FieldStrength. Achieva 3.0T enables cutting-edge applications, best-in-class MSK images

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1 FieldStrength Publication for the Philips MRI Community Issue 33 December 2007 Achieva 3.0T enables cutting-edge applications, best-in-class MSK images Palo Alto Medical Clinic Sports Medicine Center employs small field of view and large matrix to obtain high resolution images This article is part of Field Strength Issue 33 December 2007

2 Achieva 3.0T enables cutting-edge applications, best-in-class MSK images Palo Alto Medical Clinic Sports Medicine Center employs small field of view and large matrix to obtain high resolution images Chris Goumas, M.D After just over two months of using the Achieva 3.0T X-Series at the Palo Alto Sports Medicine Center at the Palo Alto campus of the Palo Alto Medical Foundation. Chris Goumas, M.D., department chief, has developed an original approach that enables him to exploit SENSE, SPAIR and asymmetric TSE to obtain exceptional high resolution images. Instead of optimizing the system for speed, Dr. Goumas focuses on a small field of view (FOV) and a high matrix to achieve maximum resolution. He says the images he is able to produce with the Achieva 3.0T X-series enable him to identify meniscal pathology and anatomic structure more easily. And referring physicians are raving about the image quality and the ability it affords to identify smaller, subtler pathology. Specifically selected for its capabilities in musculoskeletal imaging, the Achieva 3.0T X-series was installed at the Palo Alto Medical Clinic Sports Medicine Center in March It complements a full spectrum of diagnostic radiology equipment that includes an Achieva 1.5T MRI scanner, two additional 1.5T MRI scanners, Philips Brilliance 64-slice and 16-slice CT scanners, Philips ultrasound equipment, Philips DR and CR, Philips PET/CT and Nuclear medicine cameras, and Philips PACS. About 80-90% of the imaging conducted at the center is musculoskeletal with the remainder consisting of mostly brain and spine scans. Chris Goumas, M.D., department chief, says the Achieva 3.0T X-series has exceeded his expectations. The resolution of the images produced on the Achieva 3.0T X-series enables us to see small tears and subtle injuries, which leads to better and more accurate diagnoses, says Dr. Goumas. Especially with musculoskeletal, where you re dealing with very small structures a lot of the time, resolution is paramount. A 3.0T system has significantly higher SNR compared to 1.5T systems, and Dr. Goumas uses that to its full benefit. He has a clear philosophy for orthopedic imaging. He uses a smaller FOV than most, with a higher matrix. (See table). Often, a 3.0T system will be employed for speed. And if you do that, you obtain images that are comparable to a 1.5T in about half Typical resolutions for 3.0T MSK imaging at PAMF Joint FOV Coil Voxel size Knee 12 cm SENSE Knee (8) 0.28 x 0.37 x 2.5 mm Shoulder 9-10 cm SENSE Shoulder (4) 0.27 x 0.39 x 3.0 mm Wrist 7-9 cm SENSE Wrist (4) 0.23 x 0.33 x 1.5 mm Ankle 14 cm SENSE Foot/Ankle 0.23 x 0.46 x 3.5 mm 14 Field strength Issue 33 - December 2007

3 Tibial plateau fracture The sagittal asymmetric TSE, PD-weighted images both show the cordical discontinuity on the tibial plateau with the overlying cartilage appearing intact. The coronal PD-weighted image with SPAIR fat suppression correlates with the sagittal fracture location. The axial PD-weighted SPAIR image demonstrates blood/fluid levels against the contrast of the patellar cartilage. The findings suggest a posterior lateral tibial plateau facture with slight cortical offset and associated marked hemarthrosis. Acquired with SENSE Knee coil, FOV 120 mm, slice thickness 2.5 mm. The zoomed images in the right column demonstrate the excellent spatial resolution and contrast. the time, he says. Typically, I use the same time frames that I do on my 1.5T system, about 4-5 minutes per sequence, but I try to achieve the maximum resolution that I can within that time frame. I focus on the FOV that will encompass the anatomy that an orthopedic surgeon will need to determine if surgery is necessary, he continues. If there are concerns about cutting off part of the anatomy. I would suggest a separate protocol to study that portion of it. When you re looking at a joint, the FOV that encompasses that joint and its critical structures is the most important for the orthopedist to understand. Dr. Goumas was originally interested in pursuing a career in orthopedic surgery. With the emergence of MRI, he became attracted to diagnostic radiology. He enjoys the technological aspects of diagnostic radiology, as well as the ability to collaborate with orthopedic surgeons. Small FOV, large matrix and same scan time produce sharper images Dr. Goumas details his imaging strategy: In comparison to a 1.5T system, on the 3.0T system you can decrease the slice width by about 25%, and increase the matrix by about 25% in each direction, he explains. And because I use the same scan time that I would on a 1.5T system, about 5 minutes, it enables me to obtain very high resolution. So, my study time hasn t really changed, but the quality of the image has markedly improved. Achieva 3.0T X-series enables to produce higher and higher resolution images. Primary Philips techniques include SENSE, SPAIR and asymmetric TSE. Issue 33 - December 2007 Field strength 15

4 The other huge advantage is outstanding field homogenity. Philips ability to saturate fat using SPAIR technology is exquisite. For example, for a knee scan with a 12 cm FOV, the slice width went from about 4 mm thick on my 1.5T systems to 2.5 mm thick on the Achieva 3.0T X-series system, and the matrix went from 384 x 192 to 560 x 386, he says. To compensate for signal loss with fat suppression, the matrix would be about 428 x 335, voxel size 0.21 x 0.31 x 3.0 mm. I needed a system that could perform at these small fields of view, says Dr. Goumas. And I found there are a number of techniques on the Philips 3.0T X-series that enable it to produce higher and higher resolution images. Three primary Philips techniques that Dr. Goumas points out include SENSE, SPAIR and asymmetric TSE. SENSE with a speed-up factor of 1 increases SNR The gold standard in parallel imaging, SENSE offers higher temporal resolution and improved spatial resolution. While it is often used to reduce the scan time, Dr. Goumas applies it to boost SNR in a special fashion. We don t use SENSE in a traditional application for speed-up. We use SENSE with a SENSE factor of 1 (no speed up). This gives us more signal, allowing us to push the matrix higher at a small FOV. In addition, the smaller pixels let us increase the pixel shift and decrease the bandwidth. This really increases SNR, which we use to increase the matrix further. The Achieva 3.0T X-series is probably the only scanner that can do it to this extent, which is why it was selected for musculoskeletal imaging in our Sports Medicine Center, he says. Robust fat suppression with SPAIR The other huge advantage to the system is that the field homogeneity is outstanding, he adds. Philips ability to saturate fat using SPAIR technology is exquisite. SPAIR is independent of B1 and therefore achieves the most uniform fat suppression at 3.0T over the entire FOV. We can image the patient s wrist at their side and get fat saturation perfectly. We can saturate ankles perfectly in the SENSE Foot Ankle coil. It s not necessary to run a STIR sequence because the fat saturation is so robust, continues Dr. Goumas. Additionally, I can dial in the exact degree of fat saturation (as opposed to simple chemical saturation techniques) I desire for Torn Meniscus with Posterior Dislocation Top row images visualize disruption of the anterior meniscal horn body junction of the lateral meniscus with meniscal tissue of the body and posterior horn displaced posteriorly. Bottom row images are from similar slice location with and without fat suppression. Yellow arrows indicate the original location of the meniscus and posterior displacement. SENSE Knee coil, FOV 120 mm, slice thickness 2.5 mm. 16 Field strength Issue 33 - December 2007

