Ultrasound (US) imaging is an inexpensive and compact imaging modality readily available in hospitals and clinics. Medical US imaging has progressed steadily with advances in clinical applications and equipment performance, making it an indispensable tool in obstetrics and in the diagnosis and management of many diseases. With the continued improvement of the technology, miniaturization of the scanners, and increasing importance of cost containment, ultrasound imaging is expanding its role with new diagnostic and therapy guidance applications. Although many new areas of ultrasound imaging are under development and investigation, three-dimensional ultrasound (3D US) has generated great interest. Despite decades of exploration, it is only in the past 5 years that 3D US imaging has advanced sufficiently to make it useful for routine diagnostic and interventional applications. 1-3 Recent advances in computer technology and visualization techniques have allowed real-time reconstruction and visualization of 3D images and their manipulation on inexpensive desktop computers. 4-6 Only now can we begin to explore the full potential of true 3D US imaging and visualization for both diagnostic and therapeutic applications.

With increased computational power of inexpensive computers and development of advanced 3D visualization techniques, the use of 3D image-guided interventional procedures is rapidly expanding and addressing the need for increased precision, reduced invasiveness, and reduced cost. The use of 3D US has the capability of providing real-time images of the anatomy during the procedure as well as providing 3D images to monitor and guide the procedure. Research investigators and industry have begun to incorporate 3D US capability into surgical and therapy planning procedures as well as breast biopsy techniques. 7-11

ADVANTAGES OF 3D ULTRASOUND

Although conventional 2D ultrasound is a highly flexible imaging modality, allowing the user to manipulate the transducer and view the desired anatomical section, it also suffers from disadvantages that 3D US imaging attempts to address:

  • Conventional 2D ultrasound images represent thin planes of the patient. Since their location is controlled by manually manipulating the transducer’s orientation, the user must mentally integrate many 2D images to form an impression of the 3D anatomy and pathology.
  • Planning or monitoring of therapeutic procedures requires placing the 2D ultrasound image plane at a particular desired location within an organ and being able to reproduce the same location at a later time. Since conventional 2D ultrasound is controlled manually, finding the same location is very difficult.
  • Manipulation of the 3D US image allows viewing of an arbitrarily orientated image plane within the patient, even planes not accessible by 2D US.
  • Some diagnostic procedures as well as therapy and surgery planning require accurate volume measurements. The use of 2D ultrasound for measurement of organ or lesion volume is variable and at times inaccurate.

IMAGING INSTRUMENTATION

3D US systems use two basic approaches: (i) conventional 1D arrays producing 2D images, which are reconstructed into 3D images using knowledge of their relative positions, and (ii) 2D arrays generating real-time 3D images directly. Although the use of 2D arrays to produce real-time 3D ultrasound images is the most convenient, this technology is still costly, requiring specialized technology. Most 3D ultrasound systems available today use the first method, in which conventional ultrasound machines with 1D arrays are used to acquire 2D images and reconstruct them into 3D US images.

In the following sections, the main methods used to produce a 3D image are described briefly. For more detailed descriptions of 3D US approaches, the reader can refer to recent review articles and books. 1-6

Mechanical scanning mechanisms . Mechanical scanning mechanisms coupled to conventional transducers provide 2D images with accurate relative positions and orientations, allowing accurate 3D reconstructions. 1,5 Typically, the conventional ultrasound transducer is housed in a motorized mechanical assembly, which moves the transducer as 2D ultrasound images are acquired at predefined spatial or angular intervals. With accurate knowledge of the relative positions and orientations, the sequence of 2D images can be reconstructed into a 3D image very efficiently. Various kinds of mechanical 3D assemblies have been developed, which can rotate or translate the transducer over the region to be examined. Because the geometry is predefined, the reconstructed 3D image is available immediately after the acquisition.

Figure 1. Diagrams showing the motorized tilting 3D US scanning approaches using conventional ultrasound transducers. The tilting mechanism may be contained in a specially designed housing or an external fixture attached to the conventional ultrasound transducer.
Two multi-planar reformatting (MPR) approaches used to display a 3D US image of the prostate: (Figure 2a) the cube-view approach, in which the extracted planes are texture ‘painted’ on the faces of a polyhedron.
Figure 2b. The crossed-planes view, in which the extracted planes are intersecting with each other.

