Image-Guided Minimally Invasive Diagnostic and Therapeutic Interventional Procedures
Medical imaging is applied at various stages in the patient management process: screening, diagnosis, staging, therapy delivery, and therapy follow-up. The primary role for imaging has been diagnostic, but there is increasing, albeit limited, use of medical imaging modalitiesincluding endoscopy, xray fluoroscopy and computed tomography (CT), ultrasound, and recently magnetic resonance imaging (MRI)for interventional diagnostic and therapeutic uses, driven by the thrust for minimally invasive procedures. Medical imaging permits the physician to plan the therapeutic procedure more accurately before carrying it out, to guide the intervention and more correctly locate the position of the interventional tool with respect to the anatomy, to monitor the intervention as it is being carried out, and to control the intervention for optimum results.1
Although static images are sufficient for planning radiation therapy or providing some anatomical information for surgery, real-time image guidance has been fundamental to the evolution of interventional radiological procedures. More recently, the emergence of minimally invasive therapeutic procedures has encouraged the tendency for surgical procedures to shift from
1 NOTE: This chapter is adapted in large part from Jolesz and Blumenfeld, 1994; see suggested reading list at end.
large explorations toward approaches with limited access and restricted visibility. These new applications increase the demand for image guidance and compel the use of the most advanced imaging methods. It is expected that allowing medical imaging systems to play a more direct role in interventional and therapeutic proceduresthus enabling greater precision, increasing foreknowledge, and facilitating even less invasive surgical accesswill reduce the cost of patient management and improve the quality of patient outcomes.
Image-guided therapy is a new, emerging field that has close relationships to interventional radiology, minimally invasive surgery, computer-assisted visualization, and robot-assisted surgery.
12.1 Therapeutic Intervention Experience with Different Imaging Modalities
12.1.1 X-Ray Imaging
Static x-ray imaging has been used to allow the therapist to plan radiation therapy and, to a lesser extent, surgery; however, the projection nature of the image limits its value, particularly when tomographic images can be available for this purpose. For guidance, control, and monitoring operations during a procedure, fluoroscopic images are used rather than static images. Fluoroscopy has the advantage of presenting real-time images. Although it is not tomographic, overlying structures are eliminated by using contrast media to accentuate the specific anatomy (usually blood vessels) to be studied. A major disadvantage of fluoroscopic methods is that the x-ray dose delivered to the patient and to the interventionalist can be high. However, x-ray fluoroscopy is the key imaging technique used in interventional procedures.
12.1.2 Computed Tomography
Computed tomography (CT) imaging has been used in interventional applications in basically two ways: (1) to provide information for planning radiation therapy and for surgeryincluding in particular stereotactic biopsy, stereotactic craniotomy, and modern stereotactic radiosurgeryas well as for planning conventional radiation therapy; and (2) to provide guidance for image-guided biopsy of various body parts.
The advantage of CT over projection x-ray imaging is that CT is tomographic, presenting the anatomy on a slice-by-slice basis for more exact localization. It is not preferred, at this time, over standard angiographic
methods for vascular interventions, because of the difficulty of matching the passage of the bolus through the vascular tree with the acquisition of the appropriate slices. However, there are some hints that this matching may be possible and useful using spiral scanning CT. The major disadvantages of CT are that (1) like other x-ray imaging, it uses ionizing radiation, which poses a risk, particularly to the physician, and (2) it is not sensitive to parameters other than electron density and so does not provide the wealth of tissue parameter information that MRI and potentially ultrasound give. Thus CT is sensitive primarily to anatomic rather than physiological changes, although the anatomic changes it detects can also reflect physiological changes. For example, CT can be used to obtain information about abnormal myocardial wall motion through a gated study or electron beam CT. However, it is not sensitive to changes in temperature, diffusion coefficient, or perfusion in the way that MRI is, and so may not be able to provide warning of a change early enough to address the condition while it is still reversible. In addition, CT in general does not provide as good information about the lesion boundaries or margins as does MRI.
CT has much better geometric accuracy than does MRI, because of the intrinsic curved nature of the magnetic field. Thus CT may be preferred in applications for which geometric accuracy is important, such as surgical procedures that rely on prospectively derived images for navigational guidance.
The most widely used interventional CT procedure is for diagnostic biopsy, which has been applied to the head, neck, thorax, liver, pancreas, adrenal glands, kidney, pelvis, retroperitoneum, and skeletal bone. In addition, CT has been used (1) to guide other percutaneous procedures,2including discectomies, denervations, and neurolysis to relieve pain in the spinal column; (2) via neurolytic techniques, to manage pain due to cancer; (3) to manage fluid collection in the urinary system; and (4) to drain abscesses and other fluid accumulations. These techniques have been described in the book Interventional Computed Tomography listed in the suggested reading.
Ultrasound can be and has been used extensively in interventional radiology, particularly for the guidance of biopsy and to aid in fluid management. The major advantages of ultrasound are its real-time nature and its low cost compared to other imaging modalities. The images are tomographic but are
2Those performed through the skin, as with a needle.
not as clear and crisp as those obtained with CT and MRI, and thus are less acceptable to surgeons.
The advent of video endoscopic techniques3 revolutionized the field of surgery and made possible the concept of minimally invasive surgery. Originally developed as diagnostic tools, endoscopes and their allied instruments such as laparoscopes, colonoscopes, and bronchoscopes permit the physician to look into the channels of various anatomic tubes (e.g., the gastrointestinal (GI) tract, the genito-urinary (GU) system, the bronchial system) and, when used percutaneously with appropriate measures such as insufflation, into body cavities such as the abdomen. These tools provide high resolution and natural color but, unlike the diagnostic imaging modalities, they provide surface visualization only and cannot indicate how deep a lesion extends below the surface of the organ, nor exactly where in the lumen the field of view is located in relation to the rest of the anatomy.
The marriage of endoscopic surgery with on-line MR guidance promises to provide a significant improvement in the ability of the surgeon or interventional radiologist to understand the operating field and its anatomic context.
12.1.5 Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) has specific advantages, over and above its lack of ionizing radiation, for guiding, monitoring, and controlling minimally invasive diagnostic and therapeutic interventions, including the following:
· Superb tissue discrimination, enabling excellent discrimination between various organs, including blood vessels, nerves, and solid organswithout administration of intravenous contrast agents;
· Superior definition of the number and extent of lesions and discrimination between the lesions and normal tissue for optimal definition of the target for therapy;
· Best indication of anatomic context of surrounding blood vessels and nerves to provide accurate trajectory definition;
3Endoscopic techniques use fiber optics to examine the interior of a hollow organ, such as the bladder or stomach.
· Spatial resolution that extends into the submillimeter range;
· Direct acquisition of multiplanar and three-dimensional volume images;
· Image updates in well under 1 second with the new rapid image acquisition protocols, including echo-planar imaging (EPI), fast spin echo imaging, and gradient echo imaging, and various hybrid pulse sequences, enabling a close-to-real-time or real-time viewing of physiological motions and the changes induced by the interventional procedures; and
· Ability to characterize functional and physiological parameters of the treated tissues, such as diffusion, perfusion, flow, and temperature.
As a result of these characteristics, MRI can depict the tissue damage induced by various tissue ablative methods4and therefore has the unparalleled potential to not only monitor but also control interventional or minimally invasive surgical procedures.
MRI has potential appeal for most of the minimal access therapy approaches. Intravascular procedures can be significantly improved by imaging vessel walls and the associated pathologies with endoluminal coils. Vascular and endoluminal catheterization can be effectively guided within MRI systems using special localizing coils attached to the catheters.
