Three-D ultrasound evolves in interventional radiology

True or near-real-time imaging guidance is a crucial element of interventional radiology. Although CT and MR have provided interventionalists with new imaging tools and alternatives, neither can operate in real-time, and fluoroscopy's radiation dose remains a concern. Two-D ultrasound, on the other hand, has always occupied a place in the interventional radiologist's armamentarium, although hardly a preferential one.

In recent years, IR researchers have been investigating several applications for 3D and 4D sonography. As specialists anticipate an increase in minimally invasive and noninvasive treatments for solid organ tumors and other conditions, the future looks promising for 3D/4D ultrasound in interventional procedure planning, guidance, and monitoring.

IRs need real-time imaging to localize tumors and perform biopsies, ablations, and other procedures. Enhanced imaging with 3D ultrasound represents a rich area of development in this setting, said Dr. Ziv Haskal, a professor of vascular and interventional radiology at Columbia University.

GUIDING RFA

Radio-frequency ablation has evolved from a palliative treatment for unresectable liver tumors to a serious alternative to surgery. And as RFA's popularity continues to grow in the U.S., so do the means to make it more effective.

Until now, 2D ultrasound has been reliable and useful for placement of RFA needles and monitoring ablation results. But anatomy restricts 2D's already limited one-plane acoustic window, particularly in this setting, and requires a sometimes awkward placing of the transducer over the abdomen and through the rib cage. Even when visualization is possible, the risk of misplacing the RFA probe remains.

Successful guidance depends on adequate ultrasonographic visualization of the needle. Three-D's multiplanar perspective provides information on the relationship between complex anatomic structures while visualizing the probe's accurate navigation and targeting of a tumor from any direction. Real-time 4D allows a steady display of the volume and increases the visibility of the biopsy needle regardless of the transducer's movement, as long as the needle stays within the volume being scanned, said Dr. Byung Ihn Choi, radiology chair at Seoul National University Hospital in Korea.

Three-D also boosts operator confidence, according to Choi. During treatment of deep-seated small lesions, for example, longer penetration improves maneuverability of the needle. In addition, 3D displays the needle as slightly thicker than it normally is, enhancing its conspicuity.

In one of the first studies of its kind, researchers at the University of California, San Diego used 3D ultrasound to optimize RFA, percutaneous ethanol injection, and cryoablation guidance in 16 patients with a total of 23 liver tumors. They found that, compared with 2D, 3D ultrasound provided additional information that improved both placement and distribution of ablative agents to treat focal liver malignancy in 91% of patients. It helped readjust ablation devices in 45% of the procedures and overruled 2D ultrasound's orientation to a tumor in 14% of cases (Rose et al, J Vasc Interv Radiol 2001;12:507-515).

Many interventionalists use ultrasound first to determine the correct positioning of their probes and then move patients to CT to confirm positioning or for guidance. Using 4D ultrasound might simplify the procedure and make it cheaper, faster, and safer by reducing the need for CT, said Dr. Daniel B. Brown, an assistant professor of radiology and surgery at the Mallinckrodt Institute of Radiology.

GUIDING TIPS

Patients with elevated portal vein pressure, usually due to liver disease such as cirrhosis, face an increasing risk of bleeding from esophageal and gastric varices. Transjugular intrahepatic portosystemic shunting (TIPS) forms a bridge or shunt between the portal and hepatic veins, helping to lower blood pressure and avoid or control bleeding.

One of the most complicated and dangerous steps in TIPS is achieving portal vein access. Three-D ultrasound can accurately and safely define the point of access and can also identify whether the access is intra- or extrahepatic, an important distinction when choosing the type of shunt used for the procedure.

TIPS is usually performed with fluoroscopic guidance, but visualization may be incomplete or confusing. Three-D ultrasound can reproduce planes of view that help spot the correct vein access with complete confidence, said Dr. Steven C. Rose, an IR at UCSD.

"With 3D ultrasound, even before I put my devices in, I can look at the angular relationship of the portal vein bifurcation and the origin of the right hepatic vein so that I can tell whether the curve on my canula is adequately steep to be able to pass my needle from the right hepatic vein into the portal vein bifurcation area," Rose said.

Three-D ultrasound has dramatically reduced the number of steps required to perform the TIPS, reducing procedural time, radiation, and most important, the risk of bleeding complications, he said.

In his latest study, Rose and colleagues performed TIPS in 11 patients with liver cirrhosis, using 3D ultrasound to determine the correct entry point in the main portal vein. They found that 3D can confidently determine the safest portal vein entry site at a point that remains sealed by the surrounding liver tissue (J Vasc Interv Radiol 2002;13:267-273).

PLANNING DRAINAGES

Another interventional procedure in which 3D can be useful is percutaneous drainage of abdominal abscesses and other complicated fluid collections. While CT and 2D sonography are considered the imaging standards of care in this setting, both have limitations and can provide only a partial understanding of the anatomy they explore.

With the aid of either transvaginal or transrectal probes, 3D eases the drainage of complicated fluid collections and achieves success rates similar to those for drainage of simple fluid collections, which usually require just a 2D scanner for guidance, Rose said.

"Three-D guides you from one collection to the other and helps you drain fluid in the most thorough and safest way possible," he said.

In a study assessing 46 fluid collections performed in 26 patients, Rose and colleagues found that 3D ultrasound provided relevant information on the structure and spatial relationships of the targeted fluid collections and the anatomy where they originated. It also contributed useful information that led to drainage in 91% of cases. Finally, it helped select drainage access direction in 95%, determine a single access for multiple drainages in 81%, and provide curative drainage in 77% of patients (J Vasc Interv Radiol 2003;14:451-459).

Despite several proven advantages, 3D ultrasound faces a number of challenges before it is widely adopted by interventionalists. It requires image processing time, which prolongs the procedure, and it is still highly dependent on the operator's skills. In addition, it needs specialized software and probes, and the latter are usually bulkier and heavier than conventional 2D probes.

Most 3D disadvantages, however, can be easily overcome with 4D, which provides real-time reconstruction with minimal operation. Technical innovations such as slimmer and lighter transducers might eliminate these inconveniences in the near future, Choi said.

Many specialists agree that 3D/4D ultrasound demands a different set of reading and interpreting skills than other modalities, with a possibly steep learning curve. It may require development of a comprehensive standard terminology that helps physicians describe what they see and communicate it to patients and peers in the interventional suite or operation room.

Relatively few IRs have expressed interest in learning to use this technique. For Rose and others, however, 3D makes a major contribution in teaching interventionalists to handle volumetric data sets, interpret anatomy in a more realistic way, and move into other arenas where volume visualization is also applicable.

"There is a high-risk/high-gain kind of scenario in which I think the additional effort of doing a 3D ultrasound probably pays off," Rose said.

The final factor may be cost. The equipment is still too expensive and is difficult for IRs to justify. If more equipment became available through government or industry programs, specialists might be able to prove the technology's utility and useful applications, Haskal said.

"I'd love it if Acuson or GE gave me one of these for six months. I'd figure out what to do with it," he said.