Volume 34, Issue 6, June 2007
Index of content:
- Imaging Continuing Education Course: Room L100J
- CE‐Imaging: The Physics and Technology of Breast Imaging — I
34(2007); http://dx.doi.org/10.1118/1.2761195View Description Hide Description
Mammography still remains the best screening tool for early detection of breast cancer, and the recent ACRIN DMIST results draw attention to the fact that digital mammography is better at detecting cancers in dense breasts. Digital mammography equipment has continued to develop and improve and more vendors and models have entered the marketplace with FDA approval. The technology used in these systems generally falls into four main categories: direct capture detectors, indirect capture detectors, slot scanning, and cassette‐based storage phosphors(computed radiography). Additionally, a photon counting system is being developed. Each of these technologies has its advantages and disadvantages. A review of the systems currently available is presented, including the detector technology utilized. Features of these systems are also presented. This is a Self Assessment Module (SAM) to help fulfill the requirements of the Maintenance of Certification (MOC) process as defined by the American Board of Radiology (ABR). As such, audience participation through interactive questions will be provided during the course of the lecture.
1. Become familiar with the detector technologies currently used in digital mammography systems.
2. Become familiar with the digital mammography systems available from each manufacturer.
3. Become familiar with the unique elements of each system.
MO‐SAMS‐L100J‐02: Digital Mammography Quality Assurance: Comparing the Manufacturers Recommendations34(2007); http://dx.doi.org/10.1118/1.2761196View Description Hide Description
Digital mammography is quickly becoming the technology of choice for breast imaging with several FDA‐approved systems already available and more on the way. Digital detectors in mammography have different characteristics compared to the traditional screen‐film systems and require by the FDA different Quality Control tests. This lecture is going to discuss the practical issues for the medical physicist who wants to learn the differences in these Quality Control tests and how they impact the mammography facility and ACR accreditation. The lecture will be broken into several parts. The first will review currently available FFDM equipment and their respective Quality Control tests. The second part will compare similar QC tests and discuss the key differences. The third part will review quality control for review workstations and laser printers designed for digital mammography.
1. To describe current quality control procedures for FFDM systems.
2. To describe current quality control procedures for review workstations in digital mammography.
3. To describe current quality control procedures for laser printers in digital mammography.
4. To review the impact of QC on ACR accreditation for FFDM systems.
- CE‐Imaging: The Physics and Technology of Computed Tomography — II
34(2007); http://dx.doi.org/10.1118/1.2761307View Description Hide Description
The assessment of radiation dose from computed tomography has become an important issue due to the increased utilization of computed tomography in a large number of clinical applications, from CT urography to cardiac CT. The traditional metrics used for CTdose assessment (CTDI‐100) have come under assault from a number of investigators, because of its limitations in describing the radiation dose in realistic CT examinations. While there remains no real consensus in the field, a number of groups are working on the development of CTdose metrics which convey a better understanding of the radiation dose received by individual patients for specific CT examinations. In this presentation, the perspective of a number of groups will be presented. Primarily, the work of a committee of the International Commission of Radiological Units (ICRU) commissioned in 2005 will be discussed.
The ICRU committee has preliminarily defined a multi‐tier system for the assessment of radiation dose metrics in computed tomography. At the first level, machine‐dependent performance factors are described and measured, and these include the traditional CTDI‐100 metric for the 16 cm and 32 cm diameter polymethylmethacrylate dosimetry phantoms, at the center and at the periphery. There is a consensus amongst the ICRU community that radiation dose to patients undergoing CT examinations is best established using the known geometry of the CT examination, coupled with measured output characteristics of the specific CT scanner (including bow‐tie characteristics), combined with Monte Carlo computations. In the rare instance in which the CTdose to a specific patient needs to be computed, image‐based methodologies will be presented which enable these computations with a high degree of accuracy. It is emphasized that the effective dose (measured in milliseiverts) is not an appropriate measure for individual patient doses, as this metric includes population‐based radiation epidemiological data which may not be applicable to a specific individual. Therefore, the ICRU efforts towards radiation dosimetry of the individual patient focus on dosimetric units which are physical in nature, and describe the individual organdoses and the overall average dose to the patient, depending upon the specific CT examination and the patient's physical characteristics.
1. Convey some of the issues in accurate dose assessment in CT.
2. Describe some of the ongoing efforts of the ICRU committee on CT towards dose assessment.
TU‐SAMS‐L100J‐02: Determination of Dose From CT Examinations: Estimating Organ Dose Using Monte Carlo‐Based Methods34(2007); http://dx.doi.org/10.1118/1.2761308View Description Hide Description
Radiation dose from computed tomography has been an important issue for medical physicists for some time. This issue has increased in significance in the past several years due to advances in multidetector CT, which have resulted in increased utilization in areas such as pediatric CT,cardiacCT and even screening applications. Some of the key issues currently facing the medical physics community are assessing dose to patients from these various exams. One of the key building blocks to these assessments is the estimation of organ dose.
The purpose of this presentation is to describe an approach to estimating organ doses to patients using Monte Carlo‐based simulation methods. In this approach, both the scanner and patient are modeled in some detail and a CT exam may be simulated.
The detailed modeled of the CTscanner is created by including information such as the source spectra and filtration, its geometry, beam collimation and the path that the source travels around the patient (such as the path during a helical CT exam). The development, testing and validation of these models will be discussed.
Patient models generally fall into two categories. The first consists of geometric descriptions of organs (based on cones, cylinders, etc.) such as the MIRD phantom. The second consists of voxelized descriptions of patient anatomy that are created based on actual patient scans. In these, radiosensitive organs are identified in the image data to create a voxelized model of the patient geometry. For both types of models, there are challenges to create models representing patients of different sizes, ages and genders.
Once both the scanner and patient are modeled, then different scan protocols can be simulated using a Monte Carlo based software package (such as MCNP or EGS). This involves selecting a scanner model, a patient model and then selecting a set of technical parameters, such as one would do for an actual scan — including body region being examined, etc. The Monte Carlosoftware then simulates the specified scan and tallies absorbed dose in each voxel or geometric unit of the patient model, which then allows the calculation of either mean organ dose or the distribution of dose within an organ.
In this presentation, the results of this approach in several applications will be described, including: (a) dose to the fetus in pregnant women of early, middle and late gestational ages, (b) comparing dose to glandular breast tissue from thoracic CT scans both with and without tube current modulation.
1. Understand the Monte Carlo simulation based approach to estimating radiation dose to radiosensitive organs from CT scans.
2. Understand the current limitations of the Monte Carlo based approach.
3. Describe the results of some current applications of this approach to estimating fetal dose as well as breast dose reduction from tube current modulation.
34(2007); http://dx.doi.org/10.1118/1.2761309View Description Hide Description
CT technology continues to develop at a rapid pace, offering imaging options and features that can dramatically improve image quality. Multichannel systems are now commonplace and the number of channels continues to increase, allowing greater coverage per rotation, shorter scan times, and thinner images. Isotropic volumetric data acquisition permits retrospective reconstructions of many different image thicknesses and reformats can be created through multiple planes. These and many other advances have escalated and expanded the utility of CTimaging as a core diagnostic tool.
However, coupled with the improved CT technology is the increased complexity of operating the scanners and the elevated potential of increasing the radiation dose. CT operators must choose from multiple options, many of which are interdependent, for the control of the multitude of available features. The impact of each of these options on image quality and radiation dose can range from subtle to substantial, and may not necessarily be obvious to the operator.
This lecture will focus on the clinical implications of CT scan parameters and provide guidance on achieving an optimal compromise between image quality and radiation dose when constructing CT scan protocols.
1. Understand the influence of primary CT scan parameters on image quality and radiation dose.
2. Learn how to use imaging task‐specific priorities with consideration for radiation dose when determining scanner settings.
- CE‐Imaging: The Physics and Technology of Radionuclide Imaging — III
34(2007); http://dx.doi.org/10.1118/1.2761473View Description Hide Description
Positron emission tomography or PET is now considered one of the most important clinical imaging modalities, particularly in the fields of oncology, neurology and cardiology. In the past 5 years, hybrid scanners that combine the technologies of PET and computed tomography(CT) have become the instrument of choice for most imaging clinics. This presentation will discuss the instrumentation, data acquisition and image reconstruction associated with state‐of‐the‐art PET and PET‐CT systems. This talk will review the basics of positron emission, annihilation coincidence detection and instrumentation design. It will review detector designs in state‐of‐the‐art PET systems including the choice of detector materials. We will also describe both 2D and 3D PETdata acquisition modes and the merits of each. We will discuss random coincidences, scatter and attenuation correction and current approaches being used to correct for these. We will also review the methods currently sued to reconstruct these data.