Angioplasty and atherectomy devices (such as lasers) for vascular applications can be applied more effectively and more safely with on-line imaging control. Laparoscopic and endoscopic vision is limited to a relatively small surface region. This restricted direct visibility can be complemented by obtaining a larger volume to visualize beyond the operational field and below the surfaces without increasing the risk posed by the procedure.
An obvious role of MRI is in monitoring and controlling a variety of methods of interstitial ablative methods, including thermal therapy. In these applications no direct visual inspection is possible since the minimally invasive probe (needle, optical fiber, cryoprobe, RF needles, and so on) is introduced percutaneously or applied by a non-invasive, non-incisional method (e.g., focused ultrasound).
The major disadvantages of MRI in this application are its relatively high cost compared to other imaging modalities, the special environment it creates and requires because of its magnetic field and RF properties, and patient inaccessibility during an examination. In particular, the tools, instruments,
4Ablative methods include a variety of means of excising or destroying interstitial tissue.
and accessories used in the procedure room must be MR compatible and in many, but not all, cases this may require a redesign of the equipment.
12.2 The Roles of Imaging in Therapy
Underlying the use of imaging in therapy is a simple scientific concept that can be implemented at different levels of complexity using various imaging modalities from x-ray radiography to ultrasound, CT, or MRI. This concept is that information obtained from imaging devices can be used to construct ''models" of a patient's particular anatomy that can then be compared with models of normal anatomy and with the executional model in which the actual procedure and its components are described. These models are then projected back to the patient and registered within a single reference frame. The resulting integrated and superimposed models assist the physician who performs an interventional procedure.
In providing better information to the physician to make interventions more effective, imaging can contribute in three key areasplanning, guidance, and monitoring (including localization). In addition, MRI specifically provides image data, which can be used to control the interventional procedure.
Previously acquired CT and MR images (i.e., those not obtained at the time of a procedure) are being used increasingly for radiation therapy and for surgical planning. The requirements for planning differ somewhat from those for diagnostic imaging. The image sources should be high-quality, high-resolution tomographic images. If sufficient thin-slice contiguous image planes are acquired, they can then be reformatted into other planes or synthesized into three-dimensional renderings of the anatomic information in the images. The different tissues and organs within the images have been previously filtered or segmented appropriately to delineate the target and surrounding structures prior to the creation of the three-dimensional volume and surface renderings so that organs or tissues can be better distinguished and analyzed. At this point appropriate analytic and synthetic operations of trajectory simulation or planning, of probe guidance, and of surgical planning are carried out on the three-dimensional images thus created. Images from various sources, particularly MRI or CT volume rendered images, can
be put into register with each other and with real-time video of the patient, specifically for use in guidance during the image-directed procedure.
With the three-dimensional volume rendered image in hand, the physician is now able to create an appropriate surgical plan. Such a plan might delineate the safest and most appropriate route for gaining access to diseased tissue in an organ. It might involve the creation of an appropriate normal template for correcting an anatomic abnormality. Or it might be a suitable road map or series of road maps to guide the repair of a specific organ or anatomic region. Clearly the specifics of the plan will depend on the particular intervention under consideration, for example, the guidance of a biopsy needle to a diseased organ or tissue target or the delivery of a therapeutic dose of a pharmacological agent or heat energy. A range of these interventions is examined below.
Once the surgical plan has been created, the next step is to use it to guide the surgery. The interventional or therapeutic tool (a biopsy needle in the simplest case) can be guided in two ways, depending on the imaging capabilities. The preferred method is on-line guidance with rapid update of the images during the procedure to accurately monitor the position of a probe within the anatomy. Fluoroscopy- or ultrasound-guided biopsies are performed routinely and safely. However, in some cases only MRI can provide appropriate target definition and/or anatomical detail for defining the correct trajectory. Real-time MRI-guided biopsies are only now becoming clinically feasible with the advent of MRI systems with appropriately open magnets capable of rapid image update.
