Spectral CT: A Brave New World of Quantitative, Functional Imaging

G E M S T O N E S P E C T R A L I M A G I N G C L I N I C A L V A L U E Spectral CT: A Brave New World of Quantitative, Functional Imaging CT has long been considered an excellent method for viewing high-resolution images of human anatomy in a non-invasive manner. It has been used in conjunction with PET and MRI devices that can image tissue and organ function to bring together anatomic and morphologic information for a more precise patient diagnosis and treatment plan. GSI s spectral HU curve and material-basis analysis provides us with information on material characterization and quantification this is revolutionary. While CT is an excellent technique with good spatial and temporal resolution, the soft tissue contrast and ability to discriminate normal and pathological tissues is sometimes inferior to other imaging techniques, says Valentin Sinitsyn, MD, PhD, Chief of the Radiology Department at the Federal Center of Medicine and Rehabilitation (Moscow) and Professor and Chair of Radiology, School of Fundamental Medicine at Moscow State University. This limitation has led to the utilization of other functional imaging techniques in conjunction with CT imaging which may result in additional studies and potentially higher healthcare costs. However, CT is closing the gap between anatomical and alternative functional imaging thanks to advances in spectral CT and the introduction of Gemstone * Spectral Imaging (GSI) on the Discovery * CT750 HD scanner. Today, GSI is beginning to change the way radiologists across the world utilize CT imaging. Prof. Xiao-Peng Zhang 11

c l i n i c a l v a l u e G e m s t o n e S p e c t r a l I m a g i n g Gemstone Spectral Imaging Gemstone Spectral Imaging (GSI) is a dual-energy scan mode that acquires data of an object by rapidly switching between low kvp and high kvp energies in less than half a millisecond. This generates data with different attenuation values based on the corresponding energy levels. The result is a near-perfect, simultaneous dual-energy acquisition at the full 50 cm scan field of view (SFOV) producing projection (raw) data at two different energy levels that has virtually no misregistration. This enables raw data-based reconstruction of dual-energy data with the associated benefits of quantitative material decomposition and beam-hardening reduction from monochromatic energy synthesis. Projection-based reconstruction is used to process the data. Based on known attenuation curves, the process mathematically transforms low and high kvp attenuation measurements into effective material density (MD) basis-pair images. This is also known as material decomposition. GSI produces these MD pairs which are not available with conventional contrast-enhanced CT imaging. The make-up or composition of the MD pairs can be selected based on the clinical question being investigated and materials of interest, such as iodinewater, iodine-calcium, or water-calcium. GSI also produces a monochromatic image, which is synthesized from the MD images and depicts how the object would look if the X-ray source produced only X-ray photons at a single energy. Specific tools have been created in the GSI Viewer to support the data analysis of this rich information. Tissue characterization can be aided with the help of Hounsfield unit (HU) spectral curves; metal artifact reduction is enabled by interactively switching to the optimal monochromatic energy level. Additionally, material identification is made possible by displaying the effective atomic number histograms of objects, particularly kidney stones. GSI is breaking down a barrier by bringing a new type of functional imaging to CT, explains Jean-Nicolas Dacher, MD, PhD, Professor of Radiology and Diagnostic Imaging and the Chief of the Department of Diagnostic Imaging at Rouen University Hospital, (France). He routinely uses GSI, performing approximately 20 cases each week. Plus, with CT we have the advantage of a quick examination that can be more comfortable for the patient, he adds. This is compared to traditional functional imaging studies (MR, SPECT and PET) that can last a minimum of 20 minutes to one hour, an important consideration for very sick and elderly patients. After conducting more than 1,500 GSI scans and 20 pilot studies in the first three months of using GSI, Xiao-Peng Zhang, MD, Professor and Chairman of Radiology, Peking University Cancer Hospital and Institute (China), clearly sees its clinical value. We believe GSI will change the way that doctors practice and interpret CT, he says. GSI s spectral HU curve and material-basis analysis provide us with information on material characterization and quantification this is revolutionary. Revolutionary is a statement echoed by Lawrence Tanenbaum, MD, FACR, Director of MRI, CT, and Outpatient/Advanced Development, Mt. Sinai School of Medicine (USA). Spectral CT offers potentially revolutionary information when compared to traditional polychromatic CT, he says, including the ability to reduce certain artifacts, such as streak and beam hardening from metal, enhance contrast resolution that will perhaps make iodine more or less conspicuous, or improve contrast resolution by alternating energy levels to differentiate two different tissues. Tissue characterization GSI enhances tissue characterization through its ability to derive images that separate materials such as calcium, iodine, and water. What really impresses me is the rich tissue characterization capability of spectral CT, says Dr. Tanenbaum. To look at a dense brain lesion and know whether it is hemorrhagic, enhancing or calcified is very helpful, particularly in difficult case scenarios. The 101 selectable energies and monochromatic images of GSI, explains Prof. Zhang, enable easier and clearer detection of extremely tiny structures for example, the pancreatic duct and membranous structures such as the greater omentum. This is rarely achieved using conventional CT, he adds. With the GSI-generated iodine maps, I can clearly see the severity of perfusion deficit. Prof. Valentin Sinitsyn 12

