Coronary Artery Bypass Graft Imaging and Assessment of Flow

Since the report published in 1968 by Favaloro about the use of saphenous veins to restore coronary artery blood flow in 171 patients, a large number of coronary artery bypass grafting (CABG) procedures have been performed worldwide. In the United States alone, 219,000 patients underwent a total of 397,000 CABG procedures in 2010.

The left internal mammary artery (IMA) is frequently used as an arterial conduit to the left anterior descending (LAD) and its diagonal branches. Other arