Volume Ultrasound, rendering modes and clinical application

Transcription

1 GE Healthcare Volume Ultrasound, rendering modes and clinical application R. CHAOUI Center for Prenatal Diagnosis and Human Genetics, Berlin, Germany B. BENOIT Hospital Princesse Grace, Monaco, Monte Carlo

2 Introduction In recent years, three-dimensional (3D) ultrasound has become the most rapidly evolving technique in fetal imaging and meanwhile, it is currently used worldwide by more than half of all perinatologists. Some users are still using mainly 3D and 4D techniques to demonstrate the fetal face to the parents, which has made this new technique very popular as well. Volume ultrasound introduced into prenatal diagnosis a few years ago enabled a more comprehensive medical and clinical application of this new technique, opening new horizons for a future era of ultrasound. Numerous research papers were published describing the clinical potential of new techniques and rendering modes, but many colleagues may still be unfamiliar with all of these features, which are now well established in targeted prenatal diagnosis for ruling out or clearly demonstrating fetal malformations. The aim of this short review is to provide information on the potential of volume ultrasound and the application of some display modes in clinical work. Volume acquisition There are different ways to acquire a volume data set. The acquisition can be achieved either as a: static 3D Real-time 3D or 4D STIC for heart and vessels Static 3D (one volume data set): this is the 3D used in most fetal studies (face, hands etc.). The volume quality is defined by the choice of the acquisition time where max is the slowest acquisition with the highest numbers of images within a volume through high2 and high1 to min, which is then very rapid and of reduced quality. A later analysis of subtile structures from a volume data set using Tomographic Ultrasound Imaging (TUI) may necessitate a max acquisition whereas for the demonstration of large areas such as the 3D surface of the face or the body high1 or mid quality are sufficient. For the acquisition of a 3D heart volume, it is better to use the min to mid quality to avoid artifacts from wall movements. This static 3D can also be combined with Color Doppler, Power Doppler, High Definition (HD)-flow, and B-Flow depending on the question of interest. Real-time 3D or 4D-Ultrasound (many volume data set): Today this is achieved by a mechanical 3D transducer with a rapid acquisition from 1.5 to 40 volumes/sec. The advantage of a 4D examination is its easy use. The direct result on the screen enables online manipulation to acquire the best image by changing the gain and the contrast depending on the mode used. Furthermore, it allows the transducer to be moved depending on the insonation angle and it allows users to fully exploit the image. The technique is ideal for studying fetal movements and behavior throughout pregnancy, but also for use in Volume contrast Imaging (VCI) enabling online reconstruction of a volume slice of the C- or A-plane. It can be used for fetal echocardiography as well but requires a great deal of experience. STIC (one cardiac cycle including a data set up to 40 volumes): In this condition the volume of interest is not scanned in an instant. The slow acquisition of image slices (duration 7.5 to 15 sec) allows the acquisition of numerous planes including additional information from an entire cardiac cycle. The Spatial and Temporal Image Correlation (STIC) calculates the mean heart rate acquired and the images in the volume are re-arranged according to their temporal event within the heart cycle. The displayed volume then includes a single hypothetical heart cycle, which is reconstructed from single selected images of the A-Plane in the different phases of the heart cycle, whereas the B- and C-Planes are reconstructed digitally. STIC can be used with grayscale fetal echocardiography but can be also combined with color Doppler, power Doppler, High Definition (HD)-flow, B-Flow, etc. With B-flow, it can also be applied for peripheral vessels without a true calculation of the heart rate (e.g. demonstration of the umbilical cord). Once the acquisition of a volume is achieved, the information can be visualized either as single or as multiple 2D images regenerated from the volume and selected by the examiner or as a volume spatial information called 3D rendering allowing the application of different modes. Some of the actual display modes will be emphasized and illustrated in this article.