Coordinates and trajectories for biopsies or other interventional procedures (such as abscess drainage and nephrostomy) can be obtained from previously derived images. This information remains valid if the patient does not move between the time of imaging and the procedure. Using fiducial marks and measurements,5 a "free hand" biopsy can be performed with the guidance of a cross-sectional imaging system. This category of MRI-guided biopsies has been used clinically with the aid of MRI compatible needles and appropriately visible markers.
"Dead-reckoning" navigational methods are more feasible for precise biopsies and for minimally invasive surgical procedures. The stereotactic tech-
5That is, those regarded as or employed as a standard of reference, as in surveying.
niques employ acquired images with stereotactic frames or without them (frameless stereotaxy). In neurosurgery, various rigid frames are fixed to the skull to facilitate transfer of the correct coordinates from one reference system to another. Using this system, imaging data (plain film, CT, or MR) can be transferred to the operating room when the actual procedure is executed. New developments attempt to eliminate the frame while keeping track of the patient's position and the interventional tool, and relating these simply, flexibly, and relatively inexpensively to the image coordinate system. The tools (e.g., biopsy needles) can be attached to a mechanical arm with optical or electromagnetic sensors that track the movement of the tool. The image data are then registered to the patient's anatomy so that trajectories for biopsies can be defined from images or, conversely, so that the position of the interventional tool within the patient can be depicted on the displayed images. While both types of stereotactic techniques are used in neurosurgery, only frameless stereotactic methods are feasible for other parts of the body.
There are three important requirements for a guidance system to ensure that it can be used appropriately in a wide variety of interventional situations.
· Immobilization. It is often necessary to immobilize the anatomy under consideration, especially when dead-reckoning navigational guidance is used, so that the position of the target remains invariant with respect to the fiducials between the imaging process and the subsequent interventional process. The immobilization tools are different for different parts of the body. They are straightforward for the head, less so but still simple for the limbs, and more difficult for the breast. Targets such as the liver and other internal organs affected by respiratory and cardiac motion can be only partially immobilized, and further development of appropriate techniques may be necessary.
· Registration. Registration permits fusion of images in such a way that the coordinate systems of the two images are coincident. A preferred approach would eliminate the frame or other obtrusive fiducial system. Frameless stereotaxy, a proprietary method of registration without frames, permits registration of the patient coordinate system with the three-dimensional patient model or the surgical planning model. This method utilizes a video image or a laser-scanner defined surface area of the patient made during the procedure with the three-dimensional model created in the planning phase from the
prospectively acquired images. Such registration techniques not only are useful in the operating theater, but also can be used preoperatively in the planning stage to allow the physician to draw the surgical plan directly on the patient's anatomy for accurate location of the entry point and projected positions of key structures. Video registration with the three-dimensional model has been used successfully in neurosurgery and can be extended to other parts of the body.
· Tracking. A tracking method specifies the position of surgical tools or other instruments during the operation with respect to the image and patient coordinate systems, and also can be used to follow the motion of the patient's body parts, all concurrently. Probably the most versatile tracking system is optical, although the most widely used system is the ISG wand, which uses a mechanical "arm" with encoders on the joints to keep track of the position of the tip. A prototype optical system using the Pixsys FlashpointTM system is being developed for a vertical gap open interventional MRI system and could be adapted to other modalities. This system consists basically of light-emitting diodes mounted on instruments whose positions can be determined accurately by triangulation via a number of line scan video cameras mounted in the room.
12.2.3 Monitoring and Localization
On-line, real-time imaging of the delivery of therapy or of an interventional procedure allows the physician to watch his or her progress. X-ray fluoroscopy has been the modality most used for vascular interventions and for cardiac catheterizations. For needle biopsies and some other interventional procedures, real-time ultrasound represents a viable alternative.
It is only now, with MRI technology being able to acquire the data to create images in less than 1 second, that MRI can be used to monitor and localize probes on-line within the target. In addition, an open magnet configuration is necessary to provide the physician with the access to interact with the patient during an operation. Such magnet configurations with sufficient field strength, appropriate imaging pulse sequences, and vertical gaps affording direct access by the physician to the patient are now becoming available for clinical evaluations.