G e m s t o n e S p e c t r a l I m a g i n g c l i n i c a l v a l u e Pulmonary embolism GSI represents what is lacking in a typical CT study, says Prof. Sinitsyn. After the Discovery CT750 HD scanner with GSI was installed at the Federal Center in November 2009, he immediately began investigations on pulmonary emboli. Traditionally, CT provided an excellent depiction of the pulmonary vessels, thrombi and emboli, he explains. However, it could not give us full information on the severity of pulmonary embolism obstruction or perfusion deficit defects. With the GSI-generated iodine maps, I can clearly see the severity of perfusion deficit. Upon closer review, Prof. Sinitsyn discovered he could also detect tiny thrombus or embolus inside the pulmonary artery, which often causes the perfusion defect. In patients with chronic embolism, he adds, it is clear that if we see multiple perfusion defects there exists a strong indication to support surgical removal of the thrombi. He cites a recent study where the occurrence and severity of the perfusion defect as determined by spectral CT is a strong predictor of patient prognosis. Prof. Dacher also uses GSI on a daily basis to generate iodine maps for lung perfusion studies (by centering the images on the iodine). Most exciting, he says, is the capability to view the pulmonary artery anatomy for helping to detect clots at the same time he obtains a lung perfusion map. He can then use the anatomical and functional information obtained during the same study to assess the pulmonary emboli. It is very interesting to see there is no match between the extension of the pulmonary embolus and the pulmonary perfusion abnormality, he explains. In one particularly interesting case at the University Hospital of Rouen, pulmonary embolus was suspected in a patient presenting with chest pain. Prof. Dacher performed a GSI study and discovered the pulmonary artery was encased by a tumor. The flow was limited and there was hypo perfusion that was completely obvious on the GSI study because we could see the anatomy and function, he explains. If we had performed only scintigraphy historically the exam of choice for these cases we would have only noted the reduced perfusion of the upper lobe. This could have resulted in an incorrect diagnosis of pulmonary embolism and possibly led to the patient unnecessarily receiving an anticoagulant, he explains. Prof. Valentin Sinitsyn Valentin Sinitsyn, MD, PhD, is Chief of the Radiology Department at the Federal Center of Medicine and Rehabilitation (Moscow, Russia) and Professor and Chair of Radiology, School of Fundamental Medicine at Moscow State University. Prof. Sinitsyn is also the elected President of the Russian National Congress of Radiology 2011; Vice-President of the European Society of Cardiac Radiology; and a member of the ECR Program Planning Committee. His interests include cardiovascular imaging, MR, contrast media, education, and internet and computer applications in radiology. Prof. Sinitsyn has authored and co-authored more than 110 articles and eight books on radiology and internet applications, edited Russian versions of two international textbooks on MRI and CT, and serves on the editorial boards of the International Journal of Cardiovascular Imaging, the Journal of Cardiovascular Magnetic Resonance, Diagnostic Imaging (Europe) and Imaging Decisions. Images courtesy of the Federal Center of Medicine and Rehabilitation (Moscow) Figure 1A. A GSI iodine map depicts a wedge-shaped perfusion defect in the 9th segment of the left lung. Figure 1B. A small embolus inside the corresponding segmental branch of the left pulmonary artery can be seen with GSI. About the facility The Federal Center of Medicine and Rehabilitation (Moscow) is well known as a modern surgical hospital specializing in high-tech neurosurgery, abdominal surgery, gynecology, orthopedics, and sports injury rehabilitation. The Department of Radiology performs all radiological examinations, including CT, MR, and nuclear scans, around-the-clock as needed. It is also focused on scientific research regarding new imaging technologies and contrast media agents. As an academic hospital, the Center offers residency and postgraduate programs as well as clinical faculty positions in the School of Fundamental Medicine at Moscow State University. The first-in-russia Discovery CT750 HD scanner was installed here in November 2009. 13

