Postinfarction myocardial scarring in mice: molecular MR imaging with use of a collagen-targeting contrast agent

Abstract

Purpose: To prospectively evaluate a gadolinium-based collagen-targeting contrast agent, EP-3533, for in vivo magnetic resonance (MR) imaging of myocardial fibrosis in a mouse model of healed myocardial infarction (MI).

Materials and methods: All procedures were performed in accordance with protocols approved by the animal care and use committee. MI was induced in eight mice by means of occlusion of the left anterior descending coronary artery followed by reperfusion. Four MR examinations were performed in each animal: one examination before, one examination 1 day after, and two examinations 6 weeks after the MI. For the latter two examinations, electrocardiographically gated inversion-recovery gradient-echo MR images were acquired before and serially (every 5 minutes) after the intravenous injection of either gadopentetate dimeglumine or EP-3533. The image enhancement kinetic properties of the postinfarction scar, normal myocardium, and blood were compared.

Results: Dynamic T1-weighted MR imaging revealed the washout time constants for EP-3533 to be significantly longer than those for gadopentetate dimeglumine in regions of postinfarction scarring (mean, 194.8 minutes +/-116.8 [standard deviation] vs 25.5 minutes +/- 4.2; P < .05) and in normal myocardium (mean, 45.4 minutes +/- 16.7 vs 25.1 minutes +/- 9.7; P < .05). Findings on postmortem histologic sections stained for collagen correlated well with EP-3533-enhanced areas seen on inversion-recovery MR images. Fifty minutes after EP-3533 injection, the postinfarction scar tissue samples, as compared with the normal myocardium, had a twofold higher concentration of gadolinium.

Conclusion: Use of the gadolinium-based collagen-targeting contrast agent, EP-3533, enabled in vivo molecular MR imaging of fibrosis in a mouse model of healed postinfarction myocardial scarring.

Figure 1a:

Midventricular short-axis IR gradient-echo MR images (7.1/3.0/430) of LV obtained in the same heart and at a similar location (a) 40 minutes after EP-3533 injection and (b) 15 minutes after gadopentetate dimeglumine injection. Binding of EP-3533 to collagenous scar tissue (arrows) after washout from the blood pool and from the normal myocardium enables accurate differentiation between scar tissue and blood and between scar tissue and normal myocardium. IR imaging with gadopentetate dimeglumine–induced delayed enhancement confirms the region of postinfarction scarring.

Figure 1b:

Midventricular short-axis IR gradient-echo MR images (7.1/3.0/430) of LV obtained in the same heart and at a similar location (a) 40 minutes after EP-3533 injection and (b) 15 minutes after gadopentetate dimeglumine injection. Binding of EP-3533 to collagenous scar tissue (arrows) after washout from the blood pool and from the normal myocardium enables accurate differentiation between scar tissue and blood and between scar tissue and normal myocardium. IR imaging with gadopentetate dimeglumine–induced delayed enhancement confirms the region of postinfarction scarring.

Figure 2a:

(a) Midventricular short-axis black-blood gradient-echo MR image (8/3.7, 20° flip angle) of the LV obtained 6 weeks after experimental MI shows thinning of the scarred anterior wall (arrows). (b–e) IR MR images (7.1/3.0/430) of the same section obtained (b) before and (c) 5 minutes, (d) 20 minutes, and (e) 35 minutes after EP-3533 injection show EP-3533 kinetic properties, including persistent enhancement of the scar (arrows). (c) Shortly after the injection, the blood pool, normal myocardium, and scarred tissue are enhancing. As time progresses, enhancement of the blood pool and the normal myocardium (septum and inferior wall) starts to diminish until the last frame (e), in which only the scar (LV anterior and lateral walls) remains strongly enhanced.

Figure 2b:

(a) Midventricular short-axis black-blood gradient-echo MR image (8/3.7, 20° flip angle) of the LV obtained 6 weeks after experimental MI shows thinning of the scarred anterior wall (arrows). (b–e) IR MR images (7.1/3.0/430) of the same section obtained (b) before and (c) 5 minutes, (d) 20 minutes, and (e) 35 minutes after EP-3533 injection show EP-3533 kinetic properties, including persistent enhancement of the scar (arrows). (c) Shortly after the injection, the blood pool, normal myocardium, and scarred tissue are enhancing. As time progresses, enhancement of the blood pool and the normal myocardium (septum and inferior wall) starts to diminish until the last frame (e), in which only the scar (LV anterior and lateral walls) remains strongly enhanced.

Figure 2c:

(a) Midventricular short-axis black-blood gradient-echo MR image (8/3.7, 20° flip angle) of the LV obtained 6 weeks after experimental MI shows thinning of the scarred anterior wall (arrows). (b–e) IR MR images (7.1/3.0/430) of the same section obtained (b) before and (c) 5 minutes, (d) 20 minutes, and (e) 35 minutes after EP-3533 injection show EP-3533 kinetic properties, including persistent enhancement of the scar (arrows). (c) Shortly after the injection, the blood pool, normal myocardium, and scarred tissue are enhancing. As time progresses, enhancement of the blood pool and the normal myocardium (septum and inferior wall) starts to diminish until the last frame (e), in which only the scar (LV anterior and lateral walls) remains strongly enhanced.

Figure 2d:

(a) Midventricular short-axis black-blood gradient-echo MR image (8/3.7, 20° flip angle) of the LV obtained 6 weeks after experimental MI shows thinning of the scarred anterior wall (arrows). (b–e) IR MR images (7.1/3.0/430) of the same section obtained (b) before and (c) 5 minutes, (d) 20 minutes, and (e) 35 minutes after EP-3533 injection show EP-3533 kinetic properties, including persistent enhancement of the scar (arrows). (c) Shortly after the injection, the blood pool, normal myocardium, and scarred tissue are enhancing. As time progresses, enhancement of the blood pool and the normal myocardium (septum and inferior wall) starts to diminish until the last frame (e), in which only the scar (LV anterior and lateral walls) remains strongly enhanced.