Immediate visualization of the physiological effect of an intervention, such as increased blood flow, temperature change, or perfusion change, provides the physician the potential for sophisticated control of the procedure. This is of particular importance when the physiological effect is a temperature change and care must be taken to ensure that the temperature change is restricted to a specific target volume and is not too great in any specific anatomic area, and that the spatial localization of the temperature change is appropriate.
Conventional surgery uses direct visual control and eye-hand coordination, but this approach has significant drawbacks. Dissection exposes surfaces only, and so the surgeon cannot see effects below the surface and has to approach the target, dissecting carefully layer by layer. This limitation of direct visual control confined the use of surgical lasers to relatively low penetration and doomed the use of interstitial laser therapy, cryosurgery, and focused ultrasound surgery until appropriate imaging methods, combined with the capability to control energy deposition, were developed.
Direct visualization by the naked eye as is done with endoscopic, laparoscopic, or microscopic techniques utilizes color, texture, and other visual clues to attempt to distinguish normal from abnormal tissues. However, these indicators are often insufficient for accurate characterization. Nor can such modalities permit visualization of the extent of a tumor below the exposed surface or allow the effects of interventions below the surface to be followed.
As detailed above, subsurface visualization is a particular strength of MRI. Specifically, MRI, with pulse sequences that are quite sensitive to temperature changes, can provide real-time, three-dimensional, high-resolution maps of temperature changes during energy deposition or abstraction and may be able to signal when tissue proteins are denatured (coagulation) or frozen by phase changes in the image. Most important, the temperature sensitivity of MRI is sufficient to show thermal changes before irreversible tissue damage occurs. This capability allows the operator to modify the amount or rate of deposited energy in time to restrict irreversible changes in the target volume. The detection of temperature changes below the threshold of tissue damage is a great advantage of MRI over ultrasound monitoring of thermal therapy. Ultrasound monitoring is based either on the creation of "bubbles" during tissue coagulation (an adverse result for many thermal therapy procedures) or on the distinction between normal and irreversibly coagulated tissue, whereas MRI can distinguish heated tissue volumes from
12.3 Thermal Surgery
Image-guided interstitial thermal therapy or surgery is the process of optimally defining a target volume using diagnostic imaging techniques and then destroying all the tissue cells within the target volume by inducing a localized temperature change in the target volume only. Thermal therapeutic techniques use heat or freezing (cryosurgery) to induce irreversible tissue ablation by denaturing the proteins or by destroying cells with ice crystals. For control of the energy deposition, either multiple temperature probes or specialized temperature-sensitive imaging methods (particularly MRI) are employed.
Localized thermal therapy or "heat surgery" is different from classical hyperthermia treatment. Conventional hyperthermia treatment induces a relatively small temperature elevation (to ~ 41-42°C) within relatively large tissue volumes in order to exploit the differential sensitivity of neoplastic cells over normal tissue via an otherwise not well understood heat-related damage mechanism. MRI guidance for conventional hyperthermia treatment is feasible and has been tested clinically.
The physical and biological consequences of localized high temperature "thermal therapy" have been experienced by all who have inadvertently burned themselves and are well understood, clear, and not controversial. In the ideal thermal therapy procedure the targeted tissue volume is heated beyond 57-60°C, which is the threshold for protein denaturing. This treatment results in irreversible cell damage in both normal and neoplastic tissues. For this reason, localized thermal therapy is more comparable to surgery than to hyperthermia treatment. The intent is to maintain a tight target volume during the procedure. The applied temperature therefore should be localized exclusively on the target, should not exceed 100°C, and should be applied in relatively short energy pulses to reduce thermal diffusion and minimize heat removal by blood flow and/or perfusion.