5 Slap tear with Hill-Sachs fracture and anterior dislocation Left and middle image shows slap tear (yellow arrow) with anterior dislocation and Hill-Sachs fracture (red arrow). The Hill-Sachs fracture is well demonstrated on the PD-weighted SPAIR image on the right (red arrow). SENSE Shoulder coil, FOV 90 mm, slice thickness 3 mm. Slap type 2 tear in shoulder A slap, type 2 tear is seen on two consecutive axial proton density-weighted SPAIR fat suppressed slices (left and middle images). The slap tear is well visualized on the corresponding coronal SPAIR proton density scan. SENSE Shoulder coil, FOV 100 mm, slice thickness 3 mm. a study and use the extra signal to increase my matrix again, to get higher and higher resolution, says Dr. Goumas. Asymmetric TSE helps produce sharper images According to Dr. Goumas, Another major benefit of the Philips system is the asymmetric TSE scan sequence. In musculoskeletal, we make use of a lot of proton density fat saturated imaging because it shows us the bone marrow edema, cartilage, and fluid distinctly. We can see tears in menisci with an asymmetric Turbo Spin Echo sequence with a marked reduction in image blurring usually seen in TSE techniques at low TEs. Using asymmetric profile ordering to fill k-space, Philips exclusive asymmetric TSE reduces proton density (PD) TSE blurring and PD TSE acquisition time. Traditionally, the problem has been that to avoid blurring, you had to set your TE higher than you would normally like, at about 45 to 55 ms with the associated drop in SNR. And when you set your TE higher in a proton density image, your signal loss limits resolution and how far you can push the matrix up. With asymmetric TSE, I m able to drive my TEs down to 15 to 25 without blurring, because of the way the k-space is filled. It separates the filling of k-space from the TE itself. So I don t have the blurring artifact. And with a low TE, I ve got a higher SNR. That enables a higher matrix. So it all drives toward obtaining maximum resolution imaging at small FOVs. Excellent reviews from referring physicians Referring physicians have been thrilled with the images that Dr. Goumas is producing on the 3.0T X-series. For example, Amol Saxena, D.P.M., Palo Alto Sports Medicine Center, says there is a dramatic difference in the image quality when comparing foot and ankle images produced by Dr. Goumas on the 3.0T X-series to those from other 3.0T and 1.5T systems. It s almost like night and day, Dr. Saxena says. Issue 33 - December 2007 Field strength 17

6 PDw SPAIR a-tse PDw SPAIR Interference screw fragments in knee The coronal PD-weighted, asymmetric TSE, SPAIR image clearly visualizes interference screw fragments on a post ACL repair. The sagittal SPAIR PD-weighted image correlates with the coronal images and demonstrates loose interference fragments of an ACL repair screw. The high resolution of this sequence delineates the remaining threads of the largest remaining part of the screw. SENSE Knee coil, FOV 120 mm, slice thickness 2.5 mm. The 3.0T X-series has really exceeded my expectations. We ve never have had a SAR issue and I run it pretty hard with these studies. I can better see the delineation of plantar plates and joint capsules, osteochondral defects and tendons. It also helps to determine if tendons are torn or just stretched out. He adds that by having a clearer picture of the extent of injuries, he can better plan for treatment or surgery. Dr. Goumas continues, Some of the orthopedists are saying I have never seen images like this before. For example, a small meniscal tear that we would have possibly been equivocating on, we can now more definitely call. The wrist ligaments and labrum of the shoulder are particularly difficult to view, he says. With the combination of techniques we have been able to employ with the Philips system, we ve made significant progress in those areas. Dr. Goumas goes on to explain that his imaging methods also improve articulars cartilage imaging. By minimizing chemical shift artifacts, and maximizing resolution, very early and subtle pathology can be seen in cartilage views. I m seeing cartilage substantially better than I was at a 1.5T level. And I m seeing structures in it that I was not able to see before. He says that with this level of detail the possibility to identify at-risk cartilage may lead to future applications of preventative medicine. The 3.0T X-series has really exceeded my expectations, he says. We ve never have had a SAR issue on this magnet. And I run it pretty hard with these 5-minute studies. Dr. Goumas concludes, It s this combination of tools on the Philips 3.0T that enables me to improve image resolution. The flexibility of the fat saturation technique that Philips offers with SPAIR, the SENSE factor, and the asymmetric TSE that minimizes blurring, combine for a significant boost in SNR allowing for much higher matrices than would normally be achievable. The higher matrices reduce chemical shift artifacts, which allow for much narrower bandwidths. The narrow bandwidths allow for further bumps in the SNR which can be used to further maximize the matrix and resolution. 18 Field strength Issue 33 - December 2007