3 VOLUME DATA DISPLAY One plane of choice, multiplanar orthogonal planes or multiple tomographic parallel slices A volume data set consists of digital information on numerous images and thus allows the demonstration of infinite planes of choice as a so-called anyplane. From a 3D/4D data set the cross-sectional views can be obtained at any desired orientation, direction and depth. It must be borne in mind that the acquisition (A-) plane provides the best information, whereas the reconstructed planes B-, C- or others are of less quality. This should be considered during the volume acquisition. The 2D image analysis from a volume can be achieved from a 3D, 4D or STIC data set. The display format is either a single-plane view or a multiplanar view showing three planes which are perpendicular to each other. In the lateral view the intersection of the three planes is a dot and by moving the position of this dot, the examiner can navigate through the volume. The recent introduction of the multislice analysis known as Tomographic Ultrasound Imaging (TUI) is similar to the tomographic assessment known from CT and MR workstations. The examiner can define the slice thickness and the numbers of planes demonstrated. The multiplanar mode can be used to acquire a plane not directly seen on cross section 2D during live examination, mainly in cases with non-optimal fetal position, so as to demonstrate the corpus callosum, a fetal profile, a limb or the aortic arch. It can used to visualize exact midline planes after making adjustments in the two other orthogonal planes (for nasal bone assessment). One of the major advantages of a three dimensional data set is the possibility to achieve the examination off-line on few volumes at a remote station. One of the future potential uses of this mode could be the transfer of data via Internet to a remote site to get a second opinion or a complete offline evaluation without examining the patient. Multiplanar mode with the three orthogonal planes. The dot is the intersection of all 3 planes and can be used to get the best position for assessing the profile. In the C- plane (lower panel) the dot is on the nasal bone Tomographic Ultrasound Imaging TUI: with a volume of the fetal thorax and abdomen. All structures can be seen with one view Tomographic Ultrasound Imaging TUI of the brain demonstrating all important brain structures as the lateral ventricles, the cerebellum, the cavum septum pellucidum, insula and others However, since the quality of reconstructed images depends mainly on the original acquisition, the examiner should consider this aspect when acquiring volumes for future studies. We found that the four-chamber view or some intracranial structures as the lateral ventricles or cortex are best acquired in a transverse view, but the aortic arch, the spine or the nuchal translucency and midline brain structures are best viewed from a sagittal view. Organs with few details such as the stomach, urinary bladder, liver or kidneys can be acquired from any insonation angle. In a STIC volume the reconstructed cardiac volume can be displayed in the multiplanar or tomographic modes, and played in slow motion or stopped at any time for detailed analysis of specific phases of the cardiac cycle. When combined with color Doppler, events within the cardiac cycle during systole and diastole can be very well demonstrated. Tomographic Ultrasound Imaging TUI of the same volume with the demonstration of the corpus callosum Tomographic Ultrasound Imaging TUI can be used for the heart combined with STIC. Within one image mainly moving structures can be seen

4 Surface mode rendering The image rendering of the fetal surface is the most known and commonly used display modality in 3D and 4D. From the volume acquired, the skin is primarily demonstrated (surface) and not the organs inside the body. It is used to visualize a surface of a structure which is best achieved in the interlay between fluid and surface, as the face of a fetus in amniotic fluid, the valves within the heart or even the liver or abdominal organs, for example in ascites. After placing the volume box over the region of interest, the rendering should be chosen either as surface, surface smooth or gradient light or a combination of two of these options. The examiner should adjust the 2D image prior to volume acquisition to get a black amniotic fluid and a contrasty surface of the region of interest. The main advantage of the technique is its easy use and its impact on patients and doctors due to the lifelike image, or the comparison with postnatal appearance. Clinical applications are the demonstration of the whole fetus in the first trimester until 12 weeks gestation, then the demonstration of the face, limbs, etc. in order to rule out or confirm anomalies involving the skin as well as facial anomalies, spina bifida, limb anomalies and others. It is best demonstrated using 3D as well as 4D, whereas the latter can be used to analyze behavior such as fetal movements, grimacing, yawning or eye opening. Surface rendering can be applied to the fetal heart by using the interface between the cavities and the cardiac walls. It can be used in the brain to demonstrate the cavities such as the lateral ventricles, especially in the presence of brain anomalies. Surface mode rendering in early pregnancy Surface mode rendering of the face in later gestational age Surface mode rendering: can be used at the level of the heart to see the spatial appearance of the heart cavities rendering The maximum rendering mode highlights the maximal echo information of a volume data set and is an ideal tool for the 3D reconstruction of bony structures. In general, cranial bones, the ribs and other curvilinear bones can not be properly seen in a single 2D plane and are therefore better assessed in a maximum mode projection. This technique was applied in the demonstration of spine and limb abnormalities but was recently used in the assessment of the nasal bones, the cranial bones and corresponding sutures. This technique delivers a picture similar to an X-ray of the bony skeleton in the fetus. Prior to volume acquisition, the examiner should reduce the gain of the 2D image so that mainly the bones are highlighted but not the fetal skin. For 3D rendering, the examiner should then choose a narrow volume box to primarily include the region of interest, with only very little information from the neighboring tissue or skin. rendering: is used to visualize the fetal skeleton, here the bony face. On the left a normal fetus with a nasal bone and on the right a fetus with absent nasal bone rendering: demonstrating a normal hand (left) and radius aplasia (middle) and cleft hand (right)

