The heart is a 3-dimensional structure, yet nearly all cardiac imaging performed in animals relies on 2-dimensional imaging techniques such as radiography, fluoroscopy, and ultrasonography. The advancement of 3-dimensional imaging modalities over the past decade has been dramatic and techniques such as 3-dimensional echocardiography, cardiac computed tomography (cCT), and cardiac magnetic resonance imaging (CMR) allow an understanding of cardiac anatomy and an evaluation of cardiac function previously impossible to achieve. Within the vasculature, cross- sectional imaging is similarly transformative with computed tomography angiography (CTA) and magnetic resonance angiography (MRA) allowing detailed characterization of vascular malformations such as portocaval communications1, vascular ring anomalies2, and arterial-venous fistulae3. The focus of this lecture is to discuss the technical aspects of cCT, CTA, CMR, and MRA with an intent to highlight current and future capabilities of these modalities and to inform the audience on how to achieve optimal cross-sectional imaging of the heart and vasculature. Clinical cases will be used as examples.
Cardiac CT involves the timed injection of iodinated contrast into a peripheral vein, followed by image acquisition when the contrast reaches the cardiac structures of interest. In a high motion organ, such as the heart, many technical factors play an important role in optimizing image quality.4 These factors include the type of CT scanner, spatial and temporal resolution, gating and acquisition mode, and contrast administration.
Early generation CT scanners lacked sufficient spatial and temporal resolution to characterize intracardiac anatomy. The development of multi-detector CT (MDCT), also known as multi-slice CT, and cardiac gating improved both spatial and temporal resolution such that cCT is now clinically feasible. A detector is the portion of the scanner that receives the photons generated by the X-rays passing through the patient. A single detector is comprised of many elements along its circumference, but is limited to a narrow width in order to resolve structures of interest. Multiple detectors allow the radiation effect to be collected along a broader slab width, known as the z-axis, which increases the slab thickness that can be scanned at one time. The first generation MDCT scanners used 4 detectors to improve z-axis thickness, with subsequent generations increasing to 8, 16, 64, 128, 320, and even 600 detectors. A MDCT with 320 detectors can typically cover the z-axis of a human heart allowing the entire heart to be evaluated in one rotation of the CT gantry. It should be noted that the length of the z-axis that can be imaged by a given scanner is dependent on both the number of detectors as well as the width of each detector such that two different 64-slice scanners may not have the same z-axis coverage. As an example, a 64-slice scanner with detectors of 1.25 mm width could cover a z- axis thickness of 80 mm while a 64-slice scanner from another manufacturer with detectors of 1.0 mm width would yield a z-axis thickness of 60 mm with each gantry rotation. Both the width in the z-axis that can be covered by each rotation as well as the speed of gantry rotation impact the final image quality of the scanner, as will be discussed below.
Spatial resolution is defined as the ability to resolve structures that lie next to one another on an image. Spatial resolution is predominately affected by the size of the detector and the spacing between detectors; the detector must be smaller than an object to resolve it and detectors must be sufficiently close together to resolve two objects that lie close to one another in the patient. In cCT, the z-axis spatial resolution is equally important to allow reconstruction of the image in all planes without distortion, known as isotropic resolution, and for this reason MDCT scanners with a high number of detectors are desired to optimize cCT imaging. Spatial resolution is also improved by advancements in image detector technology and reconstruction software. New high-definition scanners are available that can achieve spatial resolution in the x- and y-axes to 0.23 mm using novel detector material and advanced reconstruction algorithms. As dogs and cats have smaller hearts than humans, spatial resolution of small structures (e.g., coronary vessels) remains a challenge in these species with all but the newest generation of scanners.
ECG-gated cardiac CT. The dog’s ECG is seen along the top of the image with the numbers and boxes representing the slices acquired during that cardiac cycle. Retrospective gating allows the images to be acquired at the same point in each cardiac cycle (here in early diastole). The image is constructed in the z-axis by stitching together each slab of data, ̃ 45 slices per slab with the 7 slabs together creating an image of the entire heart.
