核磁共振的结果一定准确吗

核磁共振全名是核磁共振成像（MRI），是磁矩不为零的原子核，在外磁场作用下自旋能级发生塞曼分裂，共振吸收某一定频率的射频辐射的物理过程。核磁共振波谱学是光谱学的一个分支，其共振频率在射频波段，相应的跃迁是核自旋在核塞曼能级上的跃迁。
核磁共振是处于静磁场中的原子核在另一交变磁场作用下发生的物理现象。通常人们所说的核磁共振指的是利用核磁共振现象获取分子结构、人体内部结构信息的技术。

When Gail Kislevitz had an MRI scan of her knee, it came back blurry, "uninterpretable," her orthopedist told her.

Her insurer refused to pay for another scan, but the doctor said he was sure she had torn cartilage that stabilizes the knee and suggested an operation to fix it. After the surgery, Kislevitz, 57, of Ridgewood, New Jersey, received a surprise: the cartilage had not been torn after all.

She had a long rehabilitation. And her insurer paid for the operation. But her knee is no better.

More than 95 million high-tech scans are done each year, and medical imaging, including CT, MRI and PET scans, has ballooned into a $100-billion-a-year industry in the United States, with Medicare paying for $14 billion of that. But recent studies show that as many as 20 percent to 50 percent of the procedures should never have been done because their results did not help diagnose ailments or treat patients.

"The system is just totally, totally broken," said Dr. Vijay Rao, the chairwoman of the radiology department at Thomas Jefferson University Hospital, in Philadelphia.

Radiologists say a decent MRI scan should have clearly shown whether Kislevitz's cartilage, a meniscus, was torn. But bad scans, medical experts say, are part of a growing problem with medical imaging.

Many factors contribute. Insurers pay the same for a scan done on a 10-year-old machine as for one on the latest model, though the differences in the images can be significant.

Insurers do not distinguish between scans that are done poorly or done well or read by less- or more-qualified doctors. Aside from mammography, whose standards were established by a law that went into effect more than a decade ago, the field is largely unregulated in the United States. And increasingly, doctors refer patients to scanning centers they own and profit from.

Imaging centers can, if they choose, become accredited by the American College of Radiology. That requires, among other things, scanning a phantom, a device that simulates a body part. Technologists must also be certified, and there are standards for supervising physicians. And the scanners must be regularly assessed to ensure they are properly functioning.

But many centers are not accredited, although the percentage is not known because there is no national registry of imaging centers.

Radiologists are struck by the wide variation in the quality of scans, and they say there is little patients can do other than to ask why the scan is necessary and, if it is, to ask about accreditation, the credentials of the person reading the scan and the age of the scanner.

"The studies I see coming from the outside vary from marginal quality to very good quality," said Dr. Chris Beaulieu, a Stanford radiology professor.

Interpretation can be crucial, Beaulieu said. "A good radiologist can sometimes accurately read scans off of a lower-quality scanner," he said. "I see that all the time. A good radiologist and a lower-quality scan could be better than a bad radiologist and a good scan."