It's frightful every time. And it happens a few times a year. A pitcher is struck in the head by a line drive.

Last Friday it happened to Robbie Ray of the Arizona Diamondbacks after a ball left the bat of the Cardinals' Luke Voit at 108 mph. The left-hander received stitches and went on the seven-day concussion disabled list, but it was thought early in the week that he might pitch in the team's weekend series in San Francisco. Midweek, however, Ray was sent back to Phoenix from a road trip for further testing.

The overwhelming majority of major league pitchers, presumably including Ray, wear no special head protection, just a standard, official-issue ball cap.

But several St. Louis relievers are apparently wearing safety inserts in their caps to try to minimize potential head injuries if they're struck by a liner. On Monday KSDK-TV cited the Cardinals' Matt Bowman, Zach Duke and Trevor Rosenthal in a story on head guards and Bowman said in a Fox Sports Midwest interview on Wednesday, "I'd say there are probably five of us now that have started to use it and there was a big wave of guys who ordered the insert after the Robbie Ray incident."

The insert is what Astros starter Collin McHugh has worn for years -- the Safer Sports Technologies Pro Performance head guard. And, as ESPN's Outside the Lines reported this spring, the Angels' Matt Shoemaker started wearing it in a comeback from a skull fracture and blood clot in his brain that he suffered when a liner hit him on the right side of the head last September.

Shoemaker, who required emergency brain surgery, is believed to be the only pitcher now wearing a head guard after being struck. He told OTL this week: "It's pretty simple, it's comfortable, and I don't even know it's there when I'm wearing it -- and that's what I want."

The contoured SST carbon-fiber composite partial insert -- worn inside the right side of a standard cap by righties and inside the left side by lefties -- weighs about 1.7 ounces and doesn't noticeably affect the cap's appearance. In nearly all episodes when a pitcher gets hit in the head, it's on the throwing side, since that's what is exposed by the follow-through.

In 2014 Dan Jennings, now of the Tampa Bay Rays, was also hit in the head by a liner and later started wearing the SST head guard, but he told OTL he stopped wearing it after getting into a pitching slump.

Arizona starter Robbie Ray dropped flat on the mound in a game last week after being struck by a line drive off the bat of St. Louis first baseman Luke Voit. Jeff Curry/USA TODAY Sports

MLB and the players' association have not tested or approved the SST product, but pitchers may select any protective headwear they wish, as long as it doesn't interfere with competition or licensing agreements. The only headwear that's mandatory is the official cap.

Two other companies' head safety devices have received MLB/MLBPA approval -- a hybrid cap-helmet resembling a visor that is produced by Boombang in conjunction with MLB and the union, and the bulky foam-padded isoBLOX cap. Alex Torres, who is no longer in the majors, wore the latter and received widespread attention. No MLB pitcher has worn the hybrid cap-helmet in a game.

The Kevlar-padded dome insert from Unequal Technologies was worn in 2015 by two big league pitchers who are no longer in MLB, and Shoemaker said he seriously considered using the BCL ball cap liner of carbon fiber and foam produced by former major leaguer Cliff Floyd and his father-in-law.

Contacted Friday by OTL, Matt Meier, the founder and CEO of SST, said he has reviewed the video from the plays involving Shoemaker, Ray and two pitchers who were struck in the months between those instances. Regarding Ray, Meier said, "It definitely hit him in a position that it [the SST guard] would've helped him. Thankfully, he was able to avoid major injury." Meier added that he thinks his company's insert would have lessened the impact in all four cases.

The other two were Blue Jays Triple-A pitcher T.J. House, who was hit in the head in a spring training game with Toronto's major league team, and Cardinals Triple-A pitcher Daniel Poncedeleon. House, whose incident was March 10, escaped serious injury. Poncedeleon, however, has been out of action since a May 9 liner struck him in the temple; he required emergency surgery to relieve swelling of the brain and was hospitalized for two weeks.

The SST guards, according to Meier, have been designed to reduce the risks of skull fractures and brain bleeds. But given that line drives leave the bat at 100 mph, Meier added, "There's nothing on earth that would prevent a concussion when the ball is hit at that speed."