When minimally invasive brain surgery was being invented, pioneering doctors like Minneapolis' David Tubman would sometimes slice off the ends of the flexible tubes used in heart surgery and use them in operations to fix potentially catastrophic problems in the brain.

"Honest to God, you should see what we used to use," recalled Tubman, who performed some of Minnesota's first minimally invasive brain-aneurysm repairs in the 1990s, in clinical trials at the University of Minnesota and later in regular practice at Abbott Northwestern Hospital. "It's come a long way since then, there's no doubt about that."

The skinny tools that can reach deep into the brain to treat aneurysms and strokes without cutting through the skull have advanced far enough to spur new medical specialties known as endovascular neurosurgery and interventional neuroradiology, whose practitioners wield devices backed by clinical evidence and a robust commercial market.

The market for minimally invasive tools and equipment to treat brain problems like strokes and aneurysms is worth more than $500 million in the United States today and is likely to surpass $1 billion by 2025, according to estimates from analyst Beata Blachuta with Decision Resources Group. Those estimates were considered conservative by another source.

Industry analysts say the dominant player in the neurovascular-device market is Minnesota-run Medtronic PLC, by virtue of its 2015 acquisition of industry forerunner Covidien, which had neurointerventional devices once owned by Plymouth-based firm ev3. Stacey Pugh, a registered nurse and general manager of Medtronic Neurovascular, said periodic acquisitions by larger and larger companies show the clear market appetite for the technology.

"We're just beginning to scratch the surface I think of what is possible in terms of endovascular medicine," Pugh said. "Making these devices smaller, softer, capable of being delivered in more distal regions of the brain without complication, I think we'll find ourselves in a position where you're going to get well above 90 percent of aneurysms being able to be treated without someone having to have their skull opened."

Today about 70 percent of brain aneurysms are treated with skinny tubes inserted in blood vessels lower in the body and then advanced through the blood vessels until they reach the brain. Now similar types of tools are being used to reach stroke-causing blood clots as well, but the process of rolling out that technology has been slower than for aneurysms — only about 1 in 10 stroke patients who would benefit from the highly recommended therapy get it.

Minimally invasive endovascular neurosurgical tools are built on the idea that it's possible to perform brain surgery using only surgical tools that are inserted through a small incision in the groin. The doctor uses live-motion X-rays to guide the tools to targets just a few millimeters wide in the brain. Doing it this way allows the surgeon to fix a problem from inside the vessel, avoiding the need to cut into skull to reach the exterior of the vessel.

Terri Hopp, 63, of Litchfield, is one of the people who feels she has benefited.

Earlier this month, Dr. Josser Delgado repaired Hopp's non-ruptured aneurysm at Abbott Northwestern by placing four metal coils inside the 5-millimeter balloon of weakened tissue on the left side of Hopp's internal carotid artery terminus, a major vessel in the brain. Over time, the metal coils will allow Hopp's body to form scar tissue that seals up the bulging aneurysm, greatly reducing the risk that it will burst and suddenly kill her one day.

"I'm back to work and everything's good," Hopp said Wednesday. "I'm not nervous and scared that it's going to rupture any minute."

Minimally invasive aneurysm repairs like the one Hopp received were one of the first applications of endovascular surgical tools for the brain. Blachuta estimates that minimally invasive aneurysm-repair tools have annual U.S. sales of more than $500 million.

The larger opportunity, for patients and investors alike, is the related field of ischemic stroke treatment.

Blachuta estimates that current U.S. sales of $190 million for endovascular stroke devices could grow to $450 million by the year 2025. But that growth is dependent on increasing the number of high-volume hospitals with the adequate training, and also cutting through the legal red tape that all too often requires ambulances to bring stroke victims to hospitals that can't do emergency endovascular brain surgery.

The American Heart Association says that 87 percent of all strokes are ischemic. But the device companies invented and validated treatments for hemorrhagic events much earlier, partly because it was more difficult to do studies proving medical devices can be as effective in treating ischemic stroke in a medical emergency.

"As it turns out, it works. And it works well. But it was significantly later, despite the fact that [endovascular ischemic stroke treatment] was a six- or seven-times bigger market," said Thom Gunderson, a retired analyst who spent more than 20 years studying medical technology at Piper Jaffray.

Previously, patients who'd had a major ischemic stroke were treated with a clot-busting drug known as tPA, but the drug doesn't always break up the larger clots. A series of five clinical trials published in 2015 culminated in a Medtronic-funded meta-analysis in the Journal of the American Medical Association last fall that found clear benefits to using minimally invasive tools to retrieve clots in addition to using tPA during the critical hours after a major-vessel blockage.

The analysis of 1,287 patients' data found that using tPA plus a device to retrieve the clot was associated with lower degrees of disability at three months compared to use of tPA alone for large-vessel ischemic strokes. The benefit was greatest if the clot was retrieved within two hours of the stroke; the difference became insignificant after seven hours.

Based on such data, the American Heart Association now gives the use of clot-retrievers in ischemic stroke its strongest recommendation.

"The challenge that you had for many of these studies was that the efficacy — the benefit — was so big that the trials were stopped with fairly small numbers, because there was just overwhelming benefit," Medtronic's Pugh said.

The bill to treat a stroke with tiny medical devices plus medication is higher than using tPA alone. The roughly $20,000 cost of an emergency clot-retrieval following large-vessel stroke typically includes $6,000 for the clot-retriever itself, which looks like a heart stent on the end of flexible straw. The hospital bill will include capital equipment costs for the expensive motion X-ray system that allows endovascular medicine to be possible.

But considering that strokes account for 1 in 20 U.S. deaths and are a leading cause of costly lifelong disabilities, minimally invasive devices to treat strokes and aneurysms have an easier time getting covered by insurers than other medical technologies.

"Strokes are incredibly expensive," Gunderson said. "If you can reduce the effect, or prevent having a stroke, those technologies are no-brainers when it comes to the question of who's going to pay for it. Payers will do a lot to keep a stroke from happening, or happening as hard as it could if you just leave it untreated."