Volume 34, Issue 6, June 2007
Index of content:
- Imaging Continuing Education Course: Room M100J
CE‐Imaging: The Physics and Technology of Computed Tomography — I
34(2007); http://dx.doi.org/10.1118/1.2761189View Description Hide Description
Computed tomography technology continues to evolve at a rapid pace. Recent technical innovations include dual‐source CT, 256‐channel CT, and cone‐beam angio CT. In this three‐part presentation, the impact of each of these new technologies on physics performance evaluations will be considered.
In dual‐source CT, two x‐ray tubes and two data acquisition systems are mounted orthogonally on the same rotating gantry. The system can be used to perform traditional CT scanning similar to the 64‐slice system from the same manufacturer; 85 msec temporal resolution imaging; dual source 160 kW imaging; or dual‐energy imaging. The impact of two x‐ray sources and two data acquisition systems on performance testing is relatively straightforward, as the doses from each source are simply additive. However, to assess image quality in a clinically‐relevant manner, a proper understanding of typical scan parameters is essential. Several suggested scan modes for image quality evaluation will be presented.
1. Understand the design of dual‐source CT.
2. Recognize typical dose values from each source and from clinical protocols.
3. Gain familiarity with the image quality associated with typical clinical scan protocols.
34(2007); http://dx.doi.org/10.1118/1.2761190View Description Hide Description
Medical physicists are challenged to assess image quality and patient dose for 256‐channel CT scanners. Whereas current concepts for measurement of image quality still remain valid for 256‐channel scanners, concepts for CTdosimetry will have to change fundamentally. Current methodology for CTdosimetry, e.g. assessment of CTDI with a 10 cm CTionization chamber and standard CT dose phantoms, is not compatible with the x‐ray beam geometry of the new 256‐channel CT scanner. The beam width exceeds the length of the CTionization chamber and, at the exit surface of the phantom; the beam width even exceeds the length of the body phantom. Several options to overcome dosimetric problems of cone beam CT scanners will be considered.
1. Understand the design and operation concepts a new 256‐channel axial CT scanner.
2. Understand concepts for specifying and measuring absorbed dose for a new 256‐channel axial CT scanner.
3. Understand shortcomings in current concepts for assessment of absorbed dose and and patient dose for the new 256‐channel axial CT scanner.
34(2007); http://dx.doi.org/10.1118/1.2761191View Description Hide Description
Conebeam CT using C‐arm mounted large area flat panels (FPs) is becoming more commonly used in neuro‐ and body‐interventional suites. The ability to visualize vascular geometry in 3D along with soft tissueduring an intervention is providing information that may increase accuracy, shorten procedure time and may even change the course of treatment. However, acquisition of the projection data required for volume imaging requires dose to the patient, and understanding the trade‐offs between acquisition parameters, dose and image quality is critical to the appropriate adoption of this imaging technique.
Unlike conventional CT, the beam length in conebeam CT can cover the entire length of the object to be imaged or can be varied with the use of collimation, and the concept of CTDI is not, therefore, the metric of choice for measurement of dose. In addition, most C‐arm conebeam CT systems use a short‐scan (pi plus fan‐angle) acquisition, and the dose distribution within the object is not cylindrically symmetric. Finally, since FP design has been optimized for fluoroscopy, image quality when used for CT must be carefully evaluated.
This lecture will describe a dose metric that is appropriate for conebeam CT and allows direct comparison with the CTDIw of conventional CT. A perception study using the CATPHAN 600 phantom for a range of acquisition parameters will be described, and the relationships between kVp, image noise, dose and contrast perception will be discussed.
Research sponsored by Siemens Medical Solutions AG, AX Division, by NIH grant R01 EB003524 and by the Lucas Foundation.
1. Understand how differences in acquisition geometry between conebeam and conventional CT affect dose distribution and dose measurement.
