MR elastography shakes up assumptions about body MR

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By: James Brice

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MR elastography debuted on Tuesday as a clinical application ready for diagnostic uses in the liver and many diagnostic niches elsewhere in the body.

Modality inventor Dr. Richard Ehman noted during the morning plenary session that though elasticity is the physical property of tissue most changed by disease, it has never before been considered as a target for diagnostic imaging. Yet knowledge about the importance of tissue stiffness is as old as the ancient Greeks and the use of palpation. : X/ W/ u% X) D

The propagation of mechanical waves through a region of interest can reveal the relative stiffness of anatomic structures. M/ W7 Z& v* \( i( j

"The problem is that differences are seen in the realm of nanometers," Ehman noted. "Fortunately, MR is well suited for that role." . O9 D% e0 v6 }, ?" a) A

A modified phase-contrast gradient-echo sequence was adapted for MRE. Elastograms are generated by an automated process yielding quantitative images of tissue stiffness based on a colorized linear scale calibrated into kiloPascals (kPa).

Ehman described devices developed at Mayo and Charité Clinic in Berlin to induce mechanical shear waves through the abdomen for simultaneous interrogation with MR elastography. The MRE sequence takes about 15 seconds and can be performed at the end of a standard 45-minute MRI liver exam.

Ehman listed 27 anatomic regions as potential candidates for MR elastography, including the spleen, pancreas, stomach, bowel, kidney, breast, and prostate. For the liver alone, Mayo researchers have identified 10 potential diagnostic applications: fibrosis, stenosis, portal hypertension, focal masses, diffuse masses, inflammatory disease, atrophy, metabolic conditions, pre-disease states, and response to therapy

Though elastography is still considered experimental, Ehman and colleagues at the Mayo Clinic in Rochester, MN, have performed more than 500 studies on humans. $ x. w( U5 M# N- B

"This is now the most common indication for abdominal MRI at the Mayo Clinic," he said. 0 r0 D$ ?; w* C. ~

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MR elastography detects liver fibrosis. Top row: Conventional MR images of two different patients are not capable of showing presence of liver fibrosis. Middle row: Acoustic waves are generated in abdomen with driver device and imaged with special MRI technique. Bottom row: Wave images are processed to generate quantitative images showing stiffness of tissue (elastograms). Patient on right has markedly elevated liver stiffness, averaging 7 kPa (normal value 2 kPa seen in patient on left), indicating presence of moderately advanced liver fibrosis. (Provided by R. Ehman)

It has proven especially valuable for diagnosing liver fibrosis, a silent though reversible condition that unpredictably affects about 25% of patients with liver disease. Fibrosis becomes irreversible when it progresses to cirrhosis, Ehman said. Liver biopsy is the accepted definitive diagnosis. 7 E, Z0 q! ^0 o4 j6 z

The shear stiffness of healthy liver tissue is 2 kPa. Fibrosis exhibits a stiffness value of more than 2.9 kPa.

A retrospective evaluation of 48 patients who underwent MRI and MRE studies at Mayo was presented at this year's traditional poster session. Dr. Sudhakar Venkatesh, a visiting research radiologist, used MRE to identify and grade cases on a four-point scale. MRE correctly identified 96.7% of the 30 biopsy-proven cases of fibrosis and 100% of the normal livers. One patient with stage F1 mild liver fibrosis was classified as normal with MRE.

A threshold of 4 kPa with MR elastography identifies patients with some degree of liver disease with a sensitivity of 98% and sensitivity of 99%, Ehman said. For the differentiation of low- and high-grade fibrosis, with a threshold of about 5 kPa, sensitivity and specificity are in the 85% range.

Fatty changes that typically accompany the development of liver pathology do not affect elastographic measures, Ehman said. Early experience has shown that MRE can track the fibrotic changes leading to cirrhosis. , D3 I6 H8 a% |0 ~& I3 q5 J

"This MR elastography in the liver is the first mainstream clinical application of this technology," he said. 0 A9 `- @6 W X. J; J3 `

The technology also shows potential for characterizing focal liver lesions. Benign masses such as liver hemangiomas and hepatic adenomas have an MR elastographic shear stiffness of about 4 kPa, only slightly stiffer than the surrounding normal tissue. The elastograms of malignant masses such hepatocellular carcinoma show prolonged waves through the lesions, producing a shear stiffness of more than 10 kPa.

"The data are preliminary but show significant promise for differentiating between benign lesions and malignant masses," Ehman said. 7 Q" D4 L) d5 S: @, u/ \& {

The sky's the limit for MR elastrography's potential for investigating other tissues, according to Elman. An echo-planar version of MRE has been developed that takes about nine seconds per slice to acquire to produce 3D wave data.

"MR elastrography is emerging as a versatile way to assess both diffuse and focal liver disease," Ehman said. "In the case of liver fibrosis, it is safer, less expensive, and potentially more accurate than liver biopsy, and it offers new ways to characterize liver lesions."