Gain in Signal-to-Noise for First-PassContrast-Enhanced Abdominal MR Angiography

at 3 Tesla Over Standard 1.5 Tesla: Predictionwith a Computer Model1

Elmar Max Merkle, MD, Brian Marshall Dale, PhD, Daniel Paul Barboriak, MD

Rationale and Objectives. To estimate the gain in signal-to-noise ratio (SNR) in first-pass contrast-enhanced (CE) ab-dominal 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), andgadofosveset trisodium (MS-325, EPIX Medical, Cambridge, MA). Contrast concentrations were calculated for five abdominalvessels. 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 forprotocol 1, protocol 2, and protocol 3, respectively. A gain in SNR at 3 T over 1.5 T during CE abdominal MRA of atleast 94% in all five target vessels could be achieved by applying protocol 1 or protocol 2, whereas protocol 3 provided again in SNR of 70%.

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

Key Words. Magnetic resonance angiography; 3 Tesla; contrast agent.© AUR, 2007

Magnetic resonance (MR) imaging at 3 Tesla (T) hasgained substantial interest in recent years resulting in amarket share of approximately 10% (summer of 2006) ofinstalled MR systems within the United States. In addi-tion, with the latest introduction of dedicated receiver

Acad Radiol 2007; 14:795–803

1 From the Department of Radiology (E.M.M., D.P.B.), Duke UniversityMedical Center, Box 3808, Duke North, Room 1417, Erwin Road,Durham, NC 27710; Physics and IDEA, Siemens Medical Solutions, Inc,MR R&D Collaborations, Cary, NC (B.M.D.). Received December 6, 2006;accepted March 13, 2007. Address correspondence to: E.M.M.e-mail: elmar.merkle@duke.edu

©

AUR, 2007doi:10.1016/j.acra.2007.03.007

coils, most of the standard MR examinations have be-come possible at 3 T leading to a growing interest for rou-tine clinical imaging of the abdomen and pelvis (1–10).

The main argument for investing in 3.0 T MR imagingsystems is the desire for a greater signal-to-noise ratio(SNR). As the intrinsic SNR increases proportional withthe magnetic field strength, the theoretical gain in SNR isexpected to be twofold when compared with a 1.5 T MRsystem (11). However, tissue parameters such as the lon-gitudinal relaxation time T1 and transverse relaxationtime T2 also affect the intrinsic SNR. Unfortunately, T1relaxation times increase with magnetic field strength andT2 relaxation times may slightly decrease with magnetic

field strength, with both effects having a negative impact

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on the gain in SNR at higher magnetic field strengths(12–14). In addition, field strength related changes of therelaxivity values of MR contrast agents (a measure oftheir “strength”) need to be considered as these valuesdecrease with magnetic field strength (15–20). All theseeffects combined raise the question how much contrast-enhanced (CE) MR imaging at 3 T will benefit from thehigher field strength.

Because maximum in vivo MR contrast agent concen-trations are seen during first-pass CE MR angiography(MRA), this type of MR examination seems to be well-suited to serve as a model to further elaborate this ques-tion. Therefore, the purpose of this study was to estimatethe gain in SNR in first-pass CE abdominal MRA at 3 Tcompared with 1.5 T using six different MR contrastagents, which are regularly used during CE MRA or seeka specific indication for CE MRA. Specifically, the MRcontrast agents evaluated are: gadopentetate dimeglumine(Magnevist; Berlex, Wayne, NJ), gadoteridol (Prohance;Bracco, Princeton, NJ), gadobenate dimeglumine (Multi-hance; Bracco), gadodiamide (Omniscan; AmershamHealth, Princeton, NJ), gadoversetamide (Optimark;Mallinckrodt, St. Louis, MO), and gadofosveset trisodium(MS-325; EPIX Medical, Cambridge, MA). These agentswere chosen both for their common off-label use in firstpass CE-MRA and because of available data on their re-laxivity properties (19).

MATERIALS AND METHODS

Background MR PhysicsIt is well known that the SNR in a MR image depends

critically on a variety of factors. A typical expression forthe SNR is (21):

SNR � B0 Vvoxel �TADC SSEQ (1)

where B0 is the main magnetic field strength, Vvoxel is thevolume of each voxel without interpolation, TADC is thetotal time that the analog to digital converter samples datafor the image, and SSEQ is the sequence signal expressionthat describes the contrast and signal properties of thespecific pulse sequence used.

CE-MRA is generally performed using a rapid 3Dspoiled steady-state pulse sequence like FLASH (FastLow-Angle SHot). The equation describing the signal

SFLASH from this pulse sequence is (21):

796

SFLASH �sin(�)e�TE⁄T2∗

(1 � e�TR⁄T1)

1 � cos(�)e�TR⁄T1(2)

where � is the flip angle, TR is the repetition time, TE isthe echo time, and T1 and T2* are the longitudinal andtransverse relaxation time, respectively. For short TR andreasonably large �, the FLASH signal becomes stronglyT1-weighted. Essentially only tissues with short T1 valuesare able to recover enough magnetization between repeti-tions to generate an appreciable signal.

Blood, as with most fluids, typically has a rather longT1 relaxation time, but this T1 time can be reducedthrough the use of MR contrast agents. The equation de-scribing T1 as a function of contrast agent concentrationis (19):

1

T1(C)�

1

T1(0)� RC (3)

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

MR Contrast Agents and Perfusion ModelAs mentioned previously, a total of six different MR

contrast agents were evaluated. The relaxivity values forthese six MR contrast agents for 1.5 T and 3.0 T are pro-vided in Table 1.

To generate concentration time curves in vascularcompartments, a multiorgan, whole-body pharmacokineticmodel of contrast agent distribution based on the work ofBae et al was used (22). In the published model, the per-meability of the vessels in each organ compartment tocontrast agent was assumed to be blood flow–limited, andthe permeability surface constant for each organ compart-ment was set to ten times the flow rate (KT Bae, personalcommunication). Organ and vessel blood volumes andflows were derived from standard physiology texts(23,24). For the current model, a similar procedure wasused, with the exception that the permeability surfaceconstant for vessels in the brain organ compartment wasassumed to be zero because of the blood-brain barrier.

MR contrast agents concentrations in five different ab-

Academic Radiology, Vol 14, No 7, July 2007 GAIN IN SNR AT ABDOMINAL MRA AT 3T OVER 1.5T

dominal arteries (suprarenal abdominal aorta, celiac ar-tery, proper hepatic artery, superior mesenteric artery, andrenal artery) were predicted based on this computermodel. Underlying assumptions were as follows: normalmale human adult, body weight 70 kg, height 1.73 m,blood volume 5 L, cardiac output 6.5 L/min, MR contrastagent administration via right antecubital vein. Contrastagent excretion rate was calculated as the product of therenal artery plasma concentration (assuming a hematocritof 45%) and the glomerular filtration rate (assumed to be19% of plasma flow). We assumed that all of the MRcontrast agents studied had similar permeability surfaceconstants and excretion rates—in the case of MS-325, anagent that binds reversibly to serum albumin; this as-sumption is supported by a recent study showing that im-mediately after bolus injection, most of the MS-325 is un-bound to albumin, and extravasates into tissues in a mannersimilar to other low-molecular-weight gadolinium-based con-trast agents (25). The model was implemented using JSim(National Simulation Resource, http://nsr.bioeng.washington.edu/PLN/Software).

Three different MR contrast agent injection protocolswere applied as recommended previously and described indetail in Table 2 (26–31). Specifically, injection protocol 1is 0.1 mmol/kg in 14 mL administered at 2 mL/second;this is a single dose for the Food and Drug Administra-tion (FDA)-cleared agents. Injection protocol 2 is 0.2mmol/kg in 28 mL administered at 4 mL/second, which isa double dose for the same agents. Injection protocol 3 is0.03 mmol/kg in 15 mL administered at 0.5 mL/second;this is a single dose for MS-325. Time-to-peak in the su-prarenal aorta was predicted for each contrast agent injec-tion protocol. At that time point, concentration values in

Table 1Relaxivity Values R1 and R2 and Range of Various Magnetic Reso

1.

R1 (mM second)�1

Gadopentetate dimeglumine (Magnevist) 4.1 (3.9–4.3)Gadoteridol (Prohance) 4.1 (3.9–4.3)Gadobenate dimeglumine (Multihance) 6.3 (6.0–6.6)Gadodiamide (Omniscan) 4.3 (4.4–4.8)Gadoversetamide (Optimark) 4.7 (4.4–5.0)Gadofosveset trisodium (MS-325) 19 (18–20)

R1 and R2 relaxivity values indicate the efficiency in shortening trespectively.

*All values taken from (19).

the five different target vessels were recorded.

Estimation of the Gain in SNRThe relaxation values used for blood at 1.5 T were T1 �

1,441 milliseconds and T2 � 290 milliseconds; the valuesfor blood at 3.0 T were T1 � 1,932 milliseconds and T2 �

275 milliseconds (14). Sequence parameters such as TRand TE were chosen to be as short as possible (TR 2.8milliseconds, TE 1.04 milliseconds, � 15°) and were de-rived from a vendor-optimized CE-MRA protocol (SiemensMedical Solutions, Erlangen, Germany). Equations 1–3were implemented and evaluated using Mathematica(Wolfram Research, Inc, Champaign, IL). These equationswere used to determine the SFLASH as a percentage of theequilibrium magnetization M0 at both 1.5 T and 3.0 T.The resulting signal curves were plotted as a function ofcontrast agent concentration. Based on these data, thegain in SNR at 3.0 T over 1.5 T was calculated as a per-centage of the gain in B0 and also plotted as a function ofcontrast agent concentration.

Errors from the relaxivity measurements propagatethrough equations 1–3 to cause errors in the SFLASH andgain curves. These propagated errors were estimated by

Table 2Three Different Magnetic Resonance Contrast Agent InjectionProtocols for a Human Adult With a Body Weight of 70 kgBased on Vendor Recommendations (23–28)

InjectionProtocol

Dosage(mmol/kg)

InjectionVolume (mL)

Injection Rate(mL/second)

1 0.1 14 22 0.2 28 43 0.03 15 0.5

e Contrast Agents in Plasma at Two Different Field Strengths*

3.0 T

2 (mM second)�1 R1 (mM second)�1 R2 (mM second)�1

4.6 (3.8–5.4) 3.7 (3.5–3.9) 5.2 (4.3–6.1)5.0 (4.2–5.8) 3.7 (3.5–3.9) 5.7 (4.8–6.6)8.7 (7.8–9.6) 5.5 (5.2–5.8) 11 (10–12)5.2 (4.2–6.2) 4.0 (3.8–4.2) 5.6 (4.7–6.5)5.2 (4.3–6.1) 4.5 (4.2–4.8) 5.9 (5.0–6.8)34 (32–36) 9.9 (9.4–10.4) 60 (56–64)

ngitudinal relaxation time T1 and transverse relaxation time T2,

nanc

5 T

R

he lo

calculating the difference between the curves obtained

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MERKLE ET AL Academic Radiology, Vol 14, No 7, July 2007

using the mean versus the minimum or maximum mea-sured relaxivity values given by Rohrer et al (19).

RESULTS

Peak Arterial ConcentrationsArterial concentrations for five abdominal vessels us-

ing three different MR contrast agent protocols are shownin Table 3. These concentrations were recorded at thesuprarenal aortic peak, which was seen at 14.0 secondsafter the initiation of contrast for the single- and double-dose approach (ie, contrast injection protocols 1 and 2,respectively). Using the MS-325 approach (injection pro-tocol 3), peak concentration within the suprarenal abdom-inal aorta was seen at 34.0 seconds after the initiation ofcontrast. Although peak arterial concentration using thesingle-dose approach was roughly 5 mM in all five ves-sels, this concentration doubled by using the double-dosecontrast regimen. Among the abdominal aortic branches,arterial concentration was highest within the superiormesenteric artery, followed by the renal artery, the celiacartery, and the proper hepatic artery. Using the MS-325protocol (injection protocol 3) resulted in arterial concen-trations of less than 1 mM in all five vascular territories.

Calculation of the Gain in SNR at 3.0 T Over 1.5 TApplying these concentrations as well as the relaxivity

values R1 and R2 for the various contrast agents at 1.5 Tand 3 T to Eq 3 resulted in a marked decrease of the T1relaxation time of blood as outlined in Table 4 (exempli-fied for the renal artery). The T1 time of blood showed abaseline increase of 491 ms with field strength (from 1.5 Tto 3 T). By adding the FDA-cleared MR contrast agentsusing injection protocols 1, 2, and 3, this differencedropped to 2–6 milliseconds, 2–3 milliseconds, and

Table 3Peak Arterial Concentrations Predicted by a Computer Model iMagnetic Resonance Contrast Agent Injection Protocols (Table

UpperAbdominal Aorta Celiac Trunk

Injection protocol 1 5.49 4.45Injection protocol 2 10.97 8.90Injection protocol 3 0.73 0.72

21–40 milliseconds, respectively. By administering MS-

798

325 using injection protocols 1, 2, and 3, this differencedropped to 10 milliseconds, 5 milliseconds, and 61 milli-seconds, respectively. Note in Table 4 that although allcontrast agents were simulated using all injection proto-cols, the combinations of agent and injection protocolmarked by an asterisk (*) are not used clinically nor arethey expected to be used clinically.

Equations 2 and 3 and the input parameters describedpreviously were used to calculate the signal at 1.5 T andat 3.0 T as a percentage of M0—the equilibrium longitu-dinal magnetization. The resulting signal curves wereplotted as a function of contrast agent concentration inFig 1 and 2. Note that most contrast agents follow curveswith similar shapes at 1.5 T and 3.0 T. Also note thatMS-325 performs quite differently at the two fieldstrengths.

Equations 1–3 were used to calculate the gain in SNRat 3.0 T over 1.5 T as a function of concentration for var-ious contrast agents, as shown in Fig 3. SNR gain is ex-pressed as a percentage of the gain in B0, assuming equalvoxel sizes and other sequence parameters as indicatedpreviously. In other words, if the actual field strengths are1.49 T and 2.89 T, then the gain in B0 is a factor of 1.94,and if the SNR gain in Fig 3 is 94.7%, then the actualgain in SNR would be 94.7% of 1.94 or a factor of 1.84.Note that at higher than approximately 2 or 3 mM, theSNR gain of most contrast agents is relatively indepen-dent of concentration. Notice also that MS-325 is a nota-ble exception to this trend. Note that although MS-325offered a high signal peak at lower concentrations at 1.5 T(Fig 1), it showed a markedly lower peak at 3.0 T (Fig 2),which in turn led to the relatively poor SNR gain at 3.0 Tover 1.5 T (Fig 3). MS-325 also demonstrated an optimalconcentration of approximately 4 mM, which was notreached by using the contrast injection protocol 3. The

e Different Abdominal Vessels Using Three Differentor a Human Adult With a Body Weight of 70 kg (22)

Arterial Concentration (mM)

ProperHepatic Artery

SuperiorMesenteric Artery Renal Artery

4.33 5.09 4.478.67 10.19 8.950.72 0.73 0.72

n Fiv2) f

Peak

propagated errors in signal and SNR gain from measure-

ntra

Academic Radiology, Vol 14, No 7, July 2007 GAIN IN SNR AT ABDOMINAL MRA AT 3T OVER 1.5T

ment errors in relaxivity were less than 0.5% for any clin-ical concentration.

Table 5 summarizes the SNR gain at 3.0 T over 1.5 Tfor the various MR contrast agents using three differentinjection protocols. However, instead of plotting them as

Table 4Effect of Various Magnetic Resonance ContrT1 (in milliseconds) and Transverse Relaxatioand 3 T Exemplified in the Renal Artery

Injection Proto

Blood without contrast agent NABlood and Magnevist 1Blood and Prohance 1Blood and Multihance 1Blood and Omniscan 1Blood and Optimark 1Blood and MS-325* 1Blood and Magnevist 2Blood and Prohance 2Blood and Multihance 2Blood and Omniscan 2Blood and Optimark 2Blood and MS-325* 2Blood and Magnevist* 3Blood and Prohance* 3Blood and Multihance* 3Blood and Omniscan* 3Blood and Optimark* 3Blood and MS-325 3

*This combination of injection protocol and co

Figure 1. Signal as a percentage of the equilibrium magnetiza-tion, M0, at 1.5 T. Signal is plotted as a function of contrast agentconcentration for various magnetic resonance contrast agents.The imaging parameters are repetition time 2.8 milliseconds, echotime 1.04 milliseconds, and � 15°.

a function of the contrast agent concentration as done

previously, here the concentrations are taken from Table 3.Note that in the range of contrast agent concentrations ex-pected clinically, there is less than a 1% variation in SNR

gents on the Longitudinal Relaxation Timee T2 (in milliseconds) of Blood at 1.5 T

1.5 Tesla 3.0 Tesla

T1 Time T2 Time T1 Time T2 Time

1441 290 1932 27553 42 59 3753 39 59 3435 24 40 1950 37 54 3546 37 48 3312 6 22 427 22 30 2027 21 30 1818 12 20 1026 20 28 1923 20 25 186 3 11 2

274 148 314 135274 142 314 129191 103 223 87264 139 294 130245 139 266 12770 36 131 21

st agent is not clinically applicable.

Figure 2. Signal as a percentage of the equilibrium magnetiza-tion, M0, at 3.0 T. Signal is plotted as a function of contrast agentconcentration for various magnetic resonance contrast agents.Note that most contrast agents follow curves with similar shapesat 1.5 T and 3.0 T. Also note that MS-325 performs quite differ-ently at the two field strengths (compare with Fig 1) (repetitiontime 2.8 milliseconds, echo time 1.04 milliseconds, � 15°).

ast An Tim

col

gain across all major vessels for a given contrast agent. Note

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MERKLE ET AL Academic Radiology, Vol 14, No 7, July 2007

also that these SNR gains are all higher than 94% with theexception of MS-325. Again, note in Table 5 that, althoughall contrast agents were simulated using all injection proto-cols, the combinations of agent and injection protocolmarked by an asterisk (*) are not used clinically, nor arethey expected to be used clinically.

DISCUSSION

The clinical use of contrast enhanced MRA continuesto expand rapidly (32). Over the past 10 years, major ad-vancements such as the introduction of time-resolvedMRA and parallel imaging have been made by focusingprimarily on improving the temporal resolution at the ex-pense of spatial resolution or SNR. Therefore, the intro-duction of 3 T whole-body MR systems has been metwith much anticipation because it promises twice the SNRcompared with standard 1.5 T MR systems. Unfortunately,in practice, this anticipated gain in SNR is less than twofoldbecause of the inevitable increase of the longitudinal relax-ation time T1 at higher field strengths; this increase is in theorder of 35%–40% at 3 T compared with 1.5 T (12–14).For various abdominal tissues, this effect has led to a gain inSNR at 3 T over 1.5 T ranging from only 1.16 to 1.76 onunenhanced three-dimensional T1-weighted MR imaging(33). A second effect that may further reduce the anticipatedSNR gain at 3 T for CE studies is the field strength–relateddecrease of the relaxivity values R1 and R2 of MR contrast

Figure 3. SNR gain as a function of concentration for variouscontrast agents. Signal-to-noise ratio (SNR) gain is expressed asa percentage of the gain in B0. Note that at higher than approxi-mately 2 or 3 mM, the SNR gain of most contrast agents is rela-tively independent of concentration. Notice also that MS-325 is anotable exception to this trend (repetition time 2.8 milliseconds,echo time 1.04 milliseconds, � 15°).

agents. Both effects combined raise the question whether 3

800

T will actually deliver the anticipated benefit during CE MRimaging such as CE MRA. Although the primary purpose ofour work was not to compare the performance of variouscontrast agents, but rather to examine in detail the predictedimpact of a higher main magnetic field strength on CE-MRA (1.5 T versus 3 T), it is difficult to talk about CEwithout discussing the contrast agents themselves at least tosome degree. Nevertheless, it must be clearly stated here thatit is not the purpose of this article to compare these contrastagents in detail.

Gain in SNRUsing three different contrast injection protocols, a

total of six MR contrast agents were studied. Althoughnone of these MR contrast agents is currently approvedby the FDA for CE-MRA, five agents are approved forother indications and are widely used off-label in theUnited States for CE-MRA. MS-325, on the other hand,is currently not approved for any indication by the FDA,but is seeking specific approval for CE-MRA.

The applied perfusion model revealed peak concentra-tions in the suprarenal aorta at 14.0 seconds after the ini-tiation of contrast for the single and double dose approach(ie, contrast injection protocols 1 and 2, respectively).This is in line with data from Riederer et al (34), whoreported mean bolus arrival times of 14.1 � 3.5 secondsin the renal arteries in 145 cases using a contrast injectionprotocol similar to our injection protocols 1 and 2. Usingthe MS-325 approach (injection protocol 3), peak concen-tration within the suprarenal abdominal aorta was seen at34.0 seconds after the initiation of contrast because of thesubstantially slower infusion rate. Peak concentrations in allfive abdominal target vessels were about 5 mM, 10 mM,and less than 1 mM for contrast injection protocols 1 (singledose), 2 (double dose), and 3 (MS-325), respectively.

All contrast agents were simulated using all injectionprotocols. However, in clinical practice, the current FDA-cleared agents will be used fundamentally differently thanintravascular agents such as MS-325; therefore, the indi-cated results (*) in Tables 4 and 5 should be understoodas a theoretical exercise and not as any kind of a predictionof clinical results. However, despite this caveat, it is clearthat the field strength–related T1 increase (T13.0T–T11.5T) wasalmost completely erased by applying the single- and dou-ble-dose approach, and markedly reduced by applying theMS-325 approach (Table 4), irrespective of whether onlyclinically applicable combinations of injection protocol and

contrast agent or all combinations are considered.

clinic

Academic Radiology, Vol 14, No 7, July 2007 GAIN IN SNR AT ABDOMINAL MRA AT 3T OVER 1.5T

By basically eliminating the negative impact of theelongated baseline T1 time through using the single- anddouble-dose approach, our analysis showed at least a 94%gain in SNR at 3 T over 1.5 T during CE abdominalMRA in all five target vessels (Table 5) except with MS-325, which would not be used clinically with these injec-tion protocols. This is in line with findings reported re-cently by Edelman et al and Allkemper, who both mea-sured a mean SNR gain of 2.15 in the abdominal aortaduring first-pass, three-dimensional, T1-weighted gradientecho MR imaging (33,35). Our findings are also in linewith reports from the neuroradiologic literature where cere-bral areas with the highest degree of contrast enhancementbenefited the most from the higher field strength in terms ofSNR gain (17,20,36). In Fig 4, we have included an exam-ple of two CE-MRA images, one at 1.5 T and one at 3 T,acquired with similar sequence parameters. The SNR (mea-sured on the source images) was higher by a factor of 1.87,which is almost exactly in line with the predicted factor of1.84 expected for 94.7% (Table 5) of the actual factor of1.94 change in field strength.

Although five contrast agents demonstrate a very similarbehavior in terms of T1 shortening and SNR gain at 3 T,MS-325 appears different in several ways (all figures andtables): first, its relaxivity value R1 at 1.5 T is more than

Table 5Gain in Signal-to-Noise Ratio in Five Abdominal Arteries as a P

InjectionProtocol

UpperAbdominal Aorta Celiac A

Magnevist 1 95.8 95.4Prohance 1 95.8 95.3Multihance 1 95.0 94.7Omniscan 1 97.1 96.7Optimark 1 98.0 97.8MS-325* 1 78.6 79.5Magnevist 2 97.0 96.7Prohance 2 96.9 96.6Multihance 2 95.3 95.4Omniscan 2 98.0 97.8Optimark 2 98.3 98.3MS-325* 2 70.8 74.0Magnevist* 3 89.9 89.8Prohance* 3 89.9 89.8Multihance* 3 89.3 89.3Omniscan* 3 91.9 91.8Optimark* 3 94.0 93.9MS-325 3 69.8 69.7

*This combination of injection protocol and contrast agent is not

three times higher than the R1 value of any other contrast

agent (Table 1), allowing for an exceptional performanceat doses in the range of 1 mM (Fig 1). However, the R1value of MS-325 at 3 T shows an almost 50% reductionover the R1 value at 1.5 T compared with a drop of onlyapproximately 10% in the R1 values of all the other fivecontrast agents (19). Nevertheless, the R1 value of 9.9(mM second)�1 for MS-325 is still the highest at 3 T al-lowing for a good performance, particularly at lowerdoses (Fig 2). The R1 at 3 T is only low compared withthe value at 1.5 T (Table 1), leading to a poor SNR gainat 3 T (Table 5, Fig 3) despite its good performance at 3T (Table 4, Fig 2). In addition to the high R1 value, MS-325 also has a R2 value that is four to five times higherthan the R2 of any other contrast agent. This high R2value leads to the decreasing signal as a function of con-centration for values greater than about 4 mM (Fig 1, 2).The gain in SNR at 3 T during CE abdominal MRA com-pared with 1.5 T through the administration of MS-325 isabout 70% only, thus being markedly lower than all theother contrast agents (Fig 3). This has two main reasons:first, MS-325 performance at 1.5 T is exceptional, whichbasically increases the denominator in the “gain in SNR”calculation. Second, the 50% drop of its relaxivity valueR1 at 3 T comes into play, which adversely affects thenumerator. Both effects combined explain the lower gain

