To determine whether systematic differences were present between myocardial R2* values obtained with two different decay models: truncation and exponential-plus-constant (Exp-C).

Single-center cohorts were used to compare black and bright blood sequences separately and a multi-center cohort of mixed bright and black blood studies was used to assess the generalizability. Truncated exponential estimates were calculated with CMRTools that uses a single region of interest (ROI) method. Exp-C estimates were calculated using a pixelwise approach.

No differences could be distinguished based upon whether a white or black blood sequence was examined. The two fitting algorithms gave similar R2* values, with R-squared values exceeding 0.997 and CoV of 3–4%. Results using the pixelwise method yielded a small systematic bias (~3%) that became apparent in patients with severe iron deposition. This disparity disappeared when Exp-C fitting was used on a single ROI suggesting that the use of pixelwise mapping was responsible for the bias. In the multicenter cohort the strong agreement between the two fitting approaches was reconfirmed.

Cardiac R2* values are independent of the signal model used for its calculation over clinically relevant ranges. Clinicians can compare results among centers using these disparate approaches with confidence.

Estimation of myocardial iron stores is essential for preventing cardiac disease and managing chelation treatment in patients with thalassemia, sickle cell disease, aplastic anemia, myelodysplasia, and other iron-related diseases (

The R2* value is obtained by fitting the CMR signal at different echo times (TEs) to an appropriate decay model. In heavily iron-overloaded hearts, the rapid signal loss leads to a plateau in the signal decay curve at later echo times. In this situation, fitting a simple mono-exponential model to all of the echo times produces large R2* underestimation errors (

It is important for patient management that the calculated R2* values (or T2* values) are independent from the software used. Two prior reports suggested that large differences may exist (

Two single-center cohorts and one multi-center cohort of patients were considered. The first single-center cohort included 42 patients (24 with thalassemia major, 13 with sickle cell disease, and 5 with other iron-related diseases) scanned using a black blood T2* technique. Mean age was 19.3 ± 10.1 years and 21 patients were females. The second single-center cohort included 70 patients (31 with thalassemia major, 21 with sickle cell disease, and 18 with other iron-related diseases) scanned using a white-blood T2* technique. Mean age was 18.7 ± 10.9 years and 30 patients were females. Our thalassemia cohort is 24% Chinese, 20% other Southeast Asia (Vietnamese, Laotian, Cambodian, Thai, Filipino), 17% Indian Subcontinent (Indian, Pakastani), 23% Mediterranean (Italian, Greek, Cypriot), 9% Middle Eastern (Iranian, Lebanese, Iraqi, Saudia Arabia) and 4.5% Hispanic and 2.5% other ethnic backgrounds. Our sickle cell disease cohort is 95% African descent and 5% Hispanic.

Multicenter data represented baseline measurements from a phase II clinical trial of FBS0701 (

The distribution of the patients is reported in

The protocol was approved by The Children’s Hospital of Los Angeles’ Committee for the Protection of Human Subjects (Protocol # CCI-12-00087) and the institutional review boards of all participating hospitals. The requirement for consent was waived for the retrospective data analysis. All patients in the prospective FBS0701 trial gave written informed consent.

Both single-center cohorts underwent CMR at the Children’s Hospital of Los Angeles (CHLA), but using two different 1.5T scanners: a Philips Achieva (Philips Medical Systems, Best, The Netherlands) running system 2.5.1 and a GE Signa CVi (GE Healthcare, Waukesha, WI) running system 9.1; similar phased array torso coils were used on both. Black blood multiecho gradient echo images were used to calculate R2* on the Philips while bright blood multiecho gradient echo R2* sequence was used on the GE scanner.

For the multi-center cohort gradient echo images were acquired on 1.5T scanners from all 3 major MR vendors. Black or bright blood acquisitions were used (

At the CHLA as well as in all the other sites a single short axis mid-ventricular slice was acquired. All R2* sequence parameters are indicated in

All images were processed by the same operator. Identical regions of interest (ROI’s) were drawn in the mid-ventricular septum for both techniques. Truncated exponential estimates were calculated using CMRTools. It calculates the mean signal intensity of the ROI for each image and fits the decay curve to a mono-exponential model: _{0}=initial amplitude, TE=echo time. The R-square value describing the goodness of fitting was used by the operator as guide in the application of the truncation model. If the R-square value was > 0.99, no truncation was applied. Otherwise, the last points were discarded in succession until the R-square value became > 0.99. Finally, the R2* value was calculated as 1000/T2*.

Exponential + constant (Exp-C) estimates were calculated using a rapid pseudo-pixelwise implementation written in MATLAB (The Mathworks, Natick, MA). This software, called Iron, divides the ROI into subregions of similar-relaxivity; the number of pixels in each subregion is equal to the square-root of the total number of pixels in the traced ROI. Each subregions is fitted to a Exp-C model: _{0}=initial amplitude, TE=echo time, R2* is the relaxivity, and C is the constant offset term. A distribution of R2* values is produced and the mean and median from this distribution are obtained (

Hereafter we refer to the R2* values obtained with the CMRTools as R2*_{CMRTools} and to the R2* values obtained using Iron as R2*_{Iron-mean} (mean of R2* distribution taken into account), R2*_{Iron-median} (median of R2* distribution taken into account) and R2*_{Iron_ROI_based} (single ROI).

