examples and causes of artifacts produced by modern multi‐slice CTscanners. Many of these artifacts were recognized because of a stringent quality control program implemented in our institution. The design of this quality control program will be described, along with its relative costs and benefits. Extending such a program among scanners produced by different vendors will be discussed.
1. To recognize basic artifacts in patient and phantom images generated by modern multi‐slice CTscanners.
2. To understand some basic requirements of a quality control program for multi‐slice CTscanners.
- CE: Computed Tomography Physics and Technology ‐ IV
33(2006); http://dx.doi.org/10.1118/1.2241824View Description Hide Description
The American College of Radiology accreditation program for computed tomography, introduced in 2002, is quickly gaining in popularity. It has established minimum standards for dose and image quality. Since the inception of the program, the scanners themselves have continued to evolve in complexity and capability. Medical physicists are required by the program to perform an annual survey of each scanner and are increasingly called upon to provide assistance with dose/image quality analyses. Thus, the involvement of medical physicists in CT is necessarily increasing.
To provide optimal support for facilities in the accreditation program, it is necessary that physicists be fully aware of the requirements of the program. To that end, the essentials of the program will be briefly reviewed, including personnel requirements. The quality control component of the program will be discussed, with special attention given to the annual medical physics survey. Technologist testing requirements will also be discussed.
The medical physicist can have a positive impact on the phantom images submitted as part of the accreditation process, so the site scanning instructions will be reviewed in detail, with comments made regarding common pitfalls.
With the national awareness of radiationdoses in CT, this area is perhaps where the physicist can have the largest impact clinically. In this lecture, the ACR accreditation program dosimetry requirements will be presented, including a discussion of common pitfalls and corrective measures.
The education objectives of this program are to become familiar with:
1. the general requirements for the ACR CT accreditation program
2. the role and responsibilities of the medical physicist
3. the image quality and dose measurements required by the program
4. how to calculate CTDIw, CTDIvol, Dose Length Product and Effective Dose.
- CE: MRI Physics and Technology ‐ I
33(2006); http://dx.doi.org/10.1118/1.2241397View Description Hide Description
The advances in MRItechnology are relentless. Virtually every aspect of MR scanners is being modified and optimized. There are a myriad number of factors that drive these advances. Clinical requirements are the most obvious and important driver, but are significantly influenced by the clinical setting and/or business model. Is the scanner for a large hospital radiology department, or dedicated interventional procedures scanner, or for an orthopedic, cardiac, pediatric or breast practice or a walk‐in radiological clinic? Each of these market segments places a different relative importance on the various MR system performance specifications. Consequently, the commercially available MR scanners have their own unique operating characteristics as the various MRI vendors seek to satisfy their customers' needs. This lecture will examine some of the various subsystems and discuss selected development trends, such as:
1) Utilization of novel spatialencoding mechanisms to accelerate image acquisition: parallel imaging and the coil spatial response mechanism.
2) Magnet field strength is increasing and physical magnet size is changing: shorter magnets and larger apertures.
3) Increase the effective imaging volume with advances in moving couch methods: the couch becomes another spatialencoding mechanism.
4) Demands on the reconstruction engine are constantly growing: commercial consumer electronic technology advances help provide cost effective, faster solutions.
5) Parallel receive concepts are adapted to the transmit side: transmit‐SENSE. It is not commercially available, yet.
6) More than pretty pictures: numbers. The growth in quantitative MRI and CAD.
While these technology advances are increasing system complexity, flexibility, sequence capabilities, image quality, throughput efficiencies etc, associated technology advances are also mitigating the package footprint and costs.
1. Recognizing the advances in MRI.
2. Understand the technology underlying these advances.
3. Understand the scientific/medical reasons for these advances.
4. Recognize that the commercial implementations are targeted at specific customer segments/requirements.
Conflict of Interest: The author is employed by Hitachi Medical Systems America, Inc.
- CE: MRI Physics and Technology ‐ II
33(2006); http://dx.doi.org/10.1118/1.2241489View Description Hide Description
The applications of magnetic resonance imaging(MRI) in biomedicine are undergoing rapid evolution. Typically used to produce images that are viewed and subjectively rated by a radiologist,MRI is now being utilized as a scientific apparatus capable of making noninvasive measurements in living tissues. With care, a significant number of physical and biological measurements can be performed and related to individual pixels and groups of pixels in the MRimage. This presentation will address the challenges in obtaining quantitative data from MRI.
