Spatial mapping of the percentage cellularity in human bone marrow using magnetic resonance imaging

Evaluation of dual energy quantitative CT for determining the spatial distributions of red marrow and bone for dosimetry in internal emitter radiation therapy

PURPOSE

To evaluate a three-equation three-unknown dual-energy quantitative CT (DEQCT) technique for determining region specific variations in bone spongiosa composition for improved red marrow dose estimation in radionuclide therapy.

METHODS

The DEQCT method was applied to 80/140 kVp images of patient-simulating lumbar sectional body phantoms of three sizes (small, medium, and large). External calibration rods of bone, red marrow, and fat-simulating materials were placed beneath the body phantoms. Similar internal calibration inserts were placed at vertebral locations within the body phantoms. Six test inserts of known volume fractions of bone, fat, and red marrow were also scanned. External-to-internal calibration correction factors were derived. The effects of body phantom size, radiation dose, spongiosa region segmentation granularity [single (∼17 × 17 mm) region of interest (ROI), 2 × 2, and 3 × 3 segmentation of that single ROI], and calibration method on the accuracy of the calculated volume fractions of red marrow (cellularity) and trabecular bone were evaluated.

RESULTS

For standard low dose DEQCT x-ray technique factors and the internal calibration method, the RMS errors of the estimated volume fractions of red marrow of the test inserts were 1.2-1.3 times greater in the medium body than in the small body phantom and 1.3-1.5 times greater in the large body than in the small body phantom. RMS errors of the calculated volume fractions of red marrow within 2 × 2 segmented subregions of the ROIs were 1.6-1.9 times greater than for no segmentation, and RMS errors for 3 × 3 segmented subregions were 2.3-2.7 times greater than those for no segmentation. Increasing the dose by a factor of 2 reduced the RMS errors of all constituent volume fractions by an average factor of 1.40 ± 0.29 for all segmentation schemes and body phantom sizes; increasing the dose by a factor of 4 reduced those RMS errors by an average factor of 1.71 ± 0.25. Results for external calibrations exhibited much larger RMS errors than size matched internal calibration. Use of an average body size external-to-internal calibration correction factor reduced the errors to closer to those for internal calibration. RMS errors of less than 30% or about 0.01 for the bone and 0.1 for the red marrow volume fractions would likely be satisfactory for human studies. Such accuracies were achieved for 3 × 3 segmentation of 5 mm slice images for: (a) internal calibration with 4 times dose for all size body phantoms, (b) internal calibration with 2 times dose for the small and medium size body phantoms, and (c) corrected external calibration with 4 times dose and all size body phantoms.

CONCLUSIONS

Phantom studies are promising and demonstrate the potential to use dual energy quantitative CT to estimate the spatial distributions of red marrow and bone within the vertebral spongiosa.

Comparison of vertebral bone marrow fat assessed by 1H MRS and inphase and out-of-phase MRI among family members

Inphase and out-of-phase magnetic resonance imaging is a robust and fast method which can provide similar vertebral bone marrow fat estimation as (1)H proton magnetic resonance spectroscopy, indicating that this technique is a potentially useful tool in both research and clinical practice.

INTRODUCTION

The importance of evaluating bone marrow fat lies in the fact that osteoporosis and obesity, two disorders of body composition, are growing in prevalence. Bone fat mass can be reliably assessed using proton magnetic resonance spectroscopy ((1)H MRS), but this method is technically demanding and needs advanced post-processing unlike inphase and out-of-phase magnetic resonance imaging (MRI), which is a robust and fast method.

METHODS

We compared vertebral bone marrow fat (BMF) content assessed by inphase and out-of-phase MRI and (1)H MRS using a 1.5-T MRI scanner in mothers (n = 34, aged 49.4 years), fathers (n = 31, aged 53.1 years) and their daughters (n = 40, aged 20.3 years) who participated in the CALEX family study. Signal intensity on the inphase and out-of-phase MRI was analyzed from the same location and size of the single-voxel (1)H MRS measurement.

