Kyodo news agency, quoting the Imperial Household Agency, said the emperor, 78, had undergone an angiogram on Saturday which showed that blood vessels had narrowed over the past year. He had been treated in hospital for three weeks in November for cold, fever and symptoms of bronchitis.
The Imperial Household Agency was unavailable for comment.
Kyodo said the test was performed after the emperor experienced difficulty while engaging in light exercise.
Akihito had surgery for prostate cancer in 2003 and suffered stress-related health issues in late 2008, including irregular pulse and stomach bleeding.
Akihito ascended to the throne after the death of his father Hirohito in 1989. He has spent much of the past two decades working to close the gap between the imperial family and ordinary Japanese people.
Television footage on Sunday showed the emperor greeting doctors after being discharged from hospital following the examination.