Temporal resolution describes the ability of the scanner to resolve a structure moving in time and is best measured as the time required to acquire successive images. Temporal resolution is dependent on gantry rotation speed, mode of image acquisition, gating, and pitch. The fastest scanners currently on the market can perform a full gantry rotation in ̃ 250 msec, which means that they can resolve an image in roughly 125 ms since a CT image is created/extrapolated from 1⁄2 of a gantry rotation, as the x-ray source moves from 0 degrees to 180 degrees. Adding an additional X-ray source placed 90 degrees to the other is one strategy to minimize the time required to circumferentially rotate around the patient and improve temporal resolution. These dual-source CT scanners have two unique x-ray sources (often of different energy spectra) as well as 2 sets of detectors and need to only rotate 90 degrees in order to acquire a half gantry rotation, reducing temporal resolution down to 66 msec. The mode of acquisition refers to prospective or retrospective image acquisition/reconstruction and will be discussed in the section on gating. Pitch is defined as the distance the table advances per gantry rotation divided by the total thickness of the acquired slab (in the z-axis). With MDCT, the thickness of the slab is the sum of the width of all detectors activated during acquisition. A pitch greater than 1 implies gaps in the z-axis coverage as the table moved too rapidly to allow the scanner to image every slab in the field of view. Some degree of overlap in the z-axis is preferred to adequately reconstruct images of the heart. The rapid heart rates of dogs and cats require excellent temporal resolution to achieve high-quality cCT imaging.
Gating is used to time the acquisition sequences to the cardiac cycle, thereby freezing cardiac motion in the acquired image series (Figure 1). The animal’s ECG is used to reconstruct (retrospective) or trigger (prospective) the scan. With retrospective gating, the entire cardiac cycle is scanned (usually several cardiac cycles) and the desired image reconstructed from the raw dataset based on the simultaneous ECG. This allows for reconstruction of different phases of the cardiac cycle and for dynamic analysis (Figure 2). However, retrospective gating increases radiation exposure to the patient. Prospective gating triggers the scanner to acquire images only during a portion of each cardiac cycle, typically when cardiac motion is least apparent such as mid-diastole or end-systole (for higher heart rates). This is known as a move-and-shoot technique; the scanner is only activated for small windows of time and a single portion of the cardiac cycle is recorded, limiting radiation exposure but preventing dynamic analysis. Respiratory gating, or subjecting the animal to a breath-hold or apneic period during the scan, is also frequently employed to avoid movement during scan acquisition. The ability of the scanner to appropriately gate the study relates to the temporal resolution of the scanner – at fast heart rates the scanner may not be able to acquire the images rapidly enough to adequately resolve the desired image.
Dynamic cardiac CT with retrospective gating of the left atrium and left ventricle from a dog with mitral valve dysplasia. Note the change in chamber dimensions throughout the cardiac cycle, divided into 10% increments from the start of one R wave to the next.
Cardiac MRI shares many similarities with cCT in that both allow cross-sectional imaging and reconstruction of the heart in multiple planes. The advantages of CMR over cCT relate to the lack of ionizing radiation and the ability to view the blood pool without intravenous contrast administration. Additionally, tissue characteristics can be obtained by CMR (e.g., fibrosis, fat infiltration) that cannot be detected by cCT. The downside to CMR relates to the lower spatial and temporal resolution as compared to new generation cCT scanners, as well as the requirement for general anesthesia and longer scanning times for the patient. The cost and technical expertise required to have a proficient CMR program also exceeds that for cCT.
Technical Aspects of CMR in Animals
The physics of CMR are different than that for cCT. In brief, radiofrequency (RF) pulses are transmitted into the body to alter the state of protons that are aligned to a strong magnetic field. The RF pulses change the alignment and phase of the spinning protons within the body; as they return to their base state after the RF pulse passes, they emit small bursts of RF energy that are received by coils placed around the body. These signals are received by the coils and used to generate an image; because the rate of relaxation and phase realignment differs depending on the tissue type, images of the patient’s tissue composition can be generated.