Two types of mechanical assemblies are currently available: integrated mechanisms, which are designed to accommodate the motor and transducer within the housing, and external fixtures, which are attached to the conventional ultrasound transducer (see Figure 1). The integrated mechanisms are generally smaller than the external mechanisms, allowing easier use by the operator. However, the integrated approach requires specially designed transducers and an ultrasound system capable of controlling them. These types of systems are the most popular and are used extensively in obstetrics and radiology. While external assemblies are generally bulkier, they employ conventional ultrasound transducers and can be interfaced to any conventional ultrasound machine. The external fixture approach is very flexible and has been used for controlling the movement of the transducer to generate three basic types of motion (linear, fan, and rotation scanning). In addition to vascular imaging, this approach has been successful in 3D US-guided breast biopsy and prostate cryotherapy 7 and brachytherapy 9 (see Figure 2).

Tracked free-hand . In this approach, the operator holds an assembly composed of the transducer and an attachment that provides information on the orientation and angulation of the transducer. To produce a 3D US image, the operator manipulates the assembly over the anatomy in the usual manner. The most successful approach for providing the geometrical information makes use of a six-degree of freedom magnetic positioning device. These types of systems are generally compatible with any ultrasound machine and can be purchased from two companies as well as being available in many research laboratories. These types of assemblies have been used successfully in many applications including obstetrics and vascular imaging.

Untracked free-hand .In this approach, the operator moves the transducer in a steady and regular motion, while 2-D images are digitized. Since the position and orientation of the transducer are not recorded, a linear or angular spacing between digitized images is assumed in reconstructing the 3D image. To avoid significant image distortions, the operator must be trained to move the transducer at a preselected linear or angular velocity. Nonetheless, geometric measurements such as distance or volume may be inaccurate and should not be made, limiting the utility of this approach to visualization of the anatomy only. Integration of this approach with any ultrasound system is easy and only requires software to reconstruct the series of 2D US images into a 3D image.

Real-time 3D with 2D arrays . Unlike the mechanical and free-hand 3D US systems, which use 1D arrays to produce a series of 2D images, systems using 2D arrays keep the transducer stationary and use electronic scanning to sweep an ultrasound beam over the volume-of-interest to produce 3D images in real time. The transducer is composed of a 2D phased array of elements, which are used to transmit a broad beam of ultrasound diverging away from the array and sweeping out pyramidal volumes. The returned echoes are detected by the 2D array and then processed to display in real time multiple planes from the volume or a volume-rendered view of the anatomy. The planes or the view can be chosen interactively to allow the user to view the desired region under investigation.

The most advanced clinical real-time systems used for 3D and 4D (real-time 3D) echocardiography have been developed by investigators at Duke University and by a major vendor. Since these systems require advanced technology, they are generally expensive. In addition, before this approach becomes widespread in radiology, the volume scanned must be enlarged.

IMAGE RECONSTRUCTION, DISPLAY

The reconstruction algorithm uses the acquired 2D images and knowledge of their relative positions and orientations to place each in its correct location in the volume being reconstructed. With modern desktop computers, the reconstruction procedure can be carried out within a few seconds after all the 2D images have been acquired, or even during the image acquisition. Since the 3D image is built from acquired 2D images, the gray scale values of any voxels not sampled are determined by interpolation between the appropriate acquired images. Thus, the original acquired 2D image information is preserved, allowing the physician to view these planes, as well as any other desired views using 3D visualization software.

Many algorithms have been developed to visualize and manipulate 3D images interactively. Although the quality and geometric accuracy of the 3D US image depend on the parameters and method of image acquisition, the 3D display technique often plays a dominant role in the physician’s ability to obtain the desired information. Although many 3D ultrasound display techniques have been employed, the two used most often are: multi-planar reformatting (MPR) and volume rendering (VR). 1,5

Multi-planar reformatting . This technique reformats the 3D data to a display of 2D planar surfaces. To obtain a 3D view, a utility is used to extract any desired plane from the 3D data set. The extracted planes are then displayed with 3D cues using different techniques to allow comprehension of the 3D geometry.