At the end of this presentation, the participants will be able to
1. list 3 detector materials routinely used in PET and at least one advantage or disadvantage of each,
2. describe the difference between 2D and 3D PETdata acquisition,
3. discuss 2 advantages and 2 disadvantages of CT‐based attenuation correction and list 3 approaches to reconstruction and 2 advantages of each.
34(2007); http://dx.doi.org/10.1118/1.2761474View Description Hide Description
The performance evaluation of PETscanners should follow the NEMA PET NU2‐2001 standard. This standard uses a polyethylene phantom of 700mm axial length with a line source to measure scatter fraction, count losses and randoms. The measurement of sensitivity is conducted with a line source surrounded by known absorbers, and the sensitivity with no absorbers can be found by extrapolation. The intent of the image quality measurement is to mimic a whole body scan using a torso phantom containing hot and cold spheres of various diameters (representing lesions) in a warm background. An updated standard that considers the intrinsic radiation in the scintillators has been drafted and will be discussed.
Quality assurance of PETscanners must be performed on a regular basis to maintain and confirm proper scanner performance. These procedures should track system stability and be sensitive to changes in scanner operation. The quality control and calibration of a PETscanner includes detector and electronic characterizations such as adjustment of PMT gain, definition of crystal and energy maps and coincidence timing calibration. These characterizations are applied to the PET data during acquisition. A PETquality control regimen includes system corrections such as normalization, calibration and, in the case of non‐PET/CT systems, blank scans. The accuracy of the coregistration of the PET and CTimages from a PET/CT scanner should also be monitored.
This presentation will focus on the rationale and methodology of the NEMA NU2‐2001 performance standards. The presentation will also describe the calibrations and corrections required to maintain proper system performance.
1. Describe NEMA NU2‐2001 PET performance standards.
2. Describe the calibrations required to properly detect the location of a coincident event.
3. Describe the post acquisition corrections required to minimize image artifacts.
- CE‐Imaging: Radiation Safety and Risk Management — III
34(2007); http://dx.doi.org/10.1118/1.2761613View Description Hide Description
Application of the structural shielding design techniques and goals as outlined in NCRP Reports 147: Structural Shielding Design for Medical X‐ray Imaging Facilities (2004) will be explored in this practical course. Actual facility designs will be used as examples with particular calculations demonstrated for all types of x‐ray imaging installations: Multi‐Slice CT,radiographic and fluoroscopic units. To meet the challenge of maintaining construction costs to a minimum while providing adequate radiation shielding protection requires the physicist to utilize all available materials to reduce radiation exposure to surrounding personnel and the public. Estimating future workloads as well as considering current workloads for radiographic/fluoroscopic equipment as the medical imaging community transitions from a film/screen based world to a digital world can present challenges. Practical examples of these methods of structural shielding designs will be explored in this course.
1. Understand the methods of calculation to be used for CT and radiographic/fluoroscopic installations in medical facilities.
2. Understand the effectiveness of various shielding materials found in facilities to provide required structural shielding necessary to reduce anticipated radiation exposure levels to acceptable limits.
3. Understand the calculation of anticipated workloads for radiographic/fluoroscopic equipment and the effect of these workloads on structural shielding evaluations.
34(2007); http://dx.doi.org/10.1118/1.2761614View Description Hide Description
The application of the structural shielding design techniques and goals as outlined in AAPM Task Group Report 108: PET and PET/CT Shielding RequirementsMedical Physics (Vol. 33., Issue 1 (2006)) will be the basis for this practical course. Actual facility designs will be used as the example calculations of required shielding for PET/CT installations. As the use of PET and PET/CT units expands rapidly in the medical arena, the requirements for providing adequate radiation protection for both occupational personnel in these facilities and the public in uncontrolled areas around them necessitate the involvement of a qualified medical physicist. The many areas involved in implementing a PET/CT program including the Hot Lab, Patient Uptake Rooms, Patient Restrooms, Scan Rooms, and Disposal areas will be used as practical examples of typical structural shielding designs and evaluation methods.
1. Understand the exposure factors to be used for currently used PETisotopes to determine required structural shielding to meet exposure limits for occupational personnel and the public.
2. Understand the effectiveness of existing and additional structural shielding materials that provide radiation protection and methods to calculate the required amounts of these materials.
3. Understand the methods to be used in performing the shielding calculations of PET and PET/CT installations to insure adequate shielding to be provided to meet applicable state and ALARA requirements.