2. Understand how contrast perception in conebeam CTimages depends on acquisition parameters.
CE‐Imaging: The Physics and Technology of Radionuclide Imaging — I
MO‐B‐M100J‐01: Determination of Dose to Individual Patients in Radionuclide Imaging Procedures Including Planar, SPECT, and PET34(2007); http://dx.doi.org/10.1118/1.2761201View Description Hide Description
Patient absorbed dose estimation due to internal emitters requires two separate computations prior to using the D = S*Ã formula. Here, D is the desired vector of target organdoses,S is a rectangular matrix of dose per radiodecay and Ã is a vector of cumulated activity curves for the various source organs in the patient. While S may be accessed from tabulations such as the OLINDA program from Vanderbilt University, Ã may be computed only by integrating activity curves A(t) out to sufficiently long times. Several methods are available to determine the activity in a given source organ. One may have tissues or lesions near the surface so as to use inverse square computations. At depth, the observer can use gamma cameras and a geometric mean image (GM), CT assisted matrix inversion (CAMI) or quantitative SPECTimaging. In the latter two cases, hybrid imaging devices such as SPECT/CT make the computations easier since image registration is greatly facilitated. There is also a possibility of PET/CT hybrid images being implemented with the SUV value being used to find the activity at‐depth. Lack of suitable positron labels makes PET studies problematic in the case of a general radiopharmaceutical, however. Errors in activity quantitation are typically on the order of +/− 30%, although they can be much larger in geometrically complicated cases. Two types of S values are commonly used. In type I computations, S refers to a phantom of appropriate size; e.g., adult male or female. Such dose estimates are included in applications to regulatory agencies (e.g. FDA). Additionally, type I results may be used to compare similar radiopharmaceuticals with regard to absorbed dose levels. Both diagnostic and therapeutic radiopharmaceuticals may be of interest. Type II calculations usually refer to a specific patient undergoing internal emitter therapy. Generally, alpha or beta emitters are used in this context. One may use Monte Carlo(MC) methods to find the S or correct tabulated phantom S elements to those approximating the patient. Since therapy involves short‐range emitters, S becomes diagonal and the correction is done via; S(patient) = S(phantom)*organ mass (phantom)/organ mass (patient). Such corrections can be very large — on the order of factors of two‐ or three‐fold. Finally, the uncertainty in the dose values may be estimated by combining errors in both S and Ã.
1. Understand the matrix equation D = S*Ã for estimating internal emitter radiationdoses to target organs.
4. Know the various methods to estimate source organ activity.
5. Realize that there are two types of calculations for dose; standard phantom or patient‐specific.
6. Know how to estimate uncertainties in the dose calculation.
CE‐Imaging: The Physics and Technology of Radionuclide Imaging — II
34(2007); http://dx.doi.org/10.1118/1.2761302View Description Hide Description
The general purpose gamma camera and those configured for SPECT are the bread‐and‐butter imaging instruments in nuclear medicine, and remains the principle component in systems specifically designed for cardiacSPECT. Routine maintenance, calibration, and quality control of these imaging systems is crucial for providing the nuclear physician or radiologist images of high quality without artifact, the bane of nuclear medicine imaging. Acceptance testing, for the most part, provides baseline measurements that are used as reference data for future comparisons.
Performance measurements and quality assurancetesting of gamma camera and SPECT have been quite variable. The vendors are very specific about calibration procedures that are done one site, but the quality assurancetesting by the customer are left up to the individual laboratories that vary to a large part based on the expertise of the staff. Federal and state radiation safety regulatory agencies purposely do not include any gamma cameraquality assurancetesting in their regulations. There are now two nuclear medicine accreditation programs, one by the American College of Radiology (ACR) and the other by the Intersocietal Commission for Accreditation of Nuclear Medicine Laboratories (ICANL), that have sought to standardize nuclear medicine imaging and quality assurance procedures performed. Also specified are the qualifications of the personnel who perform and interpret the imaging results.
This lecture will provide an overview of the gamma camera and SPECTcalibrations, performance measurements, and quality assurance tests, whether they are for acceptance testing or routine quality control. NEMA, ACR and ICANL accreditation program documents, and AAPM Task Group reports will be used as a backdrop for the presentation. A discussion of common image artifacts and how to avoid and correct them will also be included.
1. Learn the basic gamma camera and SPECT performance measurements and quality assurance test procedures.
2. Become familiar with the test procedures and the frequency of testing prescribed by the ACR and ICANL accreditation programs.
3. Be able to identify common gamma camera and SPECTimage artifacts and how to correct them.
CE‐Imaging: The Physics and Technology of Breast Imaging — II
TU‐B‐M100J‐01: Optimizing Mammography Image Quality and Dose: X‐Ray Spectrum and Exposure Parameter Selection34(2007); http://dx.doi.org/10.1118/1.2761315View Description Hide Description
Optimization of exposure parameters (target, filter, and kVp) in digital mammography necessitates maximization of the image signal‐to‐noise ratio (SNR), while simultaneously minimizing patient dose. The goal of this talk is to compare, for each of the major commercially available full field mammography (FFDM) systems, the impact of the selection of technique factors on imageSNR and radiation dose for a range of breast thickness and tissue types. The comparison will be based on the results of a multi‐center phantom study. The five commercial FFDM systems tested, the Senographe 2000D from GE Healthcare, the Mammomat Novation from Siemens, and the Selenia from Hologic, the Fischer Senoscan, and Fuji's 5000MA used with a Lorad M‐IV mammography unit, are located at five different university test sites. Performance was assessed using all available x‐ray target and filter combinations and nine different phantom types (three compressed thicknesses, and three tissue composition types). Each phantom type was also imaged using the automatic exposure control (AEC) of each system to identify the exposure parameters used under automated image acquisition. The figure of merit (FOM) used to compare technique factors is the ratio of the square of the imageSNR to the mean glandular dose (MGD). The results show that, for a given target/filter combination, in general FOM is a slowly changing function of kVp, with stronger dependence on the choice of target/filter combination. In all cases the FOM was a decreasing function of kVp at the top of the available range of kVp settings, indicating that higher tube voltages would produce no further performance improvement. For a given phantom type, the exposure parameter set resulting in the highest FOM value was system‐specific, depending on both the set of available target/filter combinations, and on the receptor type. Noise performance differed noticeably among the FFDM systems and played an important role in determining relative FOM values. In most cases, the AECs of the FFDM systems successfully identified exposure parameters resulting in FOM values near the maximum ones, however there were several examples where AEC performance could be improved.
1. become familiar with the effect of changing kVp, target material, and filtration on the mean glandular dose for a variety of breast.
2. become familiar with the effect of changing kVp, target material, and filtration on image signal and noise for specific commercial FFDM systems.
3. learn how the exposure technique factors selected for a variety of breast types by the AECs of current FFDM systems compare with the technique factors resulting in optimal FOM values.
CE‐Imaging: The Physics and Technology of Computed Tomography — III
34(2007); http://dx.doi.org/10.1118/1.2761464View Description Hide Description
This presentation will discuss the phantom physics tests required for ACR CT Accreditation. In particular it will focus on pitfalls to avoid in setting up the special ACR CT Phantom and performing the tests. In order to successfully complete all 0of the imaging tests, it is helpful to understand the internal construction of the ACR Phantom and it proper use. To that end, the details of the ACR CT Phantom construction will be presented. The most frequent failure is improper alignment of the phantom. Helpful methods of proper phantom alignment will be presented with examples of correct and incorrect images.
Another area of difficulty is determining and implementing appropriate “Axial” CT scan protocols as substitutes for “Helical” protocols. Examples for several different types of CT scanners (multiple detector row) and different manufacturers will be presented.
The ACR CT Phantom has 4 different imaging sections; the proper scanning of each of these sections will be presented. Commonly seen problems in imaging these sections will be presented. Finally, the radiation dose measurement process required for the ACR Physics submission will be discussed, again with commonly observed problems.
34(2007); http://dx.doi.org/10.1118/1.2761465View Description Hide Description
The ACR CT Accreditation program has existed since 2002. The CT Accreditation process includes the initial application, and the submission of clinical and phantom images and forms.