RESULTS

Positive correlations were found between (1)H MRS and inphase and out-of-phase MRI in the axial plane (r = 0.746, p < 0.001) and sagittal plane (r = 0.804, p < 0.001). The mean differences between (1)H MRS and inphase and out-of-phase MRI in the axial and sagittal planes were relatively small, at 4.13 and 2.67 %, and the agreement between techniques was 89.4 and 93.2 %, respectively. Girls had a significantly lower vertebral BMF than mothers and fathers with both methods (for all, p < 0.001).

CONCLUSIONS

We conclude that inphase and out-of-phase MRI can provide similar vertebral BMF estimation as (1)H MRS, indicating that this technique is a potentially useful tool in both research and clinical practice.

MRI measurement of bone marrow cellularity for radiation dosimetry

The current gold standard for measuring marrow cellularity is the bone marrow (BM) biopsy of the iliac crest. This measure is not predictive of total marrow cellularity, because the biopsy volume is typically small and fat fraction varies across the skeleton. MRI and localized MR spectroscopy have been demonstrated as noninvasive means for measuring BM cellularity in patients. The accuracy of these methods has been well established in phantom studies and in the determination of in vivo hepatic fat fractions but not for in vivo measurement of BM cellularity.

METHODS

Spoiled gradient-echo in vivo images of the femur, humerus, upper spine, and lower spine were acquired for 2 dogs using a clinical 3-T MRI scanner. Single-peak iterative decomposition of water and fat with echo asymmetry and least squares (SP-IDEAL) was used to derive BM fat fractions. Stimulated-echo acquisition mode spectra were acquired in order to perform multipeak IDEAL with precalibration (MP-IDEAL). In vivo accuracy was validated by comparison with histology measurements. Histologic fat fractions were derived from adipocyte segmentation.

RESULTS

Bland-Altman plots demonstrated excellent agreement between SP-IDEAL and histology, with a mean difference of -0.52% cellularity and most differences within ±2% cellularity, but agreement between MP-IDEAL and histology was not as good (mean difference, -7% cellularity, and differences between 5% and -20%).

CONCLUSION

Adipocyte segmentation of histology slides provides a measure of volumetric fat fraction (i.e., adipocyte volume fraction [AVF]) and not chemical fat fraction, because fat fraction measured from histology is invariant to the relative abundances of lipid chemical species. In contrast, MP-IDEAL provides a measure of chemical fat fraction, thus explaining the poor agreement of this method with histology. SP-IDEAL measures the relative abundance of methylene lipids, and this measure is shown to be equivalent to AVF. AVF provides the appropriate parameter to account for patient-specific cellularity in BM mass predictive equations and is consistent with current micro-CT-based models of skeletal dosimetry.

Correlations of Total Pelvic Spongiosa Volume With Both Anthropometric Parameters and Computed Tomography–Based Skeletal Size Measurements

Patient-specific dosimetry within the field of molecular radiotherapy continues to pose a challenge owing to the difficulty in predicting marrow toxicity. This study examined the correlation between total pelvic spongiosa volume (TPSV) and independent variables, which include both readily measured or calculated anthropometric parameters (AP), and image-based skeletal measurements requiring computed tomography (CT) images or skeletal radiographs. Fourteen (14) patients (5 male and 9 female) undergoing total hip arthroplasty (THA) were subjected to modified pelvic CT scans. These scans were utilized to estimate TPSV, which was comprised of the volumes of spongiosa within the L5 vertebra, os coxae, sacrum, and both proximal femurs. The APs investigated included total body height (TBH), total body mass (TBM), body mass index (BMI), body surface area (BSA), maximum effective mass (MEM), lean body mass (LBM), and fat-free mass (FFM). Skeletal measurements were also obtained from the CT images of the pelvic region. Correlation coefficients (r) were obtained for TPSV and each set of APs as well as each set of skeletal measurements. Total body height (r = 0. 80) and os coxae height (r = 0.83) had the highest correlation coefficients of all the APS and skeletal measurements, respectively. FFM (r = 0.50), LBM (r = 0.42), TBM (r = 0.11), and BSA (r = 0.11) did not correlate well with TPSV, which accounts for approximately 45% of total spongiosa seen throughout the skeleton at sites associated with active bone marrow. Skeletal height measurements appear to have a much higher correlation with TPSV than either their corresponding skeletal width measurements or parameters that are a function of an individual's TBM.