Cardiac MRI is most often performed with magnets of 1.5 Tesla and 3 Tesla field strengths with phased array coils placed around the thorax to both transmit and receive the RF pulses that will generate the image. Most CMR has been performed on 1.5 T magnets, though higher field strength magnets are becoming more popular for neurologic and musculoskeletal imaging. While 3 T magnets are the “new generation,” the transition to 3 T imaging is not comparable to the advancements in cCT – newer is not always better. The higher field strength of a 3 T magnet provides a better signal-to-noise ratio and quicker scan times, but increased artifacts and more challenging sequence optimization are difficulties for 3 T CMR imaging.5
The standard CMR image sequence for determining cardiac anatomy is 3-dimensional steady-state free precession (SSFP) imaging.6 This sequence provides a bright blood pool with good endocardial border detection and can be
acquired throughout the cardiac cycle to allow cine imaging and full chamber volume characterization. This can either be performed during induced apnea since the animal is under general anesthesia, or there are respiratory gating mechanisms that can monitor the diaphragm and control for respiratory motion. Similar to cCT, ECG-gating can be performed to acquire slices of the heart throughout the cardiac cycle. Cardiac MRI also brings novel ability to characterize tissues, with sequences for determination of tissue fibrosis (late gadolinium enhancement or T1 mapping) as well as tissue edema (T2 mapping).6 Phase-contrast sequences can be used to evaluated flow and pressure gradients6 – in the author’s experience, these are challenging to perform accurately in animals. Last, the stiffness of the myocardium can be evaluated by magnetic resonance elastography – where sound waves are introduced into the patient and their propagation monitored and analyzed by the MR system.7 Examples will be provided from clinical canine cases.
CT AND MR ANGIOGRAPHY
Both CTA and MRA have been employed to evaluate vascular structures in animals. The benefit of cross-sectional imaging such as CTA or MRA is the avoidance of selective central catheterization as only peripheral venous access is required for these techniques. Most of the published literature on these modalities in animals has focused on anatomic delineation of portosystemic shunts, though they can be used to investigate any vascular structure. Timing of the contrast administration relative to scan acquisition of the structure of interest is both challenging and paramount to obtaining a useful imaging study. Various protocols for contrast timing are available, utilizing either a test bolus scan to precisely measure the time that contrast arrives at the structure of interest and then adjusting scan time to this interval, or using a machine-specific monitoring scan that initiates the full scan once an increase in radiodensity is noted in the structure being monitored (e.g., bolus-tracking). Unique to MRA, imaging techniques are available to visualize blood clearly on the image without the administration of contrast. Contrast agents may still be used and are preferred for some studies (such as perfusion scans), but may not be necessary for anatomic delineation.
Unrelated to cCT or CMR, a new cross-sectional imaging modality is rotational angiography. Rotational angiography is performed in the catheterization laboratory with new generation fluoroscopy systems that can rapidly rotate around the patient during contrast administration and create a 3-dimensional (CT-like) image of the patient’s anatomy.8 The benefits of this technique are that injections from multiple image planes need not be recorded, thereby reducing contrast load, and improved anatomic delineation of the structure of interest can be viewed during the interventional procedure – optimizing image guidance.
Once cross-sectional imaging of the heart is obtained, either by cCT or CMR, numerous software packages are available to reconstruct the anatomy in nearly any conceivable plane or as a 3-dimensional projection. This dramatically improves understanding of spatial relationships or intracardiac defects and can be useful for planning surgical or transcatheter interventions. Additionally, these datasets can be transferred to a 3-D printer to provide hands-on objects for student teaching, resident training, or research endeavors. While echocardiography will never be replaced by these new 3D technologies, they provide the veterinary cardiologist with novel ways to assess, evaluate, and understand the hearts of animals.
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8. Pedra CA, et al. Curr Opin Cardiol. 2011;26:86–93.