The presentation will review the principles of good practice in quantification, including quality assurance,MR data collection, and analysis. Limits on precision and accuracy are discussed and solutions proposed. Three major measurement topics are considered. First, geometrical quantities, such size, position and grouping of structures are discussed. Then, MRI signal‐derived quantities such as proton density and relaxation times are considered. Third, physiological quantities such as diffusion coefficients and measures of blood flow and perfusion are explored. The principles underlying the measurement of each quantity are given along with their biological and medical significance and practical approaches for their measurement. Shortcomings of the measurement processes and a summary of potential clinical applications are also discussed. Pathological observations are also compared with MRI‐derived quantities where appropriate.
This presentation is intended as an introduction to the field of measurement in MRI for anyone who desires to use the scope of modern measurement techniques to quantitatively determine the consequences of disease, its development or its reaction to therapy from MRimages. It will be of interest to medical physicists who are considering undertaking quantitative work with MRI, as well as those already in the field.
At the end of this session the attentive participant shall:
1. Appreciate the clinically important quantities that can be measured with MRI.
2. Be familiar with the methods and techniques used for quantitative MRI.
3. Have a basic understanding of the limits on precision and sources of error in quantitative MRI.
- CE: MRI Physics and Technology ‐ III
33(2006); http://dx.doi.org/10.1118/1.2241673View Description Hide Description
In recent years, the American College of Radiology (ACR) Magnetic Resonance Accreditation Program (MRAP) has been adopted by over 3000 sites, nearly half of the estimated MRI facilities in the United States. Those sites agree to follow a weekly QC program set up and monitored by a qualified medical physicist or MR scientist. They also agree to undergo initial and annual equipment performance evaluations by a qualified medical physicist/MR scientist. There are several published documents, including the ACR Phantom Testing Guidance and the 2004 ACR MRI QC Manual, which describe the tests and the performance criteria. These documents are helpful in providing guidance on submitting phantom images for accreditation. However, they allow considerable discretion to physicists doing these tests, and the scanners change more frequently than the published guidance.
A consulting medical physicist may see a variety of scanners, each for a short period of time, and needs to provide the sites with useful recommendations beyond the pass/fail status of the phantom tests. The physicist must gather this information from existing data and tests performed with the ACR and other available phantoms. This lecture will describe information which can be derived from those data and how it may be used for improving MRimage quality.
1. Learn the current status of the ACR MRAP program and the role of the medical physicist in that program.
2. Understand how to perform required phantom and annual tests on various scanners and the performance criteria for those tests.
3. Understand how the results of those tests can be combined and analyzed to troubleshoot problems.
4. Understand how QC test and phantom availability and results may vary depending on scanner manufacturer.
- CE: MRI Physics and Technology ‐ IV
33(2006); http://dx.doi.org/10.1118/1.2241832View Description Hide Description
Every MR system has its strengths and weaknesses, often varying tremendously both within and between the various system designs. Added to that, each site will have its own priorities and styles of use of their system. For an MR physicist to best assist in optimizing imaging procedures, it is extremely important for that individual to be both well‐versed in basic MR physics, and to be able to recognize (and understand) the impacts and trade‐offs of the technology in a large variety of imaging situations. The physicist should be able to determine and evaluate problems in patient images as well as phantom tests.
For this presentation, the basic relationships between MR imaging parameters will be reviewed with an emphasis on their implementation in common clinical protocols. Examples of how they are adapted for specialized procedures will then be given to demonstrate trade‐offs between contrast requirements; imaging speed; spatial resolution; geometrical accuracy, and patient safety issues. How specific field strength and gradient configurations affect these options will also be examined. Differences in RF coil properties will be discussed from the perspective of optimizing applications for a facility.
1. Understand the primary tissuecontrastcharacteristics of the standard clinical sequences; methods of selective elimination of tissue signals; and the role of exogenous agents to alter contrast.
2. Know how to consider the interactions between SNR, spatial resolution and imaging speed and how their combined effects determine image quality for a variety of clinical situations.
3. Understand circumstances under which patient physiology and system configuration may require modification of image acquisition parameters to achieve optimal image quality.