Jurors in John W. Hinckley Jr.'s trial looked at computer-enhanced X-ray pictures of Hinckley's brain yesterday, as his lawyers turned to physical evidence to bolster their argument that he was legally insane when he shot President Reagan.

Dr. Marjorie LeMay, a Boston radiologist who specializes in the brain, flew hurriedly to Washington yesterday to testify after Judge Barrington D. Parker reversed an earlier ruling and decided to allow the jury to hear evidence about sophisticated X-rays of Hinckley's brain.

The computer-enhanced X-rays, known as CAT scans, were first taken 23 days after Hinckley shot Reagan, and then repeated last April 23. LeMay, who teaches at the Harvard Medical School, told the jury that Hinckley's brain is slightly shrunken, suggesting "organic brain disease." She did not elaborate.

LeMay testified that the appearance of Hinckley's brain was similar during both sets of X-rays, indicating "that these are permanent changes."

A CAT (Computerized Axial Tomography) scan of the brain is an image of a cross-section, or slice of the brain. It is not a photographic picture like an X-ray, but rather a precise computerized print in varying shades from black to white that shows any malformations in the brain tissue or surrounding bone. A computer produces the image using information provided from X-rays taken from varying angles.

LeMay's testimony yesterday laid the groundwork for additional defense testimony, which the jury is expected to hear today, from a psychiatrist at the National Institute of Mental Health who has studied the statistical correlation between brain abnormalities and schizophrenia.

Last week, at a hearing outside the jury's presence, Dr. Daniel R. Weinberger testified that abnormalities such as those found in Hinckley's brain are found more frequently in schizophrenics than in normal persons of the same age.

Prosecutors vigorously opposed the introduction of that evidence, and Parker had said last week that he would not allow it because the scientific community had not generally accepted the use of CAT scans to diagnose or support a finding of schizophrenia. Hinckley's defense lawyers formally rested their presentation of evidence Friday, but asked Parker to reconsider his decision.

In reversing himself yesterday, Parker said he now believed that the exclusion of that evidence would "deprive the jury of the complete picture" of Hinckley's mental condition at the time of the shooting more than a year ago.

The jury, out of the courtroom all day, finally began hearing LeMay's testimony at 4:50 p.m. yesterday, after Parker reviewed what she planned to say and the slides to be used.

Hinckley's lawyers contend that he was legally insane when he wounded Reagan and three others and should not be held criminally responsible for his acts. Three defense psychiatrists have testified that they believe that at the time of the shooting Hinckley suffered from schizophrenia, a severe break with reality characterized by delusions and deep depression.

LeMay, standing before a slide projector screen and gesturing with a pointer, explained to the jury that when she looked at Hinckley's CAT scans, she saw that the sulci, or folds, in Hinckley's brain were widened and "very prominent." LeMay also said the X-rays showed that a ventricle in Hinckley's brain was unusually large.

Both findings are symptoms of brain shrinkage or atrophy, LeMay told the jury, and are unusual in a person of Hinckley's age. He is 27.

During cross-examination by chief prosecutor Roger M. Adelman, LeMay testified that she could not say, with "reasonable medical certainty," whether the widened folds meant that Hinckley's brain had atrophied or whether his brain tissue was simply less developed than a normal person's. LeMay said she would need CAT scans taken when Hinckley was younger to determine that.

She also testified that except for relying on statistical references, a CAT scan alone could not be used to make a determination about behavior.