c l i n i c a l v a l u e G e m s t o n e S p e c t r a l I m a g i n g GSI is not simply a new study area, rather it is changing the way we think and practice CT imaging. Oncology For Prof. Zhang, the most significant use of spectral CT is to quantitatively characterize lesions via the spectral HU curve, which graphically displays the attenuation characteristic of a region across all 101 spectral energies. Using the monochromatic images, we can visualize anatomic and internal structures of lesions, which is important for early detection. says Prof. Zhang. He finds the material characterization and quantification very useful in helping him identify different types of lesions and diseases, and gaining information on cancer at different stages. Additionally, GSI provides rich and reliable hemodynamic information of tissue with iodine quantification. It helps us accurately identify infiltrated areas with the iodine-based images, adds Prof. Zhang, and with a reliable method to evaluate hemodynamic status, we can evaluate therapy results more confidently. Prof. Xiao-Peng Zhang Lymphoma Images courtesy of Peking University Cancer Hospital and Institute Figure 2A. Affected lymph node of the neck. Figure 2B. Affected lymph node of the porta pulmonis. Figure 2C. Affected lymph node of the mediastina. Figure 2D. Affected node of the spleen. 14

G e m s t o n e S p e c t r a l I m a g i n g c l i n i c a l v a l u e Graph courtesy of Peking University Cancer Hospital and Institute One clinical question that a traditional CT exam cannot answer is whether or not a lesion enhances. This is often evaluated in terms of Hounsfield units, explains Prof. Sinitsyn. Yet the value assigned may not be precise, as the enhancement may be partially due to beam hardening artifact or artificially inflated via image processing. When I see a small enhancement, I may not be able to determine if it is true or artificial, he says. By comparing water and iodine images, I get the information to quantify the area of interest based on the accumulation of iodine and objectively determine if the lesion is a concern that requires follow-up. Renal stones Prof. Sinitsyn also uses GSI to assess renal stones. Often, where there is one kidney stone, there are more, so it is important to know the material composition. GSI can help quantify the renal stone, whether it is a calcified stone or predominantly a soft stone containing uric acid, he says. The latter can be treated with techniques other than surgery, Prof. Sinitsyn adds. Prof. Xiao-Peng Zhang Xiao-Peng Zhang, MD, is the Professor of Radiology, Peking University Health Science Center, and Chairman of Radiology Department, Peking University Cancer Hospital and Institute. Prof. Zhang also holds the positions of Chairman of Oncology Imaging Committee, China Anti-Cancer Association; Managing Editor, Chinese Journal of Medical Imaging Technology; and Editor of the Chinese Journal of Radiology, Chinese Journal of Clinical Oncology, and Chinese Journal of Practical Surgery. He authored three radiology textbooks and more than 80 research papers globally. Prof. Zhang s research interests include abdominal and thoracic oncology imaging for the early detection, accurate staging, and therapy evaluation of cancer. In the Radiology Department of Peking University Cancer Hospital and Institute, a GSI team of 20 experienced radiologists and researchers, led by Professor Xiao-Peng Zhang, has conducted more than 1,500 routine GSI scans and 20 pilot studies in the first three months of using GSI. They have achieved numerous promising results, submitted 30 research abstracts and 15 research papers to prominent journals of radiology. Figure 3. Spectral HU curves show the same pattern of different affected lymph nodes in the same patient with lymphoma, which indicate these lymph nodes are of the same nature as lymphoma. About the facility Peking University Cancer Hospital and Institute is one of China s most-respected centers dedicated exclusively to cancer patient care, research, education, and prevention. Each year, more than 300,000 patients turn to the institute for cancer care in the form of surgery, chemotherapy, radiation therapy, immunotherapy, or combinations of these and other treatments. 15

c l i n i c a l v a l u e G e m s t o n e S p e c t r a l I m a g i n g Reducing artifact With the continued increase in metal instrumentation hip and knee prosthesis and spinal fusion, for example the issue of artifact degrading image quality is becoming more pronounced, explains Dr. Tanenbaum. Reducing these artifacts is an important benefit that GSI provides in his daily practice. Between 33% and 40% of our routine spine exams involve instrumentation, he says. Images courtesy of Mt. Sinai Medical Center (New York) Figure 4. Note the lack of metal artifact in patient with spinal instrumentation in the 110 kev image on the right compared to the 70 kev image on the left. 16