Thermal surgery has not been used extensively in the past because of the lack of good imaging guidance and monitoring and because there were not ways to create on-line, high-resolution, three-dimensional temperature maps of the volume of interest for monitoring or control. MRI is sensitive to temperature changes through the temperature sensitivity of the T 1relaxation time, the diffusion coefficient, or the chemical shift parameters, and recent
advances have made it possible to obtain MR images in less than 1 second, thus making it feasible to obtain and update three-dimensional temperature change "maps" of the tissue under consideration in times matched to the temporal resolution of the thermal changes so as to avoid artifacts. This rapidity allows MRI to be used to guide, monitor, and control the heat surgery.
12.3.1 Interstitial Laser Therapy
Delivery of thermal therapy can be percutaneous or non-incisional. Percutaneous delivery via optical fibers coupled to lasers is a promising application for tumors accessible via needle insertion and has been widely used and reported on. MRI monitoring of interstitial laser therapy has been suggested, and animal studies demonstrating its feasibility have been carried out. Initial clinical applications for interstitial laser therapy include stereotactic brain tumor surgery and liver surgery, and MRI localization and guidance without imaging during laser treatment have been demonstrated in a head-neck procedure. However, laser-induced thermal surgery has the drawback of a relatively high temperature gradient at the tip of the optical fiber. The tissue at the tip is heated well above 100°C, creating vapor and "smoke" and, more importantly, a non-uniform thermal lesion through the treated volume. Furthermore, the treatable volume is limited, and it is not normally matched to the target shape without moving the fiber and using multiple probes or multiple needle penetrations. Computerized control of this procedure has been suggested but has not yet been implemented because significant progress must be made in understanding the structure and biology of the target tissues, as well as their optical and thermal properties and how they change during irradiation, before laser-induced thermal surgery can be fully automated.
Freezing causes cell destruction through the development of ice crystals, which gives cryotherapy some advantages over heat surgery because the collagen structure of the tissues is not destroyed, vessel walls are preserved, and reinnervation is possible. Current cryosurgical procedures are open (liver) or percutaneous (prostate) and require imaging guidance and monitoring. Ultrasound is used to indicate the expanding freezing zone but can show only the proximal freezing boundary, so that the shape of the remote boundary
must be inferred from symmetry assumptions. These assumptions are not very accurate, particularly in patients with fibrotic tissue, such as those who have undergone radiation therapy.
Recent technological developments in the cryoprobes, which have reduced their diameter and made them MRI compatible, have made it possible to monitor cryotherapy by MRI. The lack of signal within the frozen zone sharply delineates the treated area, and the expansion of the freezing zone is well manifested on "real-time" MRI. The open magnet configuration should make cryosurgery under MRI control feasible for a wide variety of new procedures.
12.3.3 Focused Ultrasound
Focused ultrasound (FUS) evolved from lithotripsy and has affinities to localized hyperthermia techniques pioneered in the 1940s and 1950s. FUS uses an ultrasound transducer to create a point source of heat at its focus. High-aperture ultrasound transducers can create a converging beam focused to a high-intensity zone. Within this focal volume various quantities of thermal energy can be deposited without any obvious damage to the surrounding tissue. The point source of heat can be made sufficiently hot to denature protein in a cigar-like cylindrical shape varying between approximately 1-3 mm in diameter and 2-5 mm in length by "instantaneously" (under 1 s) raising the temperature to between 70°C and 100°C. In order to contain the heat in a well-defined target zone and reduce the effect of thermal diffusion, the heat is delivered in short high-energy pulses. This method results in a sharp temperature drop-off to normal body temperature outside the focal spot and so improves the control of temperature. Blood flow in the target vicinity plays a bigger role in carrying away the heat than does diffusion through the surrounding tissue.
However, experimentation in appropriate tissue and animal models will be required to better understand the most appropriate pulse amplitude and timing protocol. A key determinant will be the issue of cavitation. A high-intensity short-duration pulse may be more likely to create cavitation. Longer-duration, lower-amplitude pulses may also reduce the effect of surface heating, which may become one of the limitations in FUS.
