An ablation system, comprising: an ablation catheter comprising: an elongate shaft with a distal portion; and at least one ablation element positioned on the ablation catheter shaft distal portion and configured to deliver energy to tissue; and a diagnostic catheter comprising: an elongate shaft comprising a distal end of the shaft defining a central axis, wherein the diagnostic catheter shaft is configured to slidingly receive the distal portion of the ablation catheter shaft; an expandable assembly mounted to the diagnostic catheter shaft and configured to transition from a compacted state to an expanded state; a plurality of dipole mapping electrodes coupled to the expandable assembly; and a plurality of ultrasound transducers coupled to the expandable assembly, wherein the expandable assembly comprises a plurality of bendable splines, each spline comprising a proximal end and a distal end, wherein each spline comprises a set of spaced dipole mapping electrodes and a set of spaced ultrasound transducers, characterized in that: each spline proximal end is fixedly attached at a location proximate the diagnostic catheter elongate shaft distal end, and each spline distal end is connected in a circumferential arrangement, wherein the circumferential arrangement defines an opening, and wherein the opening is relatively centered about the axis to receive a distal end of the ablation catheter. an ablation catheter comprising: an elongate shaft with a distal portion; and at least one ablation element positioned on the ablation catheter shaft distal portion and configured to deliver energy to tissue; and an elongate shaft with a distal portion; and at least one ablation element positioned on the ablation catheter shaft distal portion and configured to deliver energy to tissue; and a diagnostic catheter comprising: an elongate shaft comprising a distal end of the shaft defining a central axis, wherein the diagnostic catheter shaft is configured to slidingly receive the distal portion of the ablation catheter shaft; an expandable assembly mounted to the diagnostic catheter shaft and configured to transition from a compacted state to an expanded state; a plurality of dipole mapping electrodes coupled to the expandable assembly; and a plurality of ultrasound transducers coupled to the expandable assembly, wherein the expandable assembly comprises a plurality of bendable splines, each spline comprising a proximal end and a distal end, wherein each spline comprises a set of spaced dipole mapping electrodes and a set of spaced ultrasound transducers, an elongate shaft comprising a distal end of the shaft defining a central axis, wherein the diagnostic catheter shaft is configured to slidingly receive the distal portion of the ablation catheter shaft; an expandable assembly mounted to the diagnostic catheter shaft and configured to transition from a compacted state to an expanded state; a plurality of dipole mapping electrodes coupled to the expandable assembly; and a plurality of ultrasound transducers coupled to the expandable assembly, wherein the expandable assembly comprises a plurality of bendable splines, each spline comprising a proximal end and a distal end, wherein each spline comprises a set of spaced dipole mapping electrodes and a set of spaced ultrasound transducers, characterized in that: each spline proximal end is fixedly attached at a location proximate the diagnostic catheter elongate shaft distal end, and each spline distal end is connected in a circumferential arrangement, wherein the circumferential arrangement defines an opening, and wherein the opening is relatively centered about the axis to receive a distal end of the ablation catheter. each spline proximal end is fixedly attached at a location proximate the diagnostic catheter elongate shaft distal end, and each spline distal end is connected in a circumferential arrangement, wherein the circumferential arrangement defines an opening, and wherein the opening is relatively centered about the axis to receive a distal end of the ablation catheter.