G e m s t o n e S p e c t r a l I m a g i n g c l i n i c a l v a l u e Traditional techniques are challenged by implanted metal hardware, leading to images with beam hardening and streak artifacts. Spectral CT generates virtually pristine images in these most challenging circumstances where traditional techniques often fail, Dr. Tanenbaum adds. We can restore the information in areas that were previously deteriorated by artifact and thus, substantially improve the imaging results in these difficult cases. Recently at the European Congress of Radiology (ECR) 2011 annual meeting, Prof. Dacher presented a study demonstrating that he could more easily obtain high quality images of the femoral arteries in patients with metallic hip prosthesis by using GSI. We cannot accept limitations in the investigation of the femoral artery, so this may be a strong advantage of GSI. Enhancing contrast resolution Iodine-based contrast material used in CT imaging is very well suited to being manipulated either enhanced or eliminated, explains Dr. Tanenbaum. Not only can we make the iodine more useful, but it provides an opportunity to deal with sub-optimal contrast administration, he adds. Radiologists can either make the contrast more conspicuous in the image by adjusting the energy or create a material-based image that eliminates visibility of the contrast. With this capability, we have additional information in situations where, historically, we ve performed both a non-contrast and post-contrast CT study, Dr. Tanenbaum adds. Prof. Dacher also sees an opportunity to use GSI in cases where optimal opacification (contrast enhancement) is not ideal, particularly in older patients or those without good venous access. When we acquire images with GSI, it is possible to decrease kev and enhance the small amount of contrast media in the patient vessel. name Dr. Lawrence of author Tanenbaum info Lawrence about N. clinic, Tanenbaum, etc. MD, FACR, is Director of MRI, CT, and Outpatient/Advanced Development, Mount Sinai School of Medicine (MSSM). The school opened its doors in the fall of 1968 and has since become one of the world s foremost centers for medical and scientific training. Located in Manhattan, MSSM works in tandem with The Mount Sinai Hospital to facilitate the rapid transfer of research developments to patient care and clinical insights back to the laboratory for further investigation. About the facility Mount Sinai Medical Center, named to U.S. News & World Report s 2009-2010 Best Hospitals Honor Roll and ranked 19th nationally, treats nearly 47,000 inpatients and 427,000 outpatients each year. Renowned for its spinal cord and brain injury rehabilitation, Mount Sinai was the first medical school to establish a Department of Geriatrics, as well as departments of environmental and occupational medicine. With more than 3,000 full-time and voluntary physicians on staff, the hospital is a regional leader in numerous specialties and the world s only center for the diagnosis and care of Jewish genetic diseases. Spectral CT generates virtually pristine images in these most challenging circumstances where traditional techniques often fail. Dr. Lawrence Tanenbaum 17

C L I N I C A L V A L U E G E M S T O N E S P E C T R A L I M A G I N G We cannot accept limitations in the investigation of the femoral artery, so this may be a strong advantage of GSI. Prof. Jean-Nicolas Dacher Images courtesy of University Hospital of Rouen (France) Figure 5. 3D reconstructed image on left at 140 kvp demonstrates lack of imaging data due to artifact from metal instrumentation. Note the GSI image (right) at 70 kev plus MAR ( metal artifact reduction) clearly depicts the right femoral artery even in the presence of metal. 18 Figure 6. GSI helps reduce artifact and provides a diagnostic-quality study even in the presence of metal instrumentation. The GSI image on the right at 70 kev plus MAR clearly demonstrates the reduction in artifact versus the 140 kvp image on the left.

