Establishing T1 values for CMR at 0.55T

Background: Cardiovascular magnetic resonance (CMR) native and post-contrast myocardial T1, as well as extracellular volume fraction (ECV), are tools that assist in phenotyping of multiple myocardial disease states [1]. T1 values may be affected by inflammatory, infiltrative, and fibrotic states. Native T1 values differ between magnetic field strengths. As 0.55T CMR grows in popularity [2], our study aimed to establish normal T1 values at 0.55T and examine the differences in T1 and ECV in different cardiovascular pathology.

Methods: CMR was performed in a group of patients referred for clinical assessment of various cardiovascular conditions and in a group of healthy volunteers. All patients agreed to participate in an IRB-approved research protocol. Some patients underwent both a 0.55T scan and a 1.5T scan with a 60-day window. The 0.55T scans were performed on either a prototype MAGNETOM Aera (Siemens Healthineers, Forcheim, Germany) or a MAGNETOM Free.Max (Siemens) with investigational high-performance gradients. All 1.5T scans were performed on the same MAGNETOM Aera system that was later upgraded into a SolaFit (Siemens).

All patients underwent MOLLI native T1 mapping in short-axis and/or long-axis views. Patients who agreed to contrast received 0.15 mmol/kg gadobutrol for each scan and underwent post-contrast T1 mapping. Regions of interest (ROIs) were drawn on the T1 maps in the myocardium and left ventricular blood pool. In patients with myocardial infarction, ROIs in both normal (remote) myocardium and infarction were measured.

Subjects were categorized into one of the following groups: 1. Healthy volunteer, 2. myocardial infarction (MI), 3. cardiomyopathy, 4. acute inflammatory process, 5. amyloid, 6. hepatic iron overload, 7. acute COVID-19 infection, 8. post-COVID, 9. other cardiovascular condition.

Results: 435 patients underwent 0.55T scans (61 healthy volunteer, 16 acute COVID, 199 post-COVID, 159 of the other categorized diagnoses), 254/435 received contrast, and 110/435 patients underwent a comparison 1.5T scan. Table 1 provides native and post-contrast T1 values from the myocardium and blood, and ECV values, for 0.55T and 1.5T. At 0.55T, the average native myocardial T1 in healthy volunteers was 704ms, compared to 1028ms at 1.5T. T1 values were strongly correlated between field strengths (Figure 1A), and ECV measurements corresponded well with Bland-Altman bias = 0.84% (Figure 1B).

Importantly, elevated T1 indicative of clinically meaningful disease process was measurable at 0.55T (mean T1 >750ms, p< 0.05 by unpaired t-test versus healthy volunteers). Figure 2 provides example T1 maps in a patient with amyloidosis and histograms showing groups with higher T1 values at 0.55T and 1.5T.

Conclusion: Our study reports on normal T1 values at 0.55T and illustrates the application of 0.55T to detect elevated ECV and T1 in a clinically referred group of patients. This study adds to the growing literature supporting 0.55T CMR.

Table 1: T1 and ECV values measured at 0.55T and 1.5T.

0.55T T1 values (ms)
	Native		Post-contrast		ECV
	Myocardium	Blood	Myocardium	Blood
Healthy volunteers	704 ± 31 (61)	1141 ± 79 (61)	395 ± 82 (6)	304 ± 101 (6)	26 ± 4 (5)
MI (remote)	705 ± 37 (34)	1154 ± 63 (34)	354 ± 38 (32)	264 ± 30 (32)	27 ± 5 (32)
MI (infarcted)	559 ± 141 (25)	--	207 ± 69 (24)	--	64 ± 21 (24)
Cardiomyopathy	717 ± 36 (19)	1126 ± 102 (19)	380 ± 71 (14)	302 ± 154 (14)	26 ± 5 (14)
Acute inflammatory process	756 ± 69 (10) *	1157 ± 113 (10)	382 ± 47 (5)	302 ± 82 (5)	28 ± 6 (5)
Amyloid	774 ± 26 (4) **	1147 ± 46 (4)	253 ± 60 (3)	265 ± 15 (3)	54 ± 18 (3)
Hepatic iron overload	731 ± 41 (43)	1175 ± 83 (43)	--	--	--
Acute COVID-19	719 ± 18 (16)	1154 ± 82 (16)	348 ± 16 (15)	243 ± 25 (15)	27 ± 3 (15)
Post-COVID-19	714 ± 22 (199)	1123 ± 105 (197)	349 ± 31 (185)	240 ± 34 (184)	26 ± 4 (184)
Other cardiovascular condition	705 ± 28 (46)	1145 ± 75 (46)	362 ± 43 (30)	256±  48 (30)	24 ± 3 (30)
1.5T T1 values (ms)
	Native		Post-contrast		ECV
	Myocardium	Blood	Myocardium	Blood
Healthy volunteers	1028 ± 34 (14)	1592 ± 103 (14)	544 ± 19 (3)	413 ± 39 (3)	27 ± 3 (3)
MI (remote)	1027 ± 60 (33)	1598 ± 31 (33)	448 ± 68 (33)	319 ± 30 (33)	28 ± 6 (33)
MI (infarcted)	948 ± 28 (24)	--	272 ± 78 (24)	--	61 ± 18 (24)
Cardiomyopathy	1029 ± 51 (19)	1520 ± 136 (19)	439 ± 39 (15)	306 ± 38 (14)	26 ± 4 (14)
Acute inflammatory process	1093 ± 74 (9) *	1676 ± 132 (9)	441 ± 56 (9)	320 ± 80 (9)	32 ± 7 (9)
Amyloid	1139 ± 50 (3) *	1607 ± 43 (3)	296 ± 68 (3)	303 ± 21 (3)	54 ± 18 (3)
Hepatic iron overload	1012 ± 57 (43)	1572 ± 100 (43)	--	--	--
Acute COVID-19	--	--	--	--	--
Post-COVID-19	998 ± 23 (5)	1498 ± 79 (5)	450 ± 18 (5)	287 ± 17 (5)	24 ± 2 (5)
Other cardiovascular conditions	1027 ± 32 (40)	1608 ± 87 (39)	453 ± 34 (26)	298 ± 39 (26)	25 ± 2 (26)

Values are reported as mean ± standard deviation (number of measurements). *p< 0.05, **p< 0.005, unpaired t-test versus healthy volunteers.Figure 1: A) Correlation of myocardium native T1, myocardium post-contrast T1, and blood native T1 between 0.55T and 1.5T. Linear regression results are shown on plots. B) Bland-Altman plot comparing ECV at 0.55T and 1.5T.
Figure 2: A) T1 maps from a patient presenting with severe left ventricular hypertrophy and exertional dyspnea. At 0.55T, myocardial native T1 values were increased at 760-790 ms (vs normal 704ms) and the extracellular volume fraction was 64%. The patient was confirmed to have cardiac amyloidosis. B) Histograms (with logistic fit) of T1 values to demonstrate elevated T1 in patients with amyloid or acute inflammation.