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Published online before print October 29, 2004, 10.1148/radiol.2333032004
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(Radiology 2004;233:921-926.)
© RSNA, 2004

Technical Developments

Myocardial Infarction: Optimization of Inversion Times at Delayed Contrast-enhanced MR Imaging1

Ankur Gupta, MD, Vivian S. Lee, MD, PhD, Yiu-Cho Chung, PhD, James S. Babb, PhD and Orlando P. Simonetti, PhD

1 From the Department of Radiology–MRI, New York University Medical Center, 530 First Ave, New York, NY 10016 (A.G., V.S.L.); and Siemens Medical Solutions, Chicago, Ill (Y.C.C., J.S.B., O.P.S.). Received December 10, 2003; revision requested February 19, 2004; revision received February 23; accepted March 23. Supported by a Seed Grant from the Society of Thoracic Radiology and a New York University General Clinical Research Center Medical Student Grant. Address correspondence to V.S.L. (e-mail: vivian.lee@med.nyu.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
Seventeen patients underwent magnetic resonance (MR) imaging for myocardial viability with a protocol approved by the institutional review board and gave written informed consent. Breath-hold cine inversion-recovery segmented k-space true fast imaging with steady-state precession sequence, referred to as inversion time (TI) mapping, was performed to determine optimal TI for myocardial infarction inversion-recovery imaging. From TI mapping, optimal TI was 180–315 msec 10–15 minutes after administration of 0.15 mmol/kg of gadolinium-based contrast material. At that optimal TI, relative signal intensity of infarcted myocardium compared with uninfarcted myocardium was maximal (mean ± standard deviation, 297.8% ± 86.5), whereas signal-to-noise ratio of uninfarcted myocardium was minimal (4.5 ± 1.2). When applied to conventional myocardial infarction inversion-recovery imaging, optimal TI resulted in nulling of signal intensity of uninfarcted myocardium in all patients and in excellent conspicuity of infarcted myocardium in all nine patients with visible infarction.

© RSNA, 2004

Index terms: Magnetic resonance (MR), contrast enhancement • Magnetic resonance (MR), inversion recovery, 51.121413 • Myocardium, infarction, 51.771


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
Discussion
 REFERENCES
 
Delayed contrast material–enhanced magnetic resonance (MR) imaging is becoming an increasingly accepted modality for the diagnosis of myocardial infarction (1–19). The imaging technique is premised on the delayed enhancement and washout of extracellular contrast agents, such as gadolinium chelates, in infarcted myocardium relative to viable myocardium. About 10–30 minutes after injection of contrast material, the infarcted myocardium will appear slightly hyperintense relative to the uninfarcted myocardium; however, this difference may be difficult to detect at routine T1-weighted MR imaging. A critical advance in this field has been the implementation of an inversion-recovery T1-weighted approach whereby the time between the 180° inversion pulse and radiofrequency excitation, referred to as the inversion time (TI), is selected to null signal intensity (SI) from the uninfarcted myocardium (20). With SI from the uninfarcted or viable myocardium nulled, the hyperintense infarcted myocardium becomes conspicuous.

The selection of an optimal TI, referred to as TI0, to null the SI of the uninfarcted myocardium can critically affect the diagnostic value of the images that depict a myocardial infarction because the TI determines the relative SI of the infarcted and the uninfarcted myocardium (Fig 1) (15,20). Choosing the TI can be difficult because differences in the contrast agent dose, patient body habitus, cardiac function, and time after contrast agent administration can cause the optimal TI to vary considerably. Typically, a TI in the range of 200–350 msec is appropriate for images obtained at 10–20 minutes after administration of approximately 0.1–0.2 mmol per kilogram body weight of a gadolinium chelate (20). Nevertheless, because the optimal TI is not known a priori, a series of trial-and-error guesses at TI intervals of 25–50 msec usually precedes successful myocardial infarction MR imaging. Thus, the purpose of our study was to assess a TI mapping sequence that helps in the determination of the optimal TI for myocardial infarction MR imaging with a single–breath-hold acquisition in 17 patients.