Three MPR approaches have been successfully used to display 3D US images. In the crossed-planes approach, a single or multiple extracted planes are presented in a view that shows these planes in their correct relative orientation. Typically, these planes intersect each other as can be seen in Figure 2b. The user can move each plane parallel or obliquely to any other plane to reveal other views of the anatomy. A second popular display technique uses the cube-view approach (Figure 2a), in which the extracted 2D ultrasound images are “painted” onto the faces of a polyhedron. The user can select any face of the polyhedron and move it parallel or obliquely to any other, while the appropriate 2D US images are extracted and “painted” in real time on the new face. In this way, the appearance of a solid-like polyhedron provides the user with 3D image-based cues relating the plane being manipulated to the other planes. Both the crossed-plane and cube view can be rotated to obtain the optimal orientation for viewing the anatomy. In a third approach, the extracted planes (typically 3 orthogonal) are shown on the computer screen beside each other. To provide 3D cues to the user, lines are drawn on each extracted plane to designate its intersection with the other planes. The user can translate or rotate these lines, to provide appropriate other desired planes to be extracted and displayed.

Volume rendering techniques (VR) . The MPR technique is the most common visualization method used to view 3D ultrasound images. Because it provides 3D viewing by showing only 2D planes with 3D cues, just a small part of the complete 3D information can be viewed at any one time. An alternative technique used frequently to view 3D CT and MRI images makes use of volume-rendering approaches, in which the entire 3D image is viewed after it has been projected onto a 2D plane. Typically, this is accomplished using ray-casting techniques, in which a 2D array of rays is projected through the 3D image. The volume elements (voxels) intersecting each ray are weighted and then summed in various ways to produce the desired effect. Common VR techniques used to view 3D US images are maximum intensity projection, translucency rendering, and surface enhancement as shown in Figure 3.

Figure 3. Two volume rendered (VR) 3D US images showing the face and hand of a fetus.

Since the VR techniques project all 3D information onto a 2D plane, interpretation of complex images is difficult. Thus, this approach is not suited for viewing 3D B-mode ultrasound images with subtle contrast between different soft tissues. However, this approach has been used most successfully to view structures in which anatomical surfaces are clearly distinguishable, such as fetal structures surrounded by amniotic fluid, 6,12 tissue/blood interfaces in the heart and large arteries, and in situations in which B-mode clutter has been removed in power or color Doppler 3D images. 13

3D ULTRASOUND APPLICATIONS

3D US imaging is now readily available on most modern ultrasound systems. With its wide availability, investigators have reported on the utility of 3D US in a wide variety of applications. Conferences focusing on medical ultrasound as well as conferences focusing on technical aspects of imaging have whole sessions devoted to 3D US imaging. Thus, in this section, we describe one application in which 3D US (obstetrics) has a demonstrated advantage over 2D US as well as two emerging applications of 3D US in image-guided therapy and surgery.

3D US in obstetrics . The most extensive applications of 3D US have been in obstetrics due to its ability to provide information not readily available with 2D US imaging. Volume rendering coupled with MPR viewing of the fetal face and skeleton offers important views that enhance the compression of the ultrasound information (see Figure 3 above). The ability to rapidly reorient the 3D view for best viewing of the fetal anatomy allows rapid identification of normal and abnormal structures. Numerous studies have been published, in which the utility of 3D US in obstetrics has been investigated. In these publications, it has generally been shown that 3D US has significant advantages compared to 2D US. These advantages include: improved comprehension of the fetal anatomy by families resulting in improved decisions about management of the pregnancy; improved maternal/fetal bonding due to improved visualization of fetal features; improved identification of fetal anomalies due to improved visualization of fetal features and ability to easily reorient the information for best viewing of the anatomy; and more accuracy in volume measurement aiding in management of the pregnancy and in determining the size and extent of the anomaly.

3D US-guided prostate therapy . The most common form of prostate brachytherapy involves the implantation of about 80 radioactive seeds into the prostate. The delivery of high conformal dose to the prostate while sparing surrounding tissues requires that the radioactive sources be positioned accurately within the gland. 3D US imaging has been used for the diagnosis of prostate cancer 8 and for guiding prostate cryosurgery. 7 Recently, this technique has been extended to 3D ultrasound-guided prostate brachytherapy with the development of semiautomated prostate contouring, needle segmentation, and seed segmentation. The use of 3D US in prostate brachytherapy allows the development of an intraoperative technique, in which all steps are carried out at one session. 9,10

Since it is difficult to place the patient in the same position during the preimplantation and implantation procedures, errors in correct positioning of the seeds can occur. In addition, since it is difficult to implant additional seeds and impossible to remove them if placed incorrectly, often errors in implantation cannot be corrected. However, performing the complete procedure intraoperatively would allow errors to be immediately detected and corrected. 3D ultrasound with rapid scanning and immediate viewing of the 3D prostate anatomy is the best candidate imaging technology, allowing a complete intraoperative procedure. This approach permits preimplant dose planning and seed implantation at the same session, thereby avoiding problems of repositioning, prostate motion, prostate size/contour changes, and image registration between imaging modalities. Availability of 3D US images during the implantation procedure would permit immediate postimplant verification to detect implantation errors and correct them immediately.