ntage of B0 Gain

ProperHepatic Artery

SuperiorMesenteric Artery Renal Artery

95.3 95.7 95.495.3 95.6 95.394.6 94.9 94.796.7 97.0 96.797.8 97.9 97.879.5 79.0 79.596.7 96.9 96.796.6 96.8 96.795.4 95.3 95.497.7 97.9 97.898.3 98.3 98.374.4 72.0 74.089.8 89.9 89.889.8 89.9 89.889.3 89.3 89.391.8 91.9 91.893.9 94.0 93.969.7 69.8 69.7

ally applicable.

erce

rtery

in SNR at 3 T of only 70% in comparison to the other

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MERKLE ET AL Academic Radiology, Vol 14, No 7, July 2007

contrast agents, which show a gain of approximately95%. This gain in SNR for MS-325 can be increased byroughly another 10% by cutting the injection time in halfas recently suggested by Hartmann et al (37).

LimitationsThree main limitations need to be mentioned in our

study design. First, we did not estimate the gain in con-trast-to-noise levels between the abdominal blood vesselsand the surrounding tissues. This parameter is generallyconsidered to be the single most important factor in termsof image quality. Fortunately, it has been shown in ani-mal studies that the gain in SNR during CE-MRA of theabdominal aorta correlates very well with the gain in con-trast-to-noise levels (35). Furthermore, background sup-

Figure 4. Clinical examples of first pass abdominal magnetic resosingle dose (0.1 mmol/kg, injection protocol 1) of Multihance. (a) Amarks the transplant renal artery) imaged at 1.5 T. Signal-to-noisemale with a history of renal artery stenosis and stenting on the righabdominal aorta is 39.2.

pression of the surrounding tissues during first-pass CE

802

MRA will most likely be even better at 3 T because ofthe longer T1 times and better fat suppression.

Assuming identical MRA sequence protocols at 1.5and 3 T represents a second main limitation, because thisdoes not reflect clinical practice (38). In particular, theflip angle at 1.5 T CE MRA will be higher, whereas thereceiver bandwidth will be lower. Both of these protocolchanges will have an impact on the SNR, as can be deter-mined by Eq 1 and 2. However, the purpose of this studywas to estimate the gain in SNR at 3 T compared with1.5 T with a focus on the impact of different MR contrastagents, and therefore the introduction of additional vari-ables would have caused more confusion than clarity.Additionally, the results of this study indicate that the fullbenefit of 3 T SNR gain is readily achievable even with-

e angiography at 1.5 T (a) and 3 T (b) after administration of aear-old male with a renal transplant in the right iliac fossa (arrowin the suprarenal abdominal aorta is 21.0. (b) A 59-year-old fe-e (arrow) imaged at 3 T. Signal-to-noise ratio in the suprarenal

nanc44-y

ratiot sid

out 3 T–specific protocol optimization.

Academic Radiology, Vol 14, No 7, July 2007 GAIN IN SNR AT ABDOMINAL MRA AT 3T OVER 1.5T

Finally, the dynamic change in contrast agent concen-tration over time in the target vessels has been ignored.However, these dynamic changes are affected by individ-ual factors (eg, cardiac output) and seem to be fairly in-dependent of the magnetic field strength. Thus an intrain-dividual estimation of the gain in MR signal at 3 T over1.5 T should not be influenced substantially.

CONCLUSION

Compared with unenhanced MR imaging, contrast-en-hanced sequences will benefit more from the higher fieldstrength of 3 T because gadolinium-based contrast agentseffectively function to counteract the baseline T1 increase.This effect becomes most obvious during first-pass CEMRA where the highest contrast agent concentrations areachieved. The five FDA-cleared contrast agents studied allfulfill the expectation of providing approximately twice theSNR in first-pass CE-MRA at 3 T versus 1.5 T.

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