The medial physicist plays a critical role in the accreditation process, along with the lead technologist and the supervising radiologist. A team approach is necessary to ensure that all information is accurate and appropriate for the examinations (both clinical and phantom) submitted for accreditation. The medical physicist should assist the facility in assessing the condition of the scanner as well as optimizing the clinical protocols.
This lecture will provide a brief overview of the application steps, and some of the most common problems encountered during accreditation.
1. Understand the basics of the ACR CT Accreditation process.
2. Understand the relationship of the medical physicist in this process.
3. Understand some of the most common pitfalls in the accreditation process.
34(2007); http://dx.doi.org/10.1118/1.2761466View Description Hide Description
Between Sept. 2002 and Dec. 2004, 829 scanners underwent the ACR CT accreditation process (178 in 2002, 396 in 2003, and 255 in 2004). Volume CTDI values (mean ± std. deviation) were 66.7±23.5, 57.8±16.6, 54.6±13.3, 58.4±17.7 (head), 17.2±9.7, 15.9±8.6, 14.0±7.0, 15.5±8.4 (pediatric abdomen), and 18.7±8.0, 19.2±8.6, 17.0±7.6, 18.4±8.3 (adult abdomen) for 2002, 2003, 2004, and 2002–2004, respectively. In every case except adult abdomen exams in 2003, both the mean dose and the std. dev. fell for each consecutive year. Similarly, the 75th and 90th %tile values and the percentage of units with doses over the reference levels consistently fell. In 2004, 22.4, 6.9, and 2.8% of sites reported doses above reference levels, compared to 49.0, 15.0, and 4.7% in 2002, for head, pediatric abdomen, and adult abdomen exams, respectively.
In July 2007, the dose reference values for the program will be based on Volume CTDI to take into account the effect of pitch. The new diagnostic reference levels will be 75 mGy (head), 20 mGy (pediatric abdomen) and 25 mGy (adult abdomen). New to the program will be maximum allowable doses. Sites with doses above these values will not be eligible for accreditation. These values will be 80 mGy (head), 25 mGy (pediatric abdomen) and 30 mGy (adult abdomen).
1. Understand the dose data collected by the ACR CT Accreditation Program.
2. Be able to describe the data trends observed in the initial three years of the program.
3. Learn the new dose reference levels and maximum allowable doses that the program will adopt in July 2007.
CE‐Imaging: The Physics and Technology of Breast Imaging — III
34(2007); http://dx.doi.org/10.1118/1.2761481View Description Hide Description
Mammography: The benefit of early detection of breast cancer by mammography has been proven by several randomized controlled trials. Mass public screening of asymptomatic women detects a prevalence rate of approximately 2–5/ 1000. Note that the prevalence of breast cancer in autopsy series may be as high as 20/1000. In spite of multiple studies, controversy still exists regarding the benefit of mammography.
Magnetic Resonance Imaging: Screening of High Risk women with MR has recently been supported by the American Cancer Society due to a few observational studies. Note that there are no randomized control trials of MR screening. Additionally, some advocate pre‐operative MR ‘screening’ for those with newly diagnosed breast cancer from core biopsy. MR may show additional foci of cancers, at the expense of increased false positives, that could alter surgical treatment from breast conservation to mastectomy. Those opposed to this idea cite the recurrence rate of 2–5% at ten years after breast conservation surgery (lumpectomy) and whole breast radiation to question the efficacy of MR, as well as the additional cost and time.
Ultrasound:Ultrasound screening is also being studied as part of a large multi‐institutional study (American College Imaging Network (ACRIN) 6666) in women at High Risk. Unique to this study is that the US results are independent to the mammographic results. Published detection rates for US are approx. 3/1000. Limitations are, again, no randomized control trials of mortality benefit and marked operator dependence making this procedure very time consuming, subject to many pitfalls, and challenging for follow‐up.
Tumor Histologies: The common types of breast cancer are the ductal, invasive ductal carcinoma, arising from the glands in the ducts, and lobular, invasive lobular, arising from the cells in the lobules. There are some unique differences to the imaging appearance of these types.
