Streak Artifact Reduction and Enhancement of Pelvic CT Image in a Patient with Hip Replacement

Our Radiation Oncologist recently showed me a pelvis CT image of a male patient who had undergone bilateral hip replacement with forged titanium alloy implants. The patient had early stage prostate cancer, and the doctor wanted to treat this patient with IMRT. The difficulty was to delineate the prostate, seminal vesicles and other organs at risk because of the streak artifacts in the CT images due to high-Z material in the patient. The question came up about how this kind of artifact can be removed or minimized so that the target organ, as well as the organs at risk and other organs can be delineated for treatment planning. I did some research and was able to come up with some good articles on this topic. I am summarizing my findings below for those who are interested to learn or as a reference for those who may have such cases in the clinic.

Streak artifacts in CT images are generated in conventional CT when implanted objects of high atomic number exist in the patient. The artifact and image degradation associated with the kilovoltage (kV) CT imaging in the presence of high atomic number material greatly hinders the ability to delineate tumors and certain organs, particularly in the treatment planning of a prostate patient with hip prostheses. Such a situation, therefore, precludes precise dose calculation. There are several techniques reported that, if used, can minimize such artifacts, thereby enhancing image visualization for the delineation of tumor and other organs.

1- Charmley et al. (1) suggested that the use of CT-MR image registration to define target volumes in pelvic radiotherapy in the presence of bilateral hip replacements could facilitate target definition of prostate patient with hip replacements. However, a number of factors were found to affect image quality and/or the accuracy of target definition. The standard MR couch, different from a CT or linac treatment couch, might result in different patient positions, and the presence of the metallic implants may create significant distortion.

2- Yazdia M. (2) suggested an adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy planning. At that time, they may require manual image post-processing and most CT scanners available in radiation oncology department are not equipped with these features.

3- The artifact image and degradation associated with the kilovoltage (kV) CT imaging in the presence of high atomic number material is greatly reduced with Megavoltage Cone Beam Computed tomography (MV-CBCT). MV-CBCT has been used in image-guided radiotherapy (IGRT) to correct patient setup immediately before treatment. Hansen et al (3) used this technique to treat paraspinous tumors in the presence of orthopedic hardware. It allows rapid acquisition of 3D images that can be registered with the planning CT with millimeter precision and enhance image visualization by exploiting the predominantly Compton scattering of high-energy photons delivered in the MV-CBCT system. Aubin et al. (4) of the Department of Radiation Oncology at UCSF did a study with the support of Siemens Oncology Care systems on the use of Megavoltage Cone Beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. They found the MV-CBCT image could be used to clearly visualize the hip prostheses and provide sufficient soft-tissue contrast to help delineate the prostate, bladder and rectum. The artifacts on the kV CT obscure the border between the prostate and anterior wall of the rectum and the interface between the prostate base and the bladder neck. However, the MV-CBCT images were particularly useful to help delineate these structures as well as the lateral extension of the prostate in the axial plane, the seminal vesicles and the lymph nodes. Also, normal anatomy such as pelvic bones, penile bulb, bladder, femoral heads, rectum and small bowel can be delineated with higher accuracy as well. They evaluate this technique for seven patients. For each patient, the MV-CBCT images were imported into the treatment planning system and registered with the original CT using body anatomy contoured on each image set. The target volumes and organs at risk for prostate treatment were contoured using both the CT and the MV-CBCT for single hip replacement, and using only the MV-CBCT for bi-lateral hip prostheses. For the full article, click on: http://bjr.birjournals.org/cgi/reprint/79/947/918

The following two figures taken from Aubin M. at el (4) show the difference between conventional CT and MV-CBCT images:

1. Chamley N. et al. The use of CT-MR image registration to define target volumes in pelvic radiotherapy in the presence of bilateral hip replacements. BJR 2005; 78:634-636.
2. Yazdia M. et al. An adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy planning: experimental and clinical studies. Int. J. Radiation Oncol Biol Physics 2005; 62(4): 1224-1231.
3. Hansen, E.K. et al. Image guided radiotherapy using Megavoltage Cone-Beam Computer Tomography for treatment of paraspinous tumors in the presence of orthopedic hardware. Int. J. Radiation Oncol Biol Physics 2006; 66(2): 323-326.
4. Aubin M. at el. Use of Megavoltage Cone-Beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. Short Communication: British Journal of Radiology 2006.

