Making Reliable Low-dose CT a Clinical Reality

Transcription

1 Making Reliable Low-dose CT a Clinical Reality By Ramakrishnan RS, MD, DNB, Consultant Radiologist 1 ; George Joseph, MD, DMRD, Consultant Radiologist 2 ; and Sundar RK, BSc, DRT, DAMIT, Clinical Applications Manager 3 ASiR * is an iterative reconstruction (IR) technique that focuses on the modeling of the noise properties which may enable noise reduction; as measured by pixel noise standard deviation in the reconstructed image and may allow for lower dose examinations even in large patients or studies requiring thinner slices. 1 MedALL Healthcare Pvt. Ltd. 2 Lakeshore Hospital and Research Centre 3 GE Healthcare 20 A GE Healthcare CT publication June 2011 Introduction We explored new methods that offer significant dose reductions, specifically the new reconstruction-based dose reduction technique, ASiR. ** Historically, common methods for achieving dose reduction were Automated Exposure Control and dedicated procedure-based protocols with low techniques for pediatric imaging, and ECG modulated ma and BMI-based for cardiac imaging. However, these dose reduction methods are based on regulated and modulated use of ma. Another method involves improving the image quality at lower ma values. This can be achieved by using softer reconstruction kernels, or by using post reconstruction image softening filters. Yet, these image space-based filter techniques do not assure any kind of dose reduction to the patient as they are applied post scanning. Filtered Back Projection (FBP) has been used as the reconstruction method for 30 years. This closed form of reconstruction involves projection based data collection, filtration, back projection, and weighting. Yet, it is based on several assumptions and does not account for all physical parameters such as detector shape, focal spot size, image voxel size, and photon flux variation between projections being transmitted from the patient s body. Thus FBP provides a calibrated, filtered image that undergoes many assumptions. ASiR is an IR technology that focuses on the modeling of the noise properties which may enable noise in the reconstructed image and may allow for lower dose examinations even in large patients or studies requiring thinner slices. In routine imaging exams at our hospital, 40% to 50% ASiR is used consistently. ** In clinical practice, the use of ASiR may reduce CT patient dose depending on the clinical task, patient size, anatomical location and clinical practice. A consultation with a radiologist and a physicist should be made to determine the appropriate dose to obtain diagnostic image quality for the particular clinical task.

2 c l i n i c a l v a l u e Routine imaging ASiR may enable improvement to the Low Contrast Detectability (LCD). Following are several patient cases demonstrating the value of using ASiR in our facility. Case 1 An abdomen study performed with 40% ASiR maintains image quality by reducing noise and dose (Figure 1). Note the improved visualization of contrast-enhanced structures. In this study, the total dose delivered is 3.35 msv (obtained by EUR EN, adult abdomen factor of x DLP). Dr. Ramakrishnan RS Ramakrishnan RS, MD, DNB, is a Consultant Radiologist at MedALL Health Care Pvt. Limited and Malar Fortis Hospitals. He received his degrees from Kilpauk Medical College and Sri Ramachandra Medical College & Research Institute. Dr. Krishnan completed a fellowship in cardiac MRI at Sankt Gertrauden Krakenhaus (Berlin). His areas of interest are body imaging, cardiac CT, and MRI. Dr. George Joseph George Joseph, MD, DMRD, is Chief Radiology Consultant at Lakeshore Hospital and Research Centre, Lissie Hospital, and Lourdes Hospital at Cochin, Kerala. Dr. Joseph received his MBBS from Government Medical College Kottayam and medical degree in Radio-diagnosis from Government Medical College Trivandrum. He was one of the first radiologists trained on a CT scanner at Sir Gangaram Hospital Delhi. Today, his primary interests in CT imaging are abdominal imaging and image-guided interventions. Sundar RK Sundar RK, BSc, DRT, DAMIT, Clinical Applications Manager, GE Healthcare Figure 1. Abdomen study with dose of 3.35 msv using 40% ASiR in the bottom image. Note the improved visualization of contrast-enhanced structures compared to the top image. A GE Healthcare CT publication June

3 Case 2 Routine chest scan conducted with 50% ASiR at 24 mas yielded a total dose of just 0.72 msv (Obtained by EUR EN adult chest factor of x DLP). Figure 2. 3D volume-rendered image (right) shows air way tree with transparency model of the lungs. We have also used ASiR to scan dense objects such as metallic surgical sutures, and implants. We have been very pleased with the visualization realized in these challenging cases. Case 3 Follow-up CTA of patient post-cerebral aneurysm surgery. With 50% ASiR at 76 mas, the effective dose was 3.1 msv (Obtained by EUR EN adult head factor of x DLP). Figure 3. 3D volume-rendered image (left) shows cerebral circulation and the positioning of the aneurysm clips. The MPVR image with MIP (center) clearly demonstrates the vasculature even in the presence of high dense metal clips. The 3D image of the clip by itself (right) depicts the clip deployment without any streak artifact. 22 A GE Healthcare CT publication June 2011

4 c l i n i c a l v a l u e Coronary CTA In our experience, the combined use of ASiR with Snapshot * Pulse prospective gated scans with adaptive gating has further reduced radiation dose by 80%, for an effective dose of 1 msv and lower, in some cases. Case 4 81 year-old male patient presented with angina on exertion. The study showed atheromatus calcified plaque in all three vessels with mild to moderate luminal narrowing. SnapShot Pulse technique was used along with 40% ASiR. The heart rate was unstable and a dynamic padding of 25 ms was given. The DLP of the study was mgy. cm, and the effective dose is 1.09 msv (EDLP, ICRP chest factor 0.014). About the facility MedALL is a leading chain of diagnostic and imaging centers across South India. MedALL was started with a vision to provide diagnostic services and endeavors to touch, serve and solve health-related challenges of its customers and to provide high level of diagnostic confidence to referral physicians. MedALL has earned a reputation among referral physicians and customers for providing the highest levels of customer satisfaction with sheer excellence and dedication in the quality of diagnostic services. MedALL started its journey by acquiring some of the leading diagnostic centers such as Precision Diagnostics in Chennai and Clumax Diagnostics in Bangalore. MedALL currently operates diagnostic centers in Bangalore, Chennai, Trichy, Tirunelveli, Melur, Pudukottai, Rajapalayam, Kurnool, Kakinada, and Vizag. A B C Figure 4. (A) 3D Volume Rendered image, (B) LAD Curved view demonstrates dense calcification along the coronary artery wall, and (C) the blue vertical block on the ECG shows the prospective triggering and exposure in the diastolic window. About the facility Lakeshore Hospital and Research Center is a multi-specialty hospital aimed at bringing international standards in healthcare to Kerala. Lakeshore Gastro Intestinal Surgery Unit is used as a training center for FRCS examinations by the Royal College of Surgeons (Edinburgh). Lakeshore Oncology Department carried out bone marrow transplantation and peripheral stem cell transplantation for blood cancer patients for the first time in the history of private hospitals in Kerala. The hospital was the first to introduce the artificial insulin pump for diabetes patients in South India. A GE Healthcare CT publication June

5 Whole body scan, poly trauma application A single CT whole body trauma evaluation is one of the most powerful tools for managing patients with multiple critical injuries. Yet, the dose has always been a consideration. This led to conducting whole-body scans on a limited basis and only for specific regions to reduce radiation exposure to the patient. With the advent of ASiR, however, we are doing our poly-trauma imaging with lower doses. Case 5 Medium-sized male patient fell from elevated height. Initial head scan showed intracranial hemorrhage and patient was sent to surgery. Patient was also referred for a whole body scan to evaluate the strong suspicion of internal organ injury and spinal fractures. Using 40% ASiR, the total whole body dose as integrated with different regional weighting factors was 3.29 msv (Each region s dose was calculated using EUR EN with adult factors of for head, for neck, for chest, for abdomen and for pelvis). Figure 5. The whole-body, contrast-enhanced scan was completed in 8 sec with mm thin slices. Conclusion ASiR is an alternative reconstruction technology where the system statistics are modeled. This may enable a reduction in pixel noise standard deviation hence allowing for reduced ma and thereby dose. ASiR was designed with dose reduction and LCD improvement in mind. In our experience, the selected ASiR level correlates, to some extent, with the dose reduction in our department. For routine imaging, we use 40% to 50% ASiR. At MedALL Healthcare and Lakeshore Hospital and Research Centre, we have found the benefits associated with lower radiation dose derived with ASiR are so encouraging that we are now using the technique for all CT scans, anatomies, and routine imaging studies. Today in our facility, the ability to image gently has become a reality with the use of ASiR. 24 A GE Healthcare CT publication June 2011