Treatment:Treatment is tailored per patient. Factors that are taken into consideration are tumor type, tumor size per breast size for cosmetic results, tumor grade, tissue markers, lymph node involvement, distal organ involvement, age of the patient, family history, risk factors, and patient wishes. Complete surgical excision is mandatory with clean margins. The addition of radiation‐ whole breast vs. partial‐ and chemotherapy vs. hormonal therapy are dependent on the above factors.
New Devices under Investigation: Examples of the following will be illustrated. 1. Tomosynthesis; 2. Optical imaging of hemoglobin concentrations (indirectly) ; 3. Electrical impedence.
1. Understand the value and limitations of mammographic screening to early detection of breast cancer.
2. Understand the value and limitations of other techniques, MR and US, to early detection of breast cancer.
3. Understand treatment options of radiation and chemotherapy.
4. Develop awareness of the common breast cancer histologies.
5. Develop awareness of new modalities being studied.
CE‐Imaging: Medical Imaging Informatics — II
TH‐A‐M100J‐01: Characteristics, Performance Evaluation, and Clinical Implementation of LCDs Used for Medical Image Display34(2007); http://dx.doi.org/10.1118/1.2761609View Description Hide Description
Accurate presentation of radiographicimages requires high‐quality electronic displays. Currently, the liquid crystal display(LCD) is the technology of choice for medical image display. This lecture will provide an overview of the technology and characteristics of LCDs as they relate to medical imaging. Fundamental characteristics that will be discussed include the pixel matrix size, optical resolution, spatialnoise, ambient light reflection, influence of viewing angle, color tone, and grayscale response function (DICOM calibration).
Assessment of many relevant LCD properties can be performed with a set of test patterns, a photometer, and a trained eye. More advanced measurements can be acquired using custom equipment, including a high‐quality CCD camera. Both standard and advanced display assessment methods will be discussed. Also, the physical and psychophysical influences of environmental lighting conditions will be introduced.
LCDs currently used for medical imaging range from consumer‐grade color to “medical imaging grade” color and gray displays. It is important that the display used is appropriate for the clinical task. For example, diagnosis from digital x‐ray images likely has greater display requirements than clinical review of MRI images. Considerations required to match the display to the imaging task will be presented.
Finally, routine display maintenance is required to ensure that LCDs used for medical imaging perform at a high level throughout their life cycle. Medical imaging grade displays have software that maintains the luminance output at an acceptable level. All LCDs require on‐site quality control periodically to help ensure performance.
1. Understand LCD technology.
2. Understand the fundamentals of display quality assessment.
3. Understand imaging requirements to match a display to a modality and task.
CE‐Imaging: Medical Imaging Informatics — III
TH‐B‐M100J‐01: Down and Dirty with DICOM. Using Two Powerful Open Source Tools to Use DICOM to Aid in Practice and Research34(2007); http://dx.doi.org/10.1118/1.2761625View Description Hide Description
DICOM is the lingua franca of medical imaging. Having a working knowledge of how to perform troubleshooting and manage images in a clinical environment is a great asset to medical physicists. Two powerful open source tools will be demonstrated in this interactive session as a method to learn DICOM as well as learn clinical uses for these tools.
The first tool is the DICOM validation toolkit which is a framework that can be used for testing DICOM transfer syntaxes. We will use clinical examples of how the tool can be used to troubleshoot for common problems such as a misbehaving modality worklist association between an imaging modality and a PACS system. The framework has a scripting language that allows for easy access to the often complex binary association language of DICOM.
The second tool is the DICOM archive tool DCM4CHEE. This open source DICOM archive is a great application that can used as the core of a research pacs, teaching, or testingarchive. DCM4CHEE is fully IHE compliant and we will test its advanced integration capabilities using DVTK.
The course will summarize installation and operations of these toolsets through practical examples and interaction with the audience. These tools are light weight and attendees are encouraged to bring a laptop to the session for practice. A basic overview of DICOM will be provided in relation to the tools presented.
1. Understand the DICOM validation toolkit (DVTK)
2. Learn how to use the DCM4CHEE DICOM archive.