All data were analyzed using SPSS version 16.0 (SPSS Inc., Chicago, IL, USA) and MedCalc for Windows version 7.2.1.0 (MedCalc Software, Mariakerke, Belgium) statistical packages.

Continuous variables were described as mean ± standard deviation (SD).

Summary data were displayed using scatter plots with regression lines. Linear regression models provided slope and intercept estimates and the R-squared measuring the goodness of the linear fit. Because R2* values were not normally distributed, a paired Wilcoxon signed rank test was applied to detect significant differences between two datasets while the Spearman correlation coefficient was used to assess their relationship. A coefficient of variation (CoV) was calculated as the ratio of the SD of the half mean square of the differences between the repeated values, to the general mean. The Bland-Altman (BA) technique was used to plot the absolute difference (standard BA) or the percent difference (relative BA) versus the average values between two datasets. The relative Bland-Altman plot was preferred when the variability of the differences increased as the magnitude of the measurements increased. Bias was the mean of the difference between the two methods and agreement was the mean ± 1.96 SDs.

A P value < 0.05 was considered statistically significant.

_{Iron-mean} values as a function of R2*_{CMRTools} values. The line of best fit had a slope of 1.031 ± 0.005, significantly different from 1 (P<0.0001), and an intercept of −1.176 ± 0.379 Hz. The R-squared value for the fit was 0.999. The R2*_{Iron-mean} values were not significantly different from R2*_{CMRTools} values (48.5 ± 54.7 Hz vs 48.2 ± 53.1 Hz, P=0.945). Bland-Altman analysis demonstrated no significant bias between the two techniques, with limits of agreement of 4.8 Hz (

Relationships between R2*_{Iron-median} and R2*_{CMRTools} as well between R2*_{Iron-ROI_based} and R2*_{CMRTools} was nearly identical to results for R2*_{Iron-mean}, being well described by a straight line. In both cases Bland-Altman analysis demonstrated good agreement and the CoV was less than 4.4% (

_{Iron-mean} values as a function of R2*_{CMRTools} values. The line of best fit had a slope of 1.025 ± 0.005, significantly different from 1 (P<0.0001), and an intercept of −0.741 ± 0.300 Hz. The R-squared value for the fit was 0.998. The R2*_{Iron-mean} values were not significantly different from R2*_{CMRTools} values (47.6 ± 37.9 Hz vs 47.2 ± 36.9 Hz, P=0.088). Bland Altman analysis demonstrated no significant bias between the two techniques, with limits of agreement of 3.5 Hz (

The results of the comparison between R2*_{Iron-median} and R2*_{CMRTools} and between R2*_{Iron-ROI_based} and R2*_{CMRTools} are indicated in

_{Iron-mean} values as a function of R2*_{CMRTools} values. The line of best fit had a slope of 0.989 ± 0.008, not significantly different from 1 (P=0.148), and an intercept of 0.108 ± 0.372 Hz. The R-squared value for the fit was 0.997. The R2*_{Iron-mean} values were not significantly different from R2*_{CMRTools} values (43.5 ± 22.6 Hz vs 43.8 ± 22.7 Hz, P=0.250). The CoV was 2.3%.

The results of the comparison between R2*_{Iron-median} and R2*_{CMRTools} and between R2*_{Iron-ROI_based} and R2*_{CMRTools} are indicated in

As can be inferred from the both the linear regression analysis and Bland-Altman plots, the pixelwise, Exp-C analysis yields slightly higher values at larger R2*s. To better characterize that effect for an acceptable number of patients (N=45), we pooled all patients with detectable cardiac iron (R2*_{CMRTools} ≥ 50 Hz).

In the last years, CMR R2* has become a widely used accurate and noninvasive technique for monitoring heart iron overload in patients with different types of hemoglobinopathies (

In all three patient cohorts, we observed that the two fitting approaches yield similar R2* values with R-squared values exceeding 0.997, regardless of whether white or black blood images were used. More importantly, the CoV was extremely low, suggesting excellent stability of both techniques. Nonetheless, the regression slopes were ~3% greater than unity and these differences became more apparent in patients with severe iron deposition (R2* > 100 Hz). This is in agreement with two previous studies involving the liver. Beaumont et al. showed that in the liver the models differences became evident for R2* values higher than 200 Hz (_{Iron-median} and R2*_{CMRTools} values was well described by a line and that results were unbiased for R2*<300 Hz, but large systematic differences in R2* appeared at higher values, with the exponential-plus-constant fits averaging ~20% higher (

We used three different cohorts to probe the differences over as a broad a patient base as possible. We could distinguish no differences based upon whether a white or black blood sequence was examined. To obtain higher generalizability, we also included a multi-center cohort of patients scanned according to the specific protocol of their center using a mixture of white and black blood techniques. We reconfirmed the strong agreement between R2* values obtained with the two softwares (

In conclusion, we showed that the cardiac R2* values are independent of the signal model (

This work was supported by a grant from the National Institutes of Health, National Heart Lung and Blood Institute (1 RO1 HL075592-01A1), the Center for Disease Control (1 U01 DD000309-1), the National Center for Research Resources, Children’s Hospital Los Angeles General Clinical Research Center (RR00043-43) and FerroKin BioSciences as sponsor of the study.