ABR Radiologic Physics Part 1 Exam – How to Prepare?

With the upcoming ABR Physics exams this summer, we have received several questions in regards to how to prepare. If you are taking Part 1 this summer, concentrate on that; don’t concern yourself with Parts 2 and the Oral examination (just yet). That said, the first thing you will want to do is review the topics covered on Part 1 of the physics exam. Fortunately, the ABR has laid out the subjects you will be tested on in the Initial Certification Study Guide. The ABR has yet (as far as we have heard) to veer from those topics, so you will not be tested on subjects outside their study guide. In fact, the guide is quite accurate in terms of the scope of the questions you will be asked. The next thing you will want to do is gather resources that cover those topics well.

The internet is strewn with information that is helpful in prepping; the resources are scattered through the Web and take time to find. One site worth bookmarking provides lecture notes on several physics topics listed in the Initial Certification Study Guide as well as a few practice questions. Another site with lecture notes on relevant topics is the course site for Diagnostic Radiology Imaging Physics at UW. More practice question can be found here and here (though at this site, you will have to register to access the free tests).

Your main allies will be your own lecture notes and good prep books. While cross-referencing is always helpful, the following texts have been helpful to other students who took the ABR Part 1 Physics exam in the past. The first is “Review of Radiologic Physics” by Walter Huda. The book is 272 pages with over 500 practice questions, and the material covered is high-yield. The next two texts are pricey, but serve as good reference texts to have in your possession. They are “The Essential Physics of Medical Imaging” by Bushberg et al and “Medical Imaging Physics” by William Hendee and E. Russell Ritenour. We have also heard that reviewing Raphex exam questions is also key in preparing. There are still a few copies of the very old exams available for purchase on Amazon.com. You can also find copies of recent exams for free on the Web: Raphex 2006 Questions and Answers, Raphex 1998 Questions, Raphex 1997 Questions and the Raphex 1997 Answers.

Last, but certainly not least, talk to people who have taken the exam within the last few years. They will be able to tell you what the ABR has been stressing on the exam these days. Ask them how they prepped and what they found to be useful. Start early, and with the resources listed above in addition to those you locate on your own, you should be well-prepared to tackle Part 1 with ease and success.

Free Medical Physics Classifieds – Sell, Buy or Advertise

Readers who have been us since the launch of MDPhysics.com in March 2009 know that we started with just a weblog. Shortly thereafter, we added a listing of funding opportunities, a medical physics job board as well as a calendar of medical physics events. In addition, a physics classifieds section has been in the works for awhile, and now with coding complete, we’re excited to add this new functionality to the site. As always, a direct link to the page can be found in the navigation bar at the top of the site. Classifieds listings, like job listings, are free. All ads expire after 50 days and can be removed at anytime using an access code you receive via email. Read more

Calculating Polarity Correction Factor

AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams contains many details to which we should pay close attention when performing necessary measurements for the calibration of high-energy beams. One such measurement is calculating the polarity correction factor (P_pol) for ionization chambers used in electron beam dosimetry. Since polarity effects vary with beam quality and other conditions (such as cable position), one must correct for these effects by making measurements each time clinical reference dosimetry is performed. To correct an ion chamber’s raw reading for polarity effects it is necessary to take readings with both polarities applied and tabulate P_pol by the following equation: Read more

Sr-90 Ophthalmic Applicator: Activity in reb/sec or in mCi?