Although this technology has been available for decades, the lack of appropriate localization and temperature-monitoring techniques has made it difficult to achieve clinically useful applications. Currently, ultrasound monitoring is used to localize and target the focused beam and to detect ther-
mally induced irreversible tissue damage.
MRI has been suggested as the optimal technique for spatially localizing, targeting, and controlling heat deposition and has been tested in animal experiments using MRI-compatible transducers positioned within the MRI patient table. The focus can be translated in three dimensions so as to enable the point source of heat to be moved through the target zone. Unlike percutaneous methods, MRI allows scanning of an extended non-spherical target without resorting to multiple punctures or an array of needles. Scanning of the focal spot of heat is computer controlled by "operating" on the MR image of the region under consideration in the patient. Although there are different ways of implementing the scan program, the basic idea is to move, on the MR image, a computer cursor that controls the positioning of the point source of heat through the target region in a sequenced program that ensures that only the desired volume is totally ablated. A test pulse of heat can first be introduced to ensure that the heat is deposited in the appropriate place in the target. This initial pulse of heat or "tracer shot" can be low enough that its effect is reversible without any permanent damage to the tissue. Upon assurance of proper targeting, the system can be switched to therapeutic mode and the target zone scanned. The deposition of heat is monitored by rapid MRI, and the effects are immediately visible on the image. This feedback enables control of the process to avoid damage to normal structures.
The key benefit of FUS is that it is non-incisional and as such has many of the characteristics of an ideal surgical tool (see Table 12.1). It destroys the target and only the target with no entry track of injury. The results are reproducible and predictable because the process can be very accurately controlled. The method results in nonhemorrhagic, sharply demarcated lesions. The surgical effect is localized, is instantaneously visible in the MR image, has no delayed or systemic effects, and is repeatable without additional risk to the patient. Thus, in addition to enabling improved outcomes, MRI-guided FUS should reduce recuperation time, infections, and morbidity.
Because ultrasound beams are blocked by air and bone, treatment of different anatomic locations will likely require different transducer shapes and application methods to optimize the treatment. Ultrasound transducers for neurosurgical procedures or for use with the thyroid will differ from those for the breast or lower abdomen. Treating the upper lobe of a liver may require a flexible transducer head that can be positioned beneath the rib cage pointing cranially. Likewise, transducers may have to be placed in multiple positions on a patient, and hence an MRI system that provides more
Table 12.1. Comparison of Focused Ultrasound with an Ideal Surgical Tool
open access and greater flexibility will be needed. Treatment of the prostate, for example, may require two transducers, one placed externally and one inserted in the rectum, somewhat analogous to the use of the combination phased array coil with insertable rectal coil. The requirements of flexible placement of transducers and simultaneous application of MRI and FUS pose significant design challenges. Realizing the full potential of FUS will require an open MRI system that permits the expanded flexibility required to perform a wide variety of procedures.
FUS systems are under clinical investigation primarily for use with benign prostatic disease and in the kidney and bladder using ultrasound guidance. An MRI-guided FUS system for treating breast tumors has been developed for clinical testing. The clinical potential of MRI-guided FUS is so significant that rapid development of the technique should be encouraged.
12.4 Research and Development Opportunities
· Definition of tumor and other surgical target margins or boundaries utilizing various medical imaging techniques (correlating with spatially registered histology to estimate the capabilities of the various imaging modalities in defining the boundaries for various anatomic regions).
· Development of real-time image-processing techniques, particularly rapid methods of model creation, three-dimensional rendering, and accurate segmentation of anatomic tissues for various imaging modalities.
· Research in the area of surgical planning and simulation, particularly trajectory planning for needle biopsy, its basic surgical application today.
· Improvement, via more complex automated technologies, of current registration or image fusion methods of different medical imaging modalities and more particularly of video-based and laser-scanning techniques with prospectively created models.