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  		Figure 1. Top: Plot of SI versus TI for infarcted myocardium, uninfarcted (normal) myocardium, and blood pool for delayed contrast-enhanced inversion-recovery T1-weighted MR imaging. Because the magnitude data were collected, the recovery curve has the appearance of the data points above the x-axis. Image contrast varies depending on the TI used. Bottom: Short-axis MR images (2.9/1.3) corresponding to each TI are shown, with area of infarction (arrows) optimally visualized at TI0, the TI at which SI of uninfarcted myocardium is nulled. At TI–3, image contrast was reversed, with SI of infarcted myocardium appearing nulled and the SI of uninfarcted myocardium high. At TIs longer than TI0, the boundary between infarcted and uninfarcted myocardium became less well defined.

 

   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
TI Mapping Sequence
We used cine MR imaging with an inversion-recovery segmented k-space gradient-echo sequence that was based on the T1 quantification experiment reported by Scheffler and Hennig (21), with slight modifications (Fig 2). Following electrocardiographic triggering, an inversion pulse (180°) was applied. Then a true fast imaging with steady-state precession (FISP) readout was implemented with a segmented k-space approach, where each segment corresponded to a different TI, so that the acquired cine MR images traced the longitudinal relaxation of the tissues imaged (Fig 1). Note that because images were gated to the electrocardiogram, consecutive images also depicted systolic and diastolic changes in cardiac morphology during the cardiac cycle. Segments were centrically reordered to reduce sensitivity of image contrast to heartbeat variation. With centric reordering of the segments, the central lines of k-space were collected for all phases (all TIs) in the first heartbeat. In our experience, this reduced the effect of variability of heart rate on image contrast caused by differences in longitudinal recovery following the inversion pulse. Also, preparation pulses were performed before the initial acquisition to reduce oscillatory magnetization for steady-state imaging.



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  		Figure 2. Diagram of MR imaging with inversion-recovery true FISP pulse sequence for TI mapping. After electrocardiographic (ECG) triggering, an inversion-recovery (IR) pulse was applied. Then a true FISP readout was performed with a segmented k-space approach; each centrically encoded segment (15 lines in our implementation) corresponded to a different TI so that the acquired cine MR images (TI interval [{Delta}TI] = 43 msec) traced the longitudinal relaxation of the tissues. Preparation pulses before the initial acquisition reduced oscillatory magnetization for steady-state imaging. Data were collected during every other heartbeat to allow adequate longitudinal relaxation. TI0 was defined visually as the TI at which SI of uninfarcted myocardium was nulled.

 
Data were collected during every other heartbeat to allow adequate longitudinal relaxation. At 1.5-T MR imaging (Magnetom Symphony [Quantum Gradients]; Siemens, Erlangen, Germany) with maximum gradient amplitude of 30 mT/m and slew rate of 125 mT · m–1 · sec–1, parameters were as follows: repetition time msec/echo time msec, 2.9/1.3; flip angle, 50°; matrix, 104 x 192; lines per segment, 15; field of view, 263 x 350 mm; section thickness, 6 mm; temporal resolution, 43 msec; and acquisition time, 14 heartbeats. A total of 16–20 images were obtained, whereby TI of the first image was 100 msec, and then TI increased at 43-msec intervals. The TI interval of 43 msec (15 lines per segment) was selected to keep acquisition times within the time frame of a breath hold while a reasonable temporal resolution for determination of the optimal TI was maintained.

Study Subjects
Seventeen consecutive patients referred for cardiac viability MR imaging (12 men, five women; mean age, 58 years ± 12 [standard deviation]; range, 41–81 years) were imaged between June 2001 and February 2003, according to a protocol approved by the institutional review board. Patients provided written informed consent to be included in this study. Patients were referred for clinical MR imaging examination for either evaluation of myocardial viability on the basis of results at electrocardiographically gated stress-rest radionuclide examinations that suggested discrepancies between fixed perfusion defects and wall motion abnormalities (n = 11) or assessment of surgical candidacy in the setting of impaired myocardial contractility (n = 6).

MR Imaging Protocol
All patients were imaged in the supine position with a four-channel torso phased-array coil, and all sequences were electrocardiographically gated.

In addition to undergoing routine cine gradient-echo MR imaging for wall motion assessment (22), all subjects underwent contrast-enhanced MR imaging after receiving an intravenous bolus of 0.15 mmol/kg gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ). Approximately 10–15 minutes after injection, the inversion-recovery TI mapping true FISP sequence was performed in one midventricular short-axis section. The optimal TI was defined visually as the TI at which the SI of the uninfarcted myocardium was nulled (Figs 1, 2). Note that the optimal TI could be determined regardless of the presence or absence of myocardial infarction on the TI mapping images.

Within five minutes of the TI mapping sequence, myocardial infarction MR imaging was performed by using the selected optimal TI and a two-dimensional segmented k-space inversion-recovery true FISP sequence, with the following parameters: 700/1.4; flip angle, 50°; lines per segment, 33; matrix, 129 x 256; field of view, 262 x 350; and acquisition time for three sections, 12 heartbeats. At least nine short-axis images (8-mm section thickness with a 2-mm gap) were acquired from the left ventricular base to the apex, as well as in the horizontal and vertical long-axis planes.

MR Image Analysis
For each subject, TI mapping images were analyzed as follows. In identical locations on images at all TIs, left ventricular uninfarcted myocardium, infarcted myocardium, and blood pool SI were measured by one investigator (A.G.) who manually defined regions of interest for each cardiac region. The areas of the infarcted myocardium were visually identified as areas that had persistent hyperintensity at later TIs. For measurements of left ventricular uninfarcted myocardium, four regions of interest were delineated on a short-axis midventricular image in anterior, lateral, inferior, and septal myocardial regions, provided sufficient uninfarcted tissue was present. The SI values from these four regions of interest were averaged to obtain SI for the uninfarcted myocardium, or SINorm. The sizes of the regions of interest in the uninfarcted myocardium ranged from 20 to 50 mm2. A large region of interest of at least 300 mm2 was drawn in the intraventricular cavity for measurements of blood pool SI, or SIBlood. For measurements of the infarcted myocardium, two regions of interest of 20–50 mm2 were drawn. The two values were averaged to obtain the SI of the myocardial infarction, or SIInf. Noise was defined as the standard deviation of the SI measured in air outside the body and measured in consistent locations for each patient, with a region of interest of at least 300 mm2.

Signal-to-noise ratios (SNRs) for different cardiac regions were calculated by dividing each mean SI by the noise. The contrast-to-noise ratio for the infarcted myocardium compared with the uninfarcted myocardium was defined as (SIInf – SINorm)/Noise. The relative SI (expressed as a percentage) of the infarcted myocardium compared with the uninfarcted myocardium was defined as [(SIInf – SINorm)/SINorm] · 100. The relative SI (expressed as a percentage) of the blood pool compared with the uninfarcted myocardium was defined as [(SIBlood – SINorm)/SINorm] · 100.

Image analysis was also performed for the myocardial infarction inversion-recovery MR images by using the same methods as were described previously. Regions of interest for uninfarcted myocardium, infarcted myocardium, blood pool, and air were drawn as described previously and only on the short-axis sections that displayed the infarcted region. Data were then averaged across sections to obtain the mean SI values for the different cardiac components and were used to calculate SNR, contrast-to-noise ratio, and relative SI values as defined previously.

Data and Statistical Analysis
The SNRs for all TI values were plotted for each subject, and the SNR for the optimal TI was compared with the SNR of other TIs. TIs preceding the optimal TI, or TI0 (the point at which nulling of SI occurred), were labeled as TI–1, TI–2, TI–3, and so on, in consecutive order, beginning with the time closest to TI0. TIs subsequent to the optimal TI were labeled as TI+1, TI+2, and so on. SNR values were calculated for each TI with the methods described previously. Contrast-to-noise ratios and relative SI values were calculated and averaged at each inversion-recovery time point for the eight patients with visible myocardial infarction at TI mapping. All values were expressed as the mean plus or minus the standard deviation.

Repeated-measures mixed-model analysis of variance was conducted to characterize temporal changes in SNR, contrast-to-noise ratio, and relative SI and to compare the mean levels observed at each TI. The correlation structure introduced by the acquisition of multiple data points per subject was modeled by assuming the experimental errors to be exchangeable within each subject and independent between subjects. In this mixed-model context, the Dunnett procedure for making pairwise comparisons to a reference was used to compare the mean levels observed at each TI with the level at TI0, with that at TI–1, or with that at TI–2 (23). This procedure allowed multiple hypothesis tests to be conducted while the familywise type I error rate for the set of comparisons at or below the nominal 5% level was maintained.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
TI Mapping
Among 17 patients, myocardial infarction was observed on the one short-axis TI mapping section in eight. At visual inspection of the inversion-recovery TI mapping images, the optimal TI was unique for each patient and ranged from 180 to 315 msec (mean, 260.6 msec ± 38.4). Five patients had values outside the 200–300-msec range. Mean SNRs of the uninfarcted myocardium, the infarcted myocardium, and the blood pool at different TIs are reported in Table 1. For TI0, defined as the TI at which SI of uninfarcted myocardium appeared visibly nulled, quantitative measurements confirmed that the SNR for uninfarcted myocardium was at its minimum (mean, 4.5 ± 1.2) compared with that at other TIs. By using the Dunnett procedure to compare all TIs with TI0 while the familywise type I error rate for the set of comparisons at or below the nominal 5% level was maintained, the SNR for uninfarcted myocardium was significantly lower at TI0 than it was at every other TI except TI–1 and TI+1. That is, the SNR of uninfarcted myocardium at TI0 could not be statistically distinguished from that at either TI–1 (P < .232) or TI+1 (P < .417).


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  		TABLE 1. Mean SNRs for Tissue and Blood Pool at TI Mapping

 
The TIs at which the SNR for the infarcted myocardium and the blood pool reached their minima preceded TI0; the minimal SNR for the infarcted myocardium occurred at TI–2, while that of the blood pool occurred at TI–1.

The contrast-to-noise ratio for the infarcted myocardium compared with uninfarcted myocardium at TI0 was 16.1 ± 5.1. Figure 3 illustrates the relative SI of the infarcted myocardium compared with uninfarcted myocardium at various TIs. The mean maximal relative SI of the infarcted myocardium was 297.8% ± 86.5 and occurred at TI0, when SI of the uninfarcted myocardium was nulled. The mean relative SI of the infarcted myocardium compared with uninfarcted myocardium was 63.9% ± 48 at TI–1 and 174.6% ± 56.6 at TI+1. By using the Dunnett procedure to compare all time points with the TI0 while the familywise type I error rate for the set of comparisons at or below the nominal 5% level was maintained, relative SI of the infarcted myocardium compared with uninfarcted myocardium was significantly higher at TI0 than it was at any other time.



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  		Figure 3. Plot shows relative SI of infarcted myocardium compared with uninfarcted myocardium for varying TIs. Means with standard error bars were plotted. Mean maximal relative SI, 297.8% ± 86.5, occurred at TI0, whereas at TI–1 mean relative SI was 63.9% ± 48, and at TI+1 it was 174.6% ± 56.6. Relative SI of infarcted myocardium compared with uninfarcted myocardium was significantly higher at TI0 than at any other time (P < .05).

 
Myocardial Infarction Imaging
When implemented by using the optimal TI determined from the TI mapping sequence, the true FISP viability MR images demonstrated myocardial infarction in nine of 17 patients, eight of whom had a myocardial infarction that was visible on the one short-axis TI mapping section. Mean SNR, contrast-to-noise ratio, and relative SI values measured from the myocardial infarction MR images are reported in Table 2. The mean SNR of the uninfarcted myocardium for 17 patients was 5.0 ± 1.2, which was comparable with the measured mean value of 4.5 ± 1.2 at optimal TI at TI mapping. The mean relative SI of the infarcted myocardium compared with uninfarcted myocardium for the nine patients with infarction was 295.4% ± 98.2, which was also comparable with the mean relative SI at TI mapping (297.8% ± 86.5). Figure 4 illustrates the viability images obtained in one of these patients.


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  		TABLE 2. Mean SNR, Contrast-to-Noise Ratio, and Relative SI Values with Myocardial Infarction MR Imaging

 


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  		Figure 4a. (a) Horizontal and (b) vertical long-axis inversion-recovery true FISP MR images (700/1.4) of same subject illustrated in Figure 1. Images were obtained at 10-15 minutes after injection of 0.15 mmol/kg gadolinium-based contrast material with TI of 229 msec, which corresponded to optimal TI determined from the TI mapping sequence. Images show subendocardial infarction (arrows), which appeared slightly hyperintense relative to blood pool and markedly hyperintense relative to nulled SI of uninfarcted myocardium.

 


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  		Figure 4b. (a) Horizontal and (b) vertical long-axis inversion-recovery true FISP MR images (700/1.4) of same subject illustrated in Figure 1. Images were obtained at 10-15 minutes after injection of 0.15 mmol/kg gadolinium-based contrast material with TI of 229 msec, which corresponded to optimal TI determined from the TI mapping sequence. Images show subendocardial infarction (arrows), which appeared slightly hyperintense relative to blood pool and markedly hyperintense relative to nulled SI of uninfarcted myocardium.

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
REFERENCES
 
Inversion-recovery sequences provide a means of nulling the SI of the uninfarcted or viable myocardium to improve the conspicuity of the infarcted myocardium on delayed contrast-enhanced images. Selection of the optimal TI to achieve this nulling of the SI, however, can be challenging. With implementation of a single–breath-hold inversion-recovery true FISP cine MR imaging sequence for TI mapping, we can determine the optimal TI for use in myocardial infarction MR imaging. In the 17 patients in this study, optimal TI was 180–315 msec. Varying the TI by increments of 43 msec had a substantial effect on the relative enhancement of the myocardial infarction. For example, the mean relative SI of the infarcted myocardium compared with uninfarcted myocardium was 63.9% ± 48 at TI–1, 297.8% ± 86.5 at TI0, and 174.6% ± 56.6 at TI+1. While we did not attempt to quantify the spatial extent of the myocardial infarction at different TIs, we did observe anecdotally that at TIs that exceeded TI0, the edges of the infarction became less well defined (Fig 1) (15).

A phase-sensitive inversion-recovery technique recently has been described as an alternative approach to TI mapping, in which the shorter T1 of the infarcted tissue resulted in increased SI relative to the uninfarcted myocardium at all TIs (15). With this approach, image contrast is much less sensitive to TI, and the dependence of apparent size of the myocardial infarction at TI is reduced. Quantification of infarction size by using this method compared with the traditional approach with TI mapping remains to be performed.

Our work has recognized limitations. Our clinical sample size was relatively small (n = 17), particularly the number of subjects with a myocardial infarction (n = 9). Nevertheless, our results did achieve statistical significance. We defined the optimal TI visually and confirmed that the SNR values of the uninfarcted myocardium were minimal at TI0 on the TI mapping images. On the myocardial infarction inversion-recovery MR images, the degree of nulling of SI of the viable myocardium and the relative SI of the infarcted myocardium to uninfarcted myocardium were comparable to those found at TI0 at TI mapping. However, in this study we did not analyze the effects of optimal TI selection on myocardial infarction detection or quantification.

In our study, we implemented a true FISP sequence for myocardial infarction MR imaging to correspond to the technique used in TI mapping. The optimal TI determined from the true FISP TI mapping sequence may not correspond exactly to the optimal value for other myocardial infarction MR imaging sequences, such as turbo fast low-angle shot MR imaging (20), depending on the timing of the acquisition of the central lines of k-space with the particular sequence. Further investigation is needed to fine-tune these adaptations.

The sequence described reflects the first iteration in its development. The precision of the optimal TI determination was limited by the temporal resolution of 43 msec for the breath-hold TI mapping sequence. New methods with parallel imaging strategies (simultaneous acquisition of spatial harmonics, or SMASH [24], and sensitivity encoding, or SENSE [25]) and corresponding innovations in coil design will likely improve the temporal resolution of the TI mapping sequence and, consequently, the precision of optimal TI measurements.


   FOOTNOTES
 
Abbreviations: FISP = fast imaging with steady-state precession, SI = signal intensity, SNR = signal-to-noise ratio, TI = inversion time

Author contributions: Guarantors of integrity of entire study, A.G., V.S.L.; study concepts, A.G., V.S.L., Y.C.C.; study design, V.S.L., Y.C.C.; literature research, A.G., V.S.L., Y.C.C., O.P.S.; clinical studies, V.S.L.; data acquisition, A.G., V.S.L.; data analysis/interpretation, A.G., V.S.L., J.S.B.; statistical analysis, J.S.B.; manuscript preparation, editing, revision/review, and final version approval, all authors; manuscript definition of intellectual content, A.G., V.S.L.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 

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RADIOLOGY RADIOGRAPHICS RSNA JOURNALS ONLINE