Low-contrast focal lesion detectability phantom for 1H MR imaging.

A phantom made from tissue-mimicking materials is reported for testing 1H MRI systems regarding their ability to detect small low-contrast focal lesions and to delineate the boundaries of larger lesions. Two sets of seven spherical simulated lesions with diameters of 2, 3, 4, 5, 6.3, 7.9, and 9.5 mm have T1 and T2 values somewhat higher than the corresponding values in the surrounding simulated normal tissue. Relaxometer determinations of T1 and T2 for the simulated normal tissue yielded 955 and 106 ms, respectively, at 22 degrees C and 1 T. The corresponding values in set #1 of the simulated lesions were 1017 and 175 ms and in set #2 were 1002 and 127 ms. These T1 and T2 values are similar to those for brain and brain lesions but are too high to represent soft tissue such as liver and muscle. The centers of the 14 lesions are coplanar, and eight alignment devices surround them facilitating superposition of the scan plane and the plane containing the centers of the lesions. Illustrative images made with a 1.5-T system are shown. All simulated lesions are detectable in T2-weighted head coil images. Corresponding T1-weighted images are also shown, the 2- and 3-mm lesions of set #2 and the 2-mm lesion of set #1 not being detectable. The phantoms could be useful for performance or acceptance testing and perhaps for quality assurance testing.

Tissue-mimicking gelatin-agar gels for use in magnetic resonance imaging phantoms.

A new variety of tissue-mimicking materials for use in 1H nuclear magnetic resonance (NMR) phantoms has been developed and extensively tested, principally at 10 MHz and at room temperature. The materials can be formed with a broad range of T1's and T2's representative of soft tissues. They are mixtures of various percentages of agar, animal hide gelatin, water, and glycerol. Small concentrations of formaldehyde and n-propanol prevent melting through 100 degrees C and prevent bacterial invasion. The materials are easily produced. A thorough description of compositions and production procedures is given. T1's exhibit about a 5%/degrees C rise in temperature. T2's exhibit less than a 1% rise/degrees C. Long-term (12 months) stability is exhibited both for NMR properties and for absence of fluid extrusion. Preliminary results indicate that T1's depend on the Larmor frequency in a similar way to that observed in soft tissues.

Transfer function measurement and analysis for a magnetic resonance imager.

The transfer function characteristics of a 1.5T imager have been determined. An edge response function (ERF) was obtained from a water/Plexiglas interface at various pixel widths ranging from 0.312 to 1.0 mm. An SE pulse sequence was used with a 5-mm transaxial slice. The ERF was smoothed, differentiated, and Fourier transformed to obtain MTF curves. The LSF was analyzed for skewness and kurtosis. The area under the MTF amplitude curves and the equivalent bandpass were calculated. All ERFs, LSFs, and MTFs were well behaved. The resulting LSF was Gaussian. All calculated MTFs had cutoff frequencies slightly less than the theoretical Nyquist limit. The MTF calculated from the theoretical Gaussian LSF is slightly superior to that calculated from experimental data and provides an upper limit to the MTF. Spatial resolution in our MR imager is dominated by the pixel size via the Nyquist sampling theorem. System performance is slightly less than theoretically predicted, possibly due to image processing algorithms during the reconstruction process.

Anthropomorphic 1H MRS head phantom.

An anthropomorphic 1H MRS head phantom has been developed which mimics the in vivo structure, metabolite concentrations, and relaxation times (for both water and metabolites) of human brain tissue. Different brain regions and two tumor types, fluid-containing ventricles, and air-filled sinus, and subcutaneous fat are all simulated. The main tissue-mimicking materials are gelatin/agar mixtures with metabolites and several other ingredients added. Their composition and method of production are thoroughly described. T1's and T2's of water in the phantom are very close to in vivo values, and metabolite T1's and T2's are considerably more realistic than those in aqueous solutions. Spectra and relaxation times for the pig brain were also acquired and compare well with those of the phantom. The realistic properties of this phantom should be useful for testing spectral quantitation and localization.