Guidance and Localization
· Development of flexible and untethered sensors to provide anatomic fiducial marks or information on the position of needles, catheters, and
surgical instruments, for tracking of instruments or for fusing patient and image coordinate systems. * Development of computational systems and algorithms to enable ''instantaneous" reconstruction, reformation, and display of the image data so as to enable real-time following of a physician's actions during a procedure (e.g., advancing a catheter or needle).
· Development of methods to update prospectively created models using real-time imaging information acquired during the interventional procedure and reflecting changes effected by the intervention. Real-time image processing and therefore high-performance computing are necessary not only to guide tools to the target but also to monitor and display the changes occurring within image volumes.
· Development of various display technologies, particularly virtual reality and three-dimensional or stereoscopic displays, to provide appropriate interfaces that enable the physician to make the best use of the data. It is expected that these technologies will advance rapidly over the next few years, but driven by other commercial products, and so they will have to be adapted to provide appropriate temporal and spatial resolution suitable for surgical and interventional applications. Such developments are required for monitoring and control tasks as well.
· Definition of the temporal resolution required for various image-guided therapeutic procedures, taking into consideration the physical characteristics of the specific imaging modalities and the dynamic properties of the monitored procedures, specifically for multislice volumetric monitoring.
· For MRI, development of new pulse sequences specifically for therapy applications rather than diagnostic applications. A particularly important need is the development of highly temperature-sensitive pulse sequences to enable monitoring of "heat surgery."
· Investigations to correlate the factors affecting energy deposition or abstraction (e.g., pulse duration, pulse energy, and power spectrum) with histological and physiological changes in the tissue and resulting image changes, for the purpose of determining mechanisms of thermal damage and the biophysical changes that take place during various thermal surgical procedures such as interstitial laser therapy, cryoablation, and high-intensity focused ultrasound treatment. Such investigations need to be done for various anatomic regions and medical conditions for which such therapy might be appropriate.
· Investigation of the range of medical conditions amenable to treatment with minimally invasive techniques made possible by expanded capabilities for visualization during a procedure via the various medical imaging modalities.
Instruments and Systems
· Although prototypical MRI systems have been created that provide direct and easy access to the patient, more research and development is required to optimize further the geometric configuration of these systems. Similar requirements are appropriate for the other imaging modalities, particularly CT.
· Development of less expensive two-dimensional detector arrays for CT and other x-ray imaging modalities, and of less expensive two-dimensional transducer arrays for ultrasound, along with appropriate means for acquiring, reconstructing, and displaying the data. * Improved methods of inexpensively shielding the magnetic field to enable inexpensive retrofitting of existing MRI systems into current operating rooms.
· Integration of imaging methods with therapeutic procedures, including feedback systems between data display devices and image information, computer-assisted image-controlled surgical tools, robotic arms, and instruments.
· Creation and development of new instruments and tools to accomplish new tasks enabled by the availability of image-guided therapy.
Compatibility of various interventional and surgical tools and imaging systems is important, especially in the case of MRI-guided therapy.
12.5 Suggested Reading
1. Cope, C., ed., Current Techniques in Interventional Radiology, 2nd Edition, Current Medicine, Philadelphia, 1995.
2. Jolesz, F.A., and Blumenfeld, S.M., Interventional use of magnetic resonance imaging, Magn. Reson. Q. 10 (1994), 85-96.
3. Kadir, S., Current Practice of Interventional Radiology, B.C. Decker, Inc., Philadelphia, 1991.
4. Kandarpa, K., and Aruny, J.E., eds., Handbook of Interventional Radiologic Procedures, 2nd Edition, Little Brown, Boston, 1995.
5. Kim, D.S., in Peripheral Vascular Interventions, Michael D. Darcy, ed., Society of Cardiovascular and Interventional Radiology, Reston, Va., 1994.
6. Seibel, R.M.M., and Gronemeyer, D.H.W., eds., Interventional Computed Tomography, Blackwell Scientific Publications, Boston, 1990.
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