5 The rendering mode chosen is then maximum mode. In some circumstances, it could be combined with surface mode. However, the threshold should be increased. Maximum mode can be used with 3D static, live 4D and elegantly with the Volume Contrast Imaging VCI-C, which is a modified live 4D in which only a defined slice of 1-20mm of a reconstructed C-plane is displayed. This technique is used to rapidly demonstrate the spine and ribs or the bony structures of the face or the skull bones. rendering: demonstrating here the skull from the side with the skull sutures rendering combined with Volume contrast imaging (VCI-C) rendering combined with Volume contrast imaging (VCI-C) Minimum mode rendering Another rendering mode is the minimum mode where the hypoechoic structures are emphasized. When this mode is selected, mainly structures with highest transparency (anechoic) can be demonstrated as a 3D projection of vessels, cysts, bladders, etc. that appear black against a surrounding of more echogenic tissue. It is preferable to choose the rendering box narrow in order to focus on including the region of interest. Within the box, the presence of amniotic fluid should be avoided as it casts a large black shadow. Images produced with this technique are similar to x-ray projection. Regions of interest are mainly the stomach, the bladder, the brain ventricles and the heart with the corresponding vessels. Minimum mode rendering demonstrating the abdomen with Bladder (BL), Gallbladder (GB), the umbilical vein (UV), the stomach (ST) the Vena Cava inferior (VCI) and aorta (AO) Minimum mode rendering used to demonstrate the aortic arch and a frontal view of the heart

6 Inversion mode rendering This recently introduced display mode starts from the minimum mode rendering and inverts merely the color of the information (similar to negative/positive film), thus presenting the hypoechoic structures as echogenic solids. It blackens most of the surrounding tissue information. By changing certain presets, such as increasing the threshold and decreasing the transparency, the image can be improved. We chose either the gradient light or the light rendering to get the most from the image. This technique was also called negative surface display and it was discovered that the images produced were similar to postmortem casting. Artifacts may result from rib shadowing or from amniotic fluid etc., but can be eliminated using the electronic scalpel Magicut. Recent reports analyzed the role of this technique in visualizing cardiac and extracardiac fluid filled structures in the fetus. Application fields are not only the heart and vessels, but also kidneys in hydronephrosis, brain ventricles and other hypoechoic cystic structures. Regions of interest in the fetus could be the fluid filled structures as the stomach, the urinary or gallbladder. The shape of the stomach and duodenum during the presence of a double bubble in duodenal atresia could be a clinical important application. The gallbladder and urinary bladder are other abdominal structures easily visualized by the inversion mode technique. The kidneys can mainly be demonstrated in anterior-posterior longitudinal projection and clinical benefit can be found in multicystic kidneys and hydronephrosis. Intracranial brain structures, especially the lateral ventricles in early pregnancy can be clearly demonstrated and malformations with disturbed anatomy of the lateral ventricles can be of clinical use with inversion mode. One of the further major fields of interest with inversion mode is the cardiovascular system. The examiner can visualize the heart and the vessels in a manner similar to 3D-Power Doppler Ultrasound at a better resolution and a more rapid acquisition rate. Particularly easily demonstrated is the crossing of the vessels of the heart or the relationship of the ventricles and their size. The main advantage of this technique is that the image is similar to the one acquired by power Doppler but without the difficulties encountered in adjusting the image. The volume can be acquired in grayscale as 3D static or as a STIC, at a high frame rate and resolution, whereas volumes with power Doppler information are at low frame rates and subject to movement artifacts. Thus, the image quality with inversion mode is superior to the quality obtained by power Doppler; however, it lacks the information of neighboring tissue demonstrated in the glass body mode. Since inversion mode can also be used for volume calculation as well, it could be easier used to calculate volumes of structures with irregular shape than with the VOCAL- Technique. The stomach demonstrated in 2D, minimum mode, inversion mode and for comparison a stomach as double bubble sign in a fetus with duodenal atresia Inversion mode in a fetus with vesico-ureteral reflux showing the dilated ureters Inversion mode of an abnormal bladder with subvesical obstruction Inversion mode to demonstrate the shape of the dilated ventricles in this fetus with spina bifida Inversion mode used to demonstrate the heart and the crossing of the great vessels

