Gain in Signal-to-Noise for First-Pass Contrast-Enhanced Abdominal MR Angiography at 3 Tesla Over Standard 1.5 Tesla Prediction with a Computer Model

Rationale and Objectives

To estimate the gain in signal-to-noise ratio (SNR) in first-pass contrast-enhanced (CE) abdominal magnetic resonance angiography (MRA) at 3.0 T compared with 1.5 T.

Materials and Methods

Three protocols were simulated using six contrast agents: gadopentetate dimeglumine (Magnevist, Berlex, Wayne, NJ), gadoteridol (Prohance, Bracco, Princeton, NJ), gadobenate dimeglumine (Multihance, Bracco, Princeton, NJ), gadodiamide (Omniscan, Amersham Health, Princeton, NJ), gadoversetamide (Optimark, Mallinckrodt, St. Louis, MO), and gadofosveset trisodium (MS-325, EPIX Medical, Cambridge, MA). Contrast concentrations were calculated for five abdominal vessels. Based on these data, the gain in SNR during CE abdominal MRA at 3.0 T over 1.5 T was estimated.

Results

In these simulations, peak concentrations in all five target vessels were about 5 mM, 10 mM, and 0.7 mM for protocol 1, protocol 2, and protocol 3, respectively. A gain in SNR at 3 T over 1.5 T during CE abdominal MRA of at least 94% in all five target vessels could be achieved by applying protocol 1 or protocol 2, whereas protocol 3 provided a gain in SNR of 70%.

Conclusions

Although five of the contrast agents studied fulfill the expectation of providing approximately twice the SNR at 3.0 T versus 1.5 T during CE abdominal MRA, MS-325 offers a gain in SNR of 70% only.

Magnetic resonance (MR) imaging at 3 Tesla (T) has gained substantial interest in recent years resulting in a market share of approximately 10% (summer of 2006) of installed MR systems within the United States. In addition, with the latest introduction of dedicated receiver coils, most of the standard MR examinations have become possible at 3 T leading to a growing interest for routine clinical imaging of the abdomen and pelvis ( ).

The main argument for investing in 3.0 T MR imaging systems is the desire for a greater signal-to-noise ratio (SNR). As the intrinsic SNR increases proportional with the magnetic field strength, the theoretical gain in SNR is expected to be twofold when compared with a 1.5 T MR system ( ). However, tissue parameters such as the longitudinal relaxation time T1 and transverse relaxation time T2 also affect the intrinsic SNR. Unfortunately, T1 relaxation times increase with magnetic field strength and T2 relaxation times may slightly decrease with magnetic field strength, with both effects having a negative impact on the gain in SNR at higher magnetic field strengths ( ). In addition, field strength related changes of the relaxivity values of MR contrast agents (a measure of their “strength”) need to be considered as these values decrease with magnetic field strength ( ). All these effects combined raise the question how much contrast-enhanced (CE) MR imaging at 3 T will benefit from the higher field strength.

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Materials and methods

Background MR Physics

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SNR∝B0VvoxelTADC−−−−−√SSEQ S

∝

where B 0 is the main magnetic field strength, V voxel is the volume of each voxel without interpolation, T ADC is the total time that the analog to digital converter samples data for the image, and S SEQ is the sequence signal expression that describes the contrast and signal properties of the specific pulse sequence used.

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SFLASH=sin()e−TE/T2∗(1−e−TR/T1)1−cos()e−TR/T1 S

sin

⁡

(

)

−

(

−

)

−

cos

⁡

(

)

−

where θ is the flip angle, TR is the repetition time, TE is the echo time, and T1 and T2 \* are the longitudinal and transverse relaxation time, respectively. For short TR and reasonably large θ , the FLASH signal becomes strongly T1-weighted. Essentially only tissues with short T1 values are able to recover enough magnetization between repetitions to generate an appreciable signal.

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1T1(C)=1T1(0)+RC 1

(

)

(

)

where C is the in vivo MR contrast agent concentration, R is the relaxivity of the MR contrast agent, T1 (0) is the baseline T1 relaxation time, and T1 ( C ) is the T1 relaxation time after the administration of the MR contrast agent. The equation for T2 as a function of concentration is identical except that T1 is replaced by T2 . Equations 2 and 3 can be combined to obtain the FLASH signal as a function of the contrast agent concentration.

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MR Contrast Agents and Perfusion Model

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Table 1

Relaxivity Values R1 and R2 and Range of Various Magnetic Resonance Contrast Agents in Plasma at Two Different Field Strengths ⁎

1.5 T 3.0 T R1 (mM second) −1 R2 (mM second) −1 R1 (mM second) −1 R2 (mM second) −1 Gadopentetate dimeglumine (Magnevist) 4.1 (3.9–4.3) 4.6 (3.8–5.4) 3.7 (3.5–3.9) 5.2 (4.3–6.1) Gadoteridol (Prohance) 4.1 (3.9–4.3) 5.0 (4.2–5.8) 3.7 (3.5–3.9) 5.7 (4.8–6.6) Gadobenate dimeglumine (Multihance) 6.3 (6.0–6.6) 8.7 (7.8–9.6) 5.5 (5.2–5.8) 11 (10–12) Gadodiamide (Omniscan) 4.3 (4.4–4.8) 5.2 (4.2–6.2) 4.0 (3.8–4.2) 5.6 (4.7–6.5) Gadoversetamide (Optimark) 4.7 (4.4–5.0) 5.2 (4.3–6.1) 4.5 (4.2–4.8) 5.9 (5.0–6.8) Gadofosveset trisodium (MS-325) 19 (18–20) 34 (32–36) 9.9 (9.4–10.4) 60 (56–64)

R1 and R2 relaxivity values indicate the efficiency in shortening the longitudinal relaxation time T1 and transverse relaxation time T2, respectively.