Figure 2e:

(a) Midventricular short-axis black-blood gradient-echo MR image (8/3.7, 20° flip angle) of the LV obtained 6 weeks after experimental MI shows thinning of the scarred anterior wall (arrows). (b–e) IR MR images (7.1/3.0/430) of the same section obtained (b) before and (c) 5 minutes, (d) 20 minutes, and (e) 35 minutes after EP-3533 injection show EP-3533 kinetic properties, including persistent enhancement of the scar (arrows). (c) Shortly after the injection, the blood pool, normal myocardium, and scarred tissue are enhancing. As time progresses, enhancement of the blood pool and the normal myocardium (septum and inferior wall) starts to diminish until the last frame (e), in which only the scar (LV anterior and lateral walls) remains strongly enhanced.

Figure 3:

Plots of mean signal-to-noise ratio (SNR) (seven hearts) for blood, normal myocardium, and scar tissue as a function of time after injection of gadopentetate dimeglumine (left) and EP-3533 (right) predominately indicate prolonged EP-3533 enhancement of the scar compared with the duration of gadopentetate dimeglumine enhancement. The plotted data also indicate greater enhancement in and slower washout from the normal myocardium after an injection of EP-3533 compared with these parameters after an injection of gadopentetate dimeglumine.

Figure 4:

Plots of mean CNR (n = 7) between scar tissue and normal myocardium and between scar tissue and blood as a function of time after EP-3533 injection. The maximal CNR between scarred tissue and blood and between scarred tissue and normal myocardium occurred 15 minutes after the injection, and the high CNR persisted for at least 50 minutes.

Figure 5:

Bar graph shows quantification of gadolinium concentration measured by using inductively coupled plasma mass spectrometry in scarred versus normal myocardium 50 minutes after injection of EP-3533. * = P < .05.

Figure 6a:

(a, b, d, e) Gradient-echo IR MR images and (c, f) corresponding picrosirius red–stained histologic sections of the LV. Arrows point to area of scarring. (a, d) Standard anatomic MR images acquired by using a double IR gradient-echo sequence (8.0/3.7/R-R interval). (b, e) Regions of contrast enhancement on midventricular short-axis MR images (7.1/3.0/430) of the LV at two section locations obtained 40 minutes after EP-3533 injection correlate closely with (c, f) photomicrographs of picrosirius red–stained tissue sections shown at nine times their original size.

Figure 6b:

(a, b, d, e) Gradient-echo IR MR images and (c, f) corresponding picrosirius red–stained histologic sections of the LV. Arrows point to area of scarring. (a, d) Standard anatomic MR images acquired by using a double IR gradient-echo sequence (8.0/3.7/R-R interval). (b, e) Regions of contrast enhancement on midventricular short-axis MR images (7.1/3.0/430) of the LV at two section locations obtained 40 minutes after EP-3533 injection correlate closely with (c, f) photomicrographs of picrosirius red–stained tissue sections shown at nine times their original size.

Figure 6c:

(a, b, d, e) Gradient-echo IR MR images and (c, f) corresponding picrosirius red–stained histologic sections of the LV. Arrows point to area of scarring. (a, d) Standard anatomic MR images acquired by using a double IR gradient-echo sequence (8.0/3.7/R-R interval). (b, e) Regions of contrast enhancement on midventricular short-axis MR images (7.1/3.0/430) of the LV at two section locations obtained 40 minutes after EP-3533 injection correlate closely with (c, f) photomicrographs of picrosirius red–stained tissue sections shown at nine times their original size.

Figure 6d:

(a, b, d, e) Gradient-echo IR MR images and (c, f) corresponding picrosirius red–stained histologic sections of the LV. Arrows point to area of scarring. (a, d) Standard anatomic MR images acquired by using a double IR gradient-echo sequence (8.0/3.7/R-R interval). (b, e) Regions of contrast enhancement on midventricular short-axis MR images (7.1/3.0/430) of the LV at two section locations obtained 40 minutes after EP-3533 injection correlate closely with (c, f) photomicrographs of picrosirius red–stained tissue sections shown at nine times their original size.

Figure 6e:

(a, b, d, e) Gradient-echo IR MR images and (c, f) corresponding picrosirius red–stained histologic sections of the LV. Arrows point to area of scarring. (a, d) Standard anatomic MR images acquired by using a double IR gradient-echo sequence (8.0/3.7/R-R interval). (b, e) Regions of contrast enhancement on midventricular short-axis MR images (7.1/3.0/430) of the LV at two section locations obtained 40 minutes after EP-3533 injection correlate closely with (c, f) photomicrographs of picrosirius red–stained tissue sections shown at nine times their original size.

Figure 6f:

(a, b, d, e) Gradient-echo IR MR images and (c, f) corresponding picrosirius red–stained histologic sections of the LV. Arrows point to area of scarring. (a, d) Standard anatomic MR images acquired by using a double IR gradient-echo sequence (8.0/3.7/R-R interval). (b, e) Regions of contrast enhancement on midventricular short-axis MR images (7.1/3.0/430) of the LV at two section locations obtained 40 minutes after EP-3533 injection correlate closely with (c, f) photomicrographs of picrosirius red–stained tissue sections shown at nine times their original size.

Figure 7:

Graph shows correlation between circumferential extent of scarring (in degrees) at EP-3533–enhanced MR imaging and circumferential extent of scarring on picrosirius red–stained histologic sections. Individual measurements (⋄) were performed at apical, midventricular, and basal locations in three hearts.