G e m s t o n e S p e c t r a l I m a g i n g c l i n i c a l v a l u e Changing clinical pathways Within one year of using GSI at the University Hospital of Rouen, Prof. Dacher and his colleagues have changed their clinical protocols. For pulmonary embolisms, metallic prosthesis, and pulmonary hypertension cases, we first utilize CT with GSI, he says. In lung perfusion, we have replaced scintigraphy and in some cases MRA with a spectral CT exam. The advantage of CT, he adds, is the speed of the exam and relative patient comfort both important considerations when evaluating very ill or elderly patients. What really impresses me, Prof. Dacher says, is the ability to quantify the amount of iodine within a voxel. This is a technical breakthrough that will continue to become more important in medicine. At Mt. Sinai, GSI is now a routine study on patients with instrumented spines or in neuroradiology cases when the radiologist suspects the presence of an aneurysm clip. Spectral CT improves the quality of some of our most challenging exams, Dr. Tanenbaum says, and it certainly has an impact on the information I can provide for surgical planning purposes. The additional information generated by GSI helps with deciding the course of treatment for the individual patient, in particular whether it should be more conservative or aggressive, explains Prof. Sinitsyn. He sees the potential to eliminate additional studies with the ability to quantify areas of interest while providing an objective and accurate diagnosis. Dual-energy will further increase the significance of CT, including when and where we use it, he adds. It is his hope that the continued use of spectral CT will enable the implementation of an objective, quantitative measurement of CT data in other words, moving away from Hounsfield units to the use of effective atomic numbers or something similar. Driven by an RSNA initiative, the aim of the radiology community is seeking to be more quantitative in imaging, explains Dr. Tanenbaum. Leveraging this capability of spectral CT should impact positively in the characterization of disease and be useful for surveillance of patients. It clearly gives us rich, more quantitative information than we had before, explains Dr. Tanenbaum. We are only touching the surface of future possibilities it remains to be seen with widespread implementation and clinical imagination where spectral CT will lead us. What it really comes down to is whether spectral CT will raise the level of diagnostic confidence and reduce the reliance on multi-modality testing resulting from inconclusive exams. GSI is not simply a new study area. Rather, it is changing the way we think and practice CT imaging, adds Prof. Zhang. It advances CT to an entirely new level and opens up a brave new world for the pioneer who embraces the spirit of discovery. Continuing enhancements to GSI Thanks to the collaboration of clinical leaders such as Prof. Dacher, Prof. Sinitsyn, Dr. Tanenbaum, and Prof. Zhang, GE Healthcare continues to refine GSI. Although Prof. Dacher has limited experience with the next iteration of GSI Viewer, he notes that it provides faster reconstruction speed and the ability to generate MIPs and MPRs. Both Prof. Dacher and Prof. Zhang see the potential to further reduce patient exposure to dose, leading to greater utilization of the technique. Prof. Jean-Nicolas Dacher Jean-Nicolas Dacher, MD, PhD, is a Professor of Radiology and Diagnostic Imaging and the Chief of the Department of Diagnostic Imaging at Rouen University Hospital. After studying medicine in Caen, France, he completed his residency in Diagnostic Imaging and Radiology at Rouen University. Prof. Dacher was a research fellow at the Free University of Brussels, Harvard Medical School (the Sachs Foundation Award) (Boston), and the University of Sherbrooke (Quebec, Canada). He has received an award from the Société Française de Radiologie and authored more than 130 articles and several book chapters in French and English textbooks. Prof. Dacher s areas of interest include non-invasive cardiovascular imaging (MR and CT) in children and adults and MR functional imaging of the kidneys. About the facility The University Hospital of Rouen is located in a city of 400,000 inhabitants made famous by the martyrdom of Joan of Arc in 1431, its Gothic Style Cathedral, and many paintings by Claude Monet. This 2,500 bed hospital in Northwest France is a full-service, multi-specialty facility that counts among its major innovations the invention of the transarterial aortic valve replacement (TAVI) by Alain Cribier, MD, Chief of the Department of Cardiology and a worldwide renowned cardiologist who implanted the first patient in Rouen in 2002. It is through the Departments of Cardiology and Cardiac Surgery and the Laboratory of Cardiovascular Pharmacology (INSERM 644) that the hospital developed a dynamic clinical and research cardiac imaging unit utilizing MDCT, MRI, and echocardiography. 19