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Table 2

Three Different Magnetic Resonance Contrast Agent Injection Protocols for a Human Adult With a Body Weight of 70 kg Based on Vendor Recommendations ( )

Injection Protocol Dosage (mmol/kg) Injection Volume (mL) Injection Rate (mL/second) 1 0.1 14 2 2 0.2 28 4 3 0.03 15 0.5

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Estimation of the Gain in SNR

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Results

Peak Arterial Concentrations

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Table 3

Peak Arterial Concentrations Predicted by a Computer Model in Five Different Abdominal Vessels Using Three Different Magnetic Resonance Contrast Agent Injection Protocols ( Table 2 ) for a Human Adult With a Body Weight of 70 kg ( )

Peak Arterial Concentration (mM) Upper Abdominal Aorta Celiac Trunk Proper Hepatic Artery Superior Mesenteric Artery Renal Artery Injection protocol 1 5.49 4.45 4.33 5.09 4.47 Injection protocol 2 10.97 8.90 8.67 10.19 8.95 Injection protocol 3 0.73 0.72 0.72 0.73 0.72

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Calculation of the Gain in SNR at 3.0 T Over 1.5 T

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Table 4

Effect of Various Magnetic Resonance Contrast Agents on the Longitudinal Relaxation Time T1 (in milliseconds) and Transverse Relaxation Time T2 (in milliseconds) of Blood at 1.5 T and 3 T Exemplified in the Renal Artery

Injection Protocol 1.5 Tesla 3.0 Tesla T1 Time T2 Time T1 Time T2 Time Blood without contrast agent NA 1441 290 1932 275 Blood and Magnevist 1 53 42 59 37 Blood and Prohance 1 53 39 59 34 Blood and Multihance 1 35 24 40 19 Blood and Omniscan 1 50 37 54 35 Blood and Optimark 1 46 37 48 33 Blood and MS-325 ⁎ 1 12 6 22 4 Blood and Magnevist 2 27 22 30 20 Blood and Prohance 2 27 21 30 18 Blood and Multihance 2 18 12 20 10 Blood and Omniscan 2 26 20 28 19 Blood and Optimark 2 23 20 25 18 Blood and MS-325 ⁎ 2 6 3 11 2 Blood and Magnevist ⁎ 3 274 148 314 135 Blood and Prohance ⁎ 3 274 142 314 129 Blood and Multihance ⁎ 3 191 103 223 87 Blood and Omniscan ⁎ 3 264 139 294 130 Blood and Optimark ⁎ 3 245 139 266 127 Blood and MS-325 3 70 36 131 21

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Table 5

Gain in Signal-to-Noise Ratio in Five Abdominal Arteries as a Percentage of B 0 Gain

Injection Protocol Upper Abdominal Aorta Celiac Artery Proper Hepatic Artery Superior Mesenteric Artery Renal Artery Magnevist 1 95.8 95.4 95.3 95.7 95.4 Prohance 1 95.8 95.3 95.3 95.6 95.3 Multihance 1 95.0 94.7 94.6 94.9 94.7 Omniscan 1 97.1 96.7 96.7 97.0 96.7 Optimark 1 98.0 97.8 97.8 97.9 97.8 MS-325 ⁎ 1 78.6 79.5 79.5 79.0 79.5 Magnevist 2 97.0 96.7 96.7 96.9 96.7 Prohance 2 96.9 96.6 96.6 96.8 96.7 Multihance 2 95.3 95.4 95.4 95.3 95.4 Omniscan 2 98.0 97.8 97.7 97.9 97.8 Optimark 2 98.3 98.3 98.3 98.3 98.3 MS-325 ⁎ 2 70.8 74.0 74.4 72.0 74.0 Magnevist ⁎ 3 89.9 89.8 89.8 89.9 89.8 Prohance ⁎ 3 89.9 89.8 89.8 89.9 89.8 Multihance ⁎ 3 89.3 89.3 89.3 89.3 89.3 Omniscan ⁎ 3 91.9 91.8 91.8 91.9 91.8 Optimark ⁎ 3 94.0 93.9 93.9 94.0 93.9 MS-325 3 69.8 69.7 69.7 69.8 69.7

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Discussion

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Gain in SNR

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Limitations

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Conclusion

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References

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Gain in Signal-to-Noise for First-Pass Contrast-Enhanced Abdominal MR Angiography at 3 Tesla Over Standard 1.5 Tesla Prediction with a Computer Model

Rationale and Objectives

Materials and Methods

Results

Conclusions

Materials and methods

Background MR Physics

MR Contrast Agents and Perfusion Model

Estimation of the Gain in SNR

Results

Peak Arterial Concentrations

Calculation of the Gain in SNR at 3.0 T Over 1.5 T

Discussion

Gain in SNR

Limitations

Conclusion

References

Further Reading

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