7 Glass-Body mode rendering If the volume data is not acquired from a simple gray scale image but combined with color Doppler, power Doppler or High- Definition (HD-) flow, the 3D volume includes the Doppler information as well. The acquisition can either be achieved as static 3D or as a STIC. Volume data can be displayed in 3 ways: either the color information alone or the gray scale information alone or a combination of both as a so called glass body mode. A prerequisite for a good volume is the optimal presetting of the color during 2D scan before acquiring a volume. Once the presetting of the 2D-power Doppler image is optimized, the acquisition can ideally be used to demonstrate the spatial relationship or size differences of the great vessels, connecting VSD, or even coarctation of the aorta or aberrant aortic arch vessels. One of the main application fields of this mode is the demonstration of the great vessels crossing or their parallel course in transposition of the great arteries. The clinical use of such new techniques, however, has yet to be evaluated. Peripheral vessels such as the umbilical cord, intraabdominal, thoracic and brain vessels can be well demonstrated. The use of Magicut permits selective removal of both grayscale and color information or one of the two, which allows for better highlighting of the region of interest. Glass body mode: on the left the color alone and on the right as glass body mode Glass body mode of the heart and abdominal vessels (left) and of the great vessels from a STIC volume (right) Glass body mode of a STIC volume combined with color demonstrating the crossing of the great vessels (left). On the right fetal heart and vessels seen with high-definition flow where gray scale information was partly removed by Magicut Volume calculation Biometry is an integral part of the antenatal ultrasound examination and has been achieved for years by measuring distances, circumferences and areas. The acquisition of a 3D volume data set allows easy reconstruction of a selected 2D plane to perform well known measurements such as nuchal translucency, biparietal diameter or femur length and others, but also offers the potential to accurately calculate the volume of a selected region of interest. Volume measurements can be achieved either using the multiplanar mode or by using the VOCAL software (VOLume CALculation). Recently, another possibility was developed for liquid-filled structures involving the threshold principle in combination with the inversion mode. Volume measurements are still time-consuming and thus limited to research purposes. Glass body mode combined with High- Definition (HD) Flow showing here the pericallosal artery (left) and the cord insertion on the placental site (right) Volume measurements and charts were reported for the placenta, the amniotic cavity, the first trimester fetus, the fetal brain, liver, and arm, but there was a special interest in measuring fetal lung volume. Fields of interest in these measurements focused chiefly on the detection of difference in volume in pregnancies complicated by chromosomal anomalies, diabetes, intrauterine growth restriction, congenital diaphragmatic hernia...

8 Conclusion Three-dimensional ultrasound has moved from a simple advertising gimmick directed at parents to a powerful tool in prenatal diagnosis. The different display modes available can be used for the demonstration of the spatial appearance of surface structures as well as the projection of bony structures for a better understanding of skeletal findings and others. The multiplanar mode and tomographic imaging allows the reconstruction of planes not directly seen on the screen and offers new insight into fetal anatomy similar to images made known by MRI as demonstrated for brain structures. Some of the other rendering displays present on the machine were not emphasized in this article. The numerous enthusiastic articles on 3D written in recent years confirm that we are rapidly moving today from the era of sonography in 2D planes to volume ultrasound. With this article, we aimed to stimulate the reader s curiosity with regard to this revolutionary technology and we hope to encourage the examiner to use volume ultrasound more often and to clinically apply the different rendering modes. GE Medical Systems Ultrasound United Kingdom Fax: (+44) , Tel: (+44) GE Medical Systems America: Milwaukee, WI, USA - Fax: (+1) GE Medical Systems Asia: Tokyo, Japan Fax: (+81) Shanghai, China Fax:(+86) Visit us online at: General Electric Company All rights reserved. GE Healthcare, a division of General Electric Company. General Electric Company reserves the right to make changes in specifications and features shown herein, or discontinue the product described at any time without notice or obligation. Contact your GE representative for the most current information. General Electric Company, doing business as GE Healthcare. GE, GE Monogram, Voluson and CrossXBeam CRI are trademarks of General Electric Company. Healthcare Re-imagined GE is dedicated to helping you transform healthcare delivery by driving critical breakthroughs in biology and technology. Our expertise in medical imaging and information technologies, medical diagnostics, patient monitoring systems, drug discovery, and biopharmaceutical manufacturing technologies is enabling healthcare professionals around the world discover new ways to predict, diagnose and treat disease earlier. We call this model of care "Early Health." The goal: to help clinicians detect disease earlier, access more information and intervene earlier with more targeted treatments, so they can help their patients live their lives to the fullest. Re-think, Re-discover, Re-invent, Re-imagine. GE Ultraschall Deutschland GmbH & Co. KG Beethovenstr. 239, D Solingen Fax: (+49) Tel: (+49) GE imagination at work Printed in Austria U022E