Red marrow dosimetry for radiolabeled antibodies that bind to marrow, bone, or blood components

Hematologic toxicity limits the radioactivity that may be administered for radiolabeled antibody therapy. This work examines approaches for obtaining biodistribution data and performing dosimetry when the administered antibody is known to bind to a cellular component of blood, bone, or marrow. Marrow dosimetry in this case is more difficult because the kinetics of antibody clearance from the blood cannot be related to the marrow. Several approaches for obtaining antibody kinetics in the marrow are examined and evaluated. The absorbed fractions and S factors that should be used in performing marrow dosimetry are also examined and the effect of including greater anatomical detail is considered. The radiobiology of the red marrow is briefly reviewed. Recommendations for performing marrow dosimetry when the antibody binds to the marrow are provided.

An automated iterative algorithm for water and fat decomposition in three-point Dixon magnetic resonance imaging

An iterative, outlier exclusion, second-order surface fitting algorithm has been developed to solve the well-known phase wraparound problem associated with in vivo applications of the three-point Dixon magnetic resonance imaging method. The technique was optimized for speed by reducing the problem to a pair of planar fits. The spatial misalignment between water and fat components due to the chemical shift was handled on a subpixel level by invoking the shift theorem of Fourier transformation. From the chemical shift corrected water and fat images, high quality recombined MR images were generated. The algorithm was validated in both phantom and patient studies. In vivo breast images and pelvic images are provided as a demonstration of the method.

Quantitative assessment of bone marrow hematopoiesis using parametric magnetic resonance imaging

Spatial maps of the percentage cellularity in pelvic bone marrow were calculated at a resolution of 15.6 mm3 from six volunteers and 10 patients treated for documented hematologic disease using a three-point Dixon MRI pulse sequence. The percentage cellularity calculation was aided by analyzing a two-dimensional feature space consisting of the apparent water fraction (Wa), and the T2 relaxation time of water (T2w). An extracellular water fraction was assigned to each voxel on the basis of a two-component T2w algorithm. In six cases, the method was compared to results obtained from core biopsies or aspirates of the posterior iliac crest. The results indicate that segmentation schemes that combine high-quality phase-contrast imaging with nuclear relaxation time measurements can potentially identify the true fractional marrow volume occupied by hematopoietic elements in a variety of clinical situations.

Magnetic resonance properties of ex vivo breast tissue at 1.5 T

The magnetic resonance absorption spectrum, T1 and T2 relaxation time distributions, and magnetization transfer properties of ex vivo breast tissue have been characterized at 1.5 T and 37 degrees C. The fraction of fibroglandular tissue within individual tissue samples (n = 31) was inferred from the tissue volumetric water content obtained by integration of resolvable broad-line fat and water resonances. The spectroscopically estimated water content was strongly correlated with that extracted enzymatically (Pearson correlation coefficient 0.98, P < < 0.01), which enabled the assignment of principal relaxation components for fibroglandular tissue (T2=0.04+/-0.01, T1=1.33+/-0.24 s), and for adipose tissue (T2=0.13+/-0.01, T1=0.23+/-0.01 s, and T2=0.38+/-0.03, T1=0.62+/-0.16 s). Th e relaxation components for fibroglandular tissue exhibited strong magnetization transfer, whereas those for adipose tissue showed little magnetization transfer effect. These results ultimately have applicability to the optimization of clinical magnetic resonance imaging and research investigations of the breast.