We provide here a description of the system and the general approach used in estimating the four evaluated compartments. A previous description of the EchoMRI-AH QMR system at this level is lacking and leaves an important gap when interpreting the studies that follow. The earlier Echo Medical Systems’ QMR instruments and DXA were developed and validated against classic carcass chemical analysis for use in small animals (7 (
link),8 (
link)). The reported precision (CV) for fat measurement in mice was 0.34–0.71% for QMR compared with 3.06–12.60% for DXA (8 (
link)).
The QMR system has a patient capacity of up to 250 kg. The resistive magnet bore size is 29 × 29 inches and external dimensions are
L ×
W ×
H is 144 × 57 × 60 inches. The field of view is 27.5 × 27.5 × 63 inches (
X ×
Y ×
Z axis) and the system is self-shielded. Static magnetic field homogeneity in the QMR system over the whole body is about 0.3% as measured by a non-NMR method. Measurement of an ECHO WIDTH for samples weighing ~100 kg (i.e., large adult human) demonstrates a static magnetic field homogeneity of ~0.2%. The QMR operating system is based on Windows XP Professional Edition. Measuring time is typically <3 min, with three or four repeat measurements taking about 10 min; there is a recommended daily system test in the most recent software. The system output includes fat mass, lean tissue mass, free water mass, and total water mass in units of kilograms.
The QMR system produces an optimized sequence of radio pulses consisting of several segments separated by pauses of varying duration that are designed to capture relevant relaxation time scales. The captured signals, as shown in the human example presented in
Figure 1a, reflect the combined signals of TBF, lean, and free water. Examples of separate measurements of canola oil, lean pork, and water phantoms normalized to unit mass are shown in
Figure 1b.
The key principal of the QMR method is that each scan produces a record of NMR responses (echoes) to a radiofrequency pulse sequence. The sequence is composed of several periodic Carr-Purcell-Meiboom-Gill parts separated by pauses of different duration. The lengths of the periodic parts and the pause durations are designed to capture all relevant characteristic (relaxation) time scales of the NMR responses (transverse, “T
2”, and longitudinal, “T
1”, relaxation) typical for fat, lean, and free water. The whole-body signal is a linear combination of fat, lean, and free water contributions, and the differences between the relaxation rates of the three basic substances make it possible to use linear regression analysis formulas calibrated to fat (canola oil), lean (chicken breast, small animals; lean pork, larger animals), and free water (tap water) phantoms. The algorithm for optimizing these regression formulas is a variant of multivariate calibration, which is typical of chemometric analyses (9 ), employing partial least squares optimization combined with principal component analysis for high-dimensional regressions.
Total water is calculated from the same records but in a different way as the contributions of protons associated with proteins and other “solid” materials are negligible at the time scales employed by the QMR system. The “lean” signal therefore comes mainly from water bound within the lean tissues. There is also a substantial contribution from protons in fat molecules. The difference between an estimate of the total amount of protons participating in the record and an estimate of fat found by regression analysis yields an estimate of the amount of total water included in lean together with free water. Therefore, fat mass and TBW are not measured independently of each other.
In sum, the main QMR pulse sequence is used to derive fat, lean mass, and free water using multiple linear regression prediction formulas calibrated against canola oil, lean animal tissues, and tap water, respectively. QMR body mass (BM) is then estimated as the sum of fat mass, lean mass, and free water mass; bone mineral, with high calcium and phosphorus content and gastrointestinal solids are not detected using the QMR system. QMR BM thus reflects the sum of three measured compartments, fat, lean, and free water mass.
Gallagher D., Thornton J.C., He Q., Wang J., Yu W., Bradstreet T.E., Burke J., Heymsfield S.B., Rivas V.M, & Kaufman R. (2010). Quantitative Magnetic Resonance Fat Measurements in Humans correlate With Established Methods but Are Biased. Obesity (Silver Spring, Md.), 18(10), 2047-2054.