We thank all patients for their cooperation.

Typical black blood (left) and bright blood (right) short-axis mid-ventricular images at the first echo time.

Comparison between R2*_{Iron-mean} values and R2*_{CMRTools} values for a) the first single-center cohort (black blood images), b) the second single-center cohort (bright blood images) and c) the multi-center cohort. Left: Scatter plot with regression line (solid line). The dotted line is the line of identity. Right: Bland-Altman plot of absolute differences. Dotted lines indicate the limits of agreement.

Agreement between the two methods for all patients with detectable cardiac iron. Relative Bland-Altman plots for a) R2*_{Iron-mean} and R2*_{CMRTools} values, b) R2*_{Iron-median} and R2*_{CMRTools} values and c) R2*_{Iron-ROI_based} and R2*_{CMRTools} values.

R2* sequence parameters for all the sites involved in this study.

Single center | Multi center | ||||||||
---|---|---|---|---|---|---|---|---|---|

Los Angeles, USA | Boston, | Cagliari, | Genoa, | Izmir, | London, | Oakland, | Siriraj, | ||

# patients | 42 | 70 | 4 | 15 | 6 | 11 | 6 | 7 | 13 |

Scanner | Philips | GE Signa | Philips | Siemens | GE LX | Siemens | Siemens | Philips | Philips |

Field strength | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 |

Software | 2.6.3.5 | 9.1 | 2.6.3.5 | VB15 | 15.0 M4 | VA30A | VB17 | 2.6.3 | 2.6.3.5 |

Coil | body | torso | 4-channel | 8-channel | 8-channel | Phased- | 4-channel | Phased | 5-channel |

Sequence | Black blood | Bright | Black blood | Bright | Bright | Bright | Bright | Black blood | Black blood |

Min TE (ms) | 1.28 | 1.46 | 2 | 2.59 | 1.4 | 2.6 | 2.59 | 1.92 | 2 |

ΔTE (ms) | 1.09 | 2.38 | 2.14 | 2.23 | 1 | 2.69 | 2.23 | 2.01 | 2.02 |

# echoes | 16 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 |

FA (°) | 20 | 20 | 20 | 20 | 20 | 20 | 20 | 20 | 20 |

Matrix (pixels) | 256 × 256 | 256 × 256 | 128 × 128 | 128 × 256 | 128 × 96 | 192 × 75 | 256 × 96 | 125 × 156 | 128 × 256 |

Field of view | 30–36 | 30–36 | 25–32 | 35–40 | 31–40 | 40 | 40 | 23–36 | 40 |

Bandwith | 1563 | 488 | 723 | 810 | 648 | 810 | 810 | 720 | 382 |

Slice thk (mm) | 8 | 8 | 8 | 10 | 8 | 10 | 10 | 8 | 10 |

R2*_{Iron-median} and R2*_{Iron-ROI_based} versus R2*_{CMRTools} for the three cohorts of patients.

Paired t-test | Regression Analysis | Bland Altman | CoV | ||||||
---|---|---|---|---|---|---|---|---|---|

Mean | P | Slope | P for | Intercept | R- | Mean | Limits | ||

_{Iron-median}_{CMRTools} | 48.9±55.0 | 0.258 | 1.036±0.007 | <0.0001 | −1.007±0.473 | 0.998 | 0.7 | −5.0 to | 4.37 |

_{Iron-ROI_based}_{CMRTools} | 48.9±52.7 | 0.063 | 0.993±0.007 | 0.336 | 1.058± 0.512 | 0.998 | 0.7 | −4.1 to | 3.66 |

_{Iron-median}_{CMRTools} | 47.7±38.2 | 0.085 | 1.035±.006 | <0.0001 | −1.190±0.373 | 0.998 | 0.4 | −4.0 to | 3.45 |

_{Iron-ROI_based}_{CMRTools} | 47.5±37.4 | 0.050 | 1.013±0.004 | 0.002 | −0.245±0.247 | 0.999 | 0.4 | −2.3 to | 2.06 |

_{Iron-median}_{CMRTools} | 43.7±22.4 | 0.989 | 0.982±0.007 | 0.015 | 0.705±0.359 | 0.997 | −0.1 | −2.7 to | 2.25 |

_{Iron-ROI_based}_{CMRTools} | 44.0±22.5 | 0.207 | 0.989±0.007 | 0.131 | 0.681±0.352 | 0.997 | 0.2 | −2.3 to | 2.07 |