Intraoperative 3D ultrasound. Ultrasound imaging is used for intraoperative imaging because it does not require a specialized operating room or a specialized ultrasound machine. In addition, its use does not lengthen surgical procedures except for the few minutes required for the imaging itself, and it provides good contrast for a broad range of tumor types. However, the issues discussed above limit ultrasound imaging.

When employing an image-guidance system, the locations of the tracked surgical tools should be displayed in some form on the images. When using 2D ultrasound images, the tip of the surgical tool must be within the plane of the image, restricting the tool motion along a single trajectory that is within the 2D image plane. 11 This approach is suitable for biopsy but not for tumor resection. 3D US imaging avoids this problem by allowing viewing of the surgical tool in any plane.

The use of 3D US in intraoperative neurosurgical application is being investigated in many research laboratories. 14 Generally, in these systems the surgeon acquired the 3D US image via a freehand technique, and then either displayed the volume as axial, sagittal, and coronal slices adjacent to the preoperative MRI slices, or alternatively displayed a slice through the volume that corresponded to the trajectory of the tool that was being tracked by the guidance system. In addition to the use of 3D US for tracking of surgical tools, it has also been used effectively to register intraoperative 3D US images with preoperative 3D MR images to correct for brain shift during tumor resection.

Aaron Fenster, PhD, is director, Robarts Research Institute, London, Ontario, Canada.

References:

  1. Nelson TR, Downey DB, Pretorius DH, Fenster A. Three-Dimensional Ultrasound. Philadelphia: Lippincott, Williams, and Wilkins; 1999.
  2. Downey DB, Fenster A, Williams JC. Clinical utility of three-dimensional US. Radiographics. 2000;20:559-571.
  3. Baba K, Jurkovic D. Three-Dimensional Ultrasound in Obstetrics and Gynecology. New York: Parthenon Publishing Group; 1997.
  4. Nelson TR, Pretorius DH: Three-dimensional ultrasound imaging. Ultrasound Med Biol. 1998;24:1243-70.
  5. Fenster A, Downey D, Cardinal N. Review: 3-dimensional ultrasound imaging. Phys Med Biol. 2001;46:R67-R99.
  6. Nelson TR, Pretorius DH, Sklansky M, Hagen-Ansert S. Three-dimensional echocardiographic evaluation of fetal heart anatomy and function: acquisition, analysis, and display. J Ultrasound Med. 1996;15:1-9.
  7. Pretorius DH, Nelson TR. Fetal face visualization using three-dimensional ultrasonography. J Ultrasound Med. 1995;14:349-356.
  8. Downey DB, Fenster A. Vascular imaging with a three-dimensional power Doppler system. AJR Am J Roentgenol. 1995;165:665-668.
  9. Wei Z, Wan G, Gardi L, Mills GR, Downey DB, Fenster A. Robot-assisted 3D-TRUS guided prostate brachytherapy: system integration and validation. Med Phys. 2004;31:539-548.
  10. Ladak H, Mao F, Wang Y, Downey D, Steinman D, Fenster A. Prostate boundary segmentation from 2D ultrasound images. Med Phys. 2000;27:1777-1788.
  11. Smith WL, Surry KJM, Mills GR, Downey DB, Fenster A. Three-dimensional ultrasound-guided core needle breast biopsy. Ultrasound Med Biol. 2000;27:1025-1034.
  12. Chin JL, Downey DB, Mulligan M, Fenster A. Three-dimensional transrectal ultrasound guided cryoablation for localized prostate cancer in nonsurgical candidates: a feasibility study and report of early results. J Urol. 1998;159:910-914.
  13. Tong S, Downey DB, Cardinal HN, Fenster A. A three-dimensional ultrasound prostate imaging system. Ultrasound Med Biol. 1996;22:735-746.
  14. Unsgaard G, Ommedal S, Muller T, Gronningsaeter A, Nagelhus Hernes TA. Neuronavigation by intraoperative three-dimensional ultrasound: initial experience during brain tumor resection. Neurosurgery. 2002;50:804-812.