Have you ever seen radioactive material labeled with the units Roentgen – Equivalent – Beta rays/second (reb/sec)?  A medical physicist recently told me he came across these units on a Strontium-90 source at his new job while he was taking inventory of radioactive materials. This particular source was labeled with its model and serial number, as usual, but its radioactivity (the strength of the source) was given in Roentgen – Equivalent – Beta rays/second (reb/sec) instead of millicuries (mCi). Since the convention is to use miC when recording source strength in the inventory log book, he was wondering how to convert these units to mCi. Since I had not worked with Sr-90, I didn’t know the answer myself. I spoke to a couple of experienced physicists I know, and surprisingly no one had the answer. Like any good scientist, this peaked my curiosity…so I did some research. I am guessing many physicists may not know the answer, so I am sharing the fruits of my labor and the result of my due diligence in this post. This is for those who, like me, are curious and are interested to learn: Read more

Physics to Medical Physics: Switching Paths

We were recently contacted by an individual who was studying physics at the doctoral level and was interested in switching to a career in medical physics. Switching to a career in medical physics with a Ph.D. in any branch of physics was a relatively easy task, say 20 years ago, but has become increasingly more difficult with the growing number of medical physics degree programs and the restrictions of residency admissions to those who have specifically graduated from an academic program in medical physics. It’s certainly an exciting time to be in medical physics, but it’s become difficult (albeit, not impossible) for those who have not specifically trained in medical physics to join the party. It’s worth mentioning that many past (and current) leaders in our field did not graduate from medical physics degree programs, which makes one wonder how many talented individuals with the potential to contribute to our community are unable to become medical physicists simply because they chose to study a different branch of physics instead. Read more

Open Medical Physics Residency Positions 2009-2010

It’s that time of the year again. No, I’m not talking about the holiday season. I’m talking about medical physics residency application season! Ho, ho ho! Medical physics residency programs are looking to fill positions for next year, so as a reminder to those who are currently applying, here is a round up of CAMPEP-accredited programs (in alphabetical order) currently accepting applications and their deadlines. Direct links are provided to official application information for programs that provide such information on their websites. Also, be sure to check the listing of medical physics job openings for more residency positions as they are announced. Read more

Practice ABR Radiologic Physics Oral Board Exam

The University of Texas MD Anderson Cancer Center is offering a Diagnostic Radiologic Physics Mock Oral Board Exam the weekend of April 17-18, 2010. Participation is limited, so sign up early. The practice exam dates have been added to the MDPhysics Event Calendar. The mock exam aims to provide the student with a simulation of the ABR oral exam environment by using similar exam format, questions and board-certified examiners.

This mock exam has been offered internally to physics residents and junior faculty with great success, and participation is now being extended outside MD Anderson. All proceeds (tuition is $500) will support medical physics educational programs at the University of Texas MD Anderson Cancer Center.

After taking the mock board exam, the student will be familiar with his or her individual areas of weakness to improve with additional study before the actual board exam. In addition, the mock board exam will allow for practicing the expression of coherent answers in a risk-free setting. Examiners will provide some guidance and feedback to the student on his or her performance.

For more information proceed to Imaging Physics Department’s education page or contact Georgeann Moore at gmoore@di.mdacc.tmc.edu.

The course brochure and application can be found here.

How Well Do You Know Varian and Elekta?

Varian and Elekta are two world leading (and competing) companies pioneering significant clinical solutions for treating cancer in radiation oncology. Any medical physicist is as familiar with the terms “Varian” and “Elekta” as he is with “x-ray” and “radiation.” In essence, Varian and Elekta are the “giants” in our industry…but how much do you really know about these companies whose names are ubiquitous with medical physics and whose products most of us use everyday? Read more

Lack of HDR Training in Residency Program (For Physicians and Medical Physicists)

If you graduate from a residency program that does not offer or use an HDR after-loader for patient treatments, what problems, if any, could you encounter at your first job? The answer is simple, if you are expected to treat patients with an HDR after-loader as part of your job responsibilities, you are not qualified to be listed as an Authorized User (AU) in the HDR material license, and hence you cannot independently treat patients with an HDR after-loader. That is, you are not legally able to fulfill this part of your job description. It’s a potentially embarrassing circumstance in addition to one that may inconvenience physician scheduling. Read more

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