Non-contrast-enhanced T1 -weighted MRI of myocardial radiofrequency ablation lesions

Abstract

Purpose: To demonstrate imaging of radiofrequency ablation lesions with non-contrast-enhanced T₁ -weighted (T1w) MRI.

Methods: Fifteen swine underwent left ventricular ablation followed by MRI using different preparations: endocardial or epicardial ablation of naïve animal, or endocardial ablation of animal with myocardial infarction. Lesion imaging was performed using free-breathing, non-contrast-enhanced, T1w sequence with long inversion time (TI). Also acquired were T₁ maps and delayed contrast-enhanced (DCE) imaging. Hearts were excised for ex vivo imaging, and sliced for gross pathology and histology.

Results: All ablations were visibly enhanced in non-contrast-enhanced T1w imaging using TI = 700 ms. T1w enhancement agreed with regions of necrosis in gross pathology and histology. Enhanced lesion cores were surrounded by dark bands containing contraction band necrosis, hematoma, and edema. In animals with myocardial infarction, chronic scar was hypointense in T1w, whereas acute ablations were enhanced, allowing discrimination between chronic scar and acute lesions, unlike DCE. Contrast was sufficient to create 3D volume renderings of lesions after minor postprocessing.

Conclusions: Non-contrast-enhanced T1w imaging with long TI promises to be an effective method for visualizing necrosis within radiofrequency ablation lesions. Enhancement is more specific and stationary than that from DCE. The imaging can be repeated as needed, unlike DCE, and may be especially useful for assessing ablations during or after a procedure. Magn Reson Med 79:879-889, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Keywords: RF ablation; T1-weighted imaging; cardiac MRI; lesion assessment; non-contrast-enhanced imaging.

FIG. 1

TWILITE pulse sequence. (a) Use of 2RR triggering to increase T₁ contrast. (b) The Simulated signal evolution for three tissues of interest.

FIG. 2

Signal (a) and contrast (b) obtained through simulation of the TWILITE sequence. For TI <700 ms (shaded region), the blood signal can be rectified in magnitude images, compromising the contrast with myocardium. Increasing TI above 700 ms slightly increases contrast between normal myocardium and lesion. LV blood suppression is strongest for lower TI values, and TI =700 ms was chosen as a good compromise.

FIG. 3

Typical lesion appearance from the 3D TWILITE sequence (TI =700 ms, no contrast agent), acquired 9 min (a) and 80 min (b) after septal LV ablation. Lesion core is enhanced and surrounded by a dark transition band of edema, hematoma, and CBN. Anatomy and treatment may be rendered in 3D using simple thresholding and pseudo-coloring (c), which may be useful for treatment monitoring, targeting, or navigation.

FIG. 4

Images acquired using different inversion times show the effects on contrast. Lesion core enhancement is seen on anterior wall (red circle) in this sagittal view. Shorter TI results in greater LV blood suppression, unless rectified, whereas higher TI slightly increases contrast between normal myocardium and blood.

FIG. 5

Consecutive slices from endocardial ablation study showing multiple lesions in the LV (opposite RV) and one in the RV (arrow). Native enhancement is visible even in thin RV wall. The top row is non-contrast-enhanced T1w with TI =700 ms; middle row is early-phase contrast enhancement with TI =300 ms, sequence starting 1 min after contrast injection; bottom row is the same 20 min later (DCE). Note the changing appearance of the lesions from early to later-phase contrast enhancement. A rotating 3D volume rendering of the T1w images from the top row is provided in Supporting Video S1.

FIG. 6

Epicardial lesions administered during an open-chest experiment. (a, b) Lesion core is identifiable with beige color, surrounded by a heterogeneous transition band tissue with dark red color, and a band of lighter grayish color, likely made up of CBN and edema. (c) Long-axis image from the proposed non-contrast-enhanced T1w technique shows enhanced lesion core and surrounding hypoin-tense band (arrow). LV blood is suppressed, facilitating identification of ablated tissue. (d) Contrast-enhanced image acquired on same slice. Animal was given a second dose of contrast agent and sacrificed shortly afterward. (e) Corresponding image from high-resolution ex vivo scan. (f) Volume rendering of the 3D slices gives good depiction of lesion cores and surrounding hypointense regions in anatomical context. A rotating view of this rendering is provided in Supporting Video S2.

FIG. 7

(a) Multiplanar reformat of images from previous figure, showing short-axis view through lesions in zoom inset of Figure 6b. (b) Photograph of tissue slice through same lesions, showing correspondence. (c, d) Zoom of subregion from (a) and (b). (e) Corresponding tissue slice after Masson’s trichrome stain. Lighter region indicates coagulation necrosis (CN) at lesion core; arrows denote transition band of CBN or hematoma.

FIG. 8

Survival experiment showing lesion enhancement over 21 days. Images were acquired weekly and animal was sacrificed on day 21. The image from day 0 appears more diastolic because of the lower heart rate. Lesion core enhancement, morphology, and size do not change appreciably during this period. The dark surrounding band dissipates during the first week. Similar results were obtained in all survival experiments.

FIG. 9

Images of infarcted animal before and after endocardial ablation. Two days before ablation: scar is hypointense in T1w (a), and enhanced in DCE (b). After ablation: (c) T1w with window level adjusted for viewing of normal and ablated myocardium, lesion core enhancement seen at approximately 11:00 in a region containing scar, and 5:00 in a normal region; (d) brightness increased to show hypointense regions of scar; (e) color map applied for better visualization of scarred (low dynamic range), normal, and ablated regions (high dynamic range). (f) T2w images acquired with TIRM sequence; (g) color map applied for better visualization of T₂ enhancement in scar (yellow/red), and lower, less-selective enhancement of lesion core and surrounding edematous area (green). (h) Early contrast enhancement showing scar regions enhanced and lesion cores unenhanced, whereas delayed imaging (i) shows contrast agent entering the lesion cores. (j) Ex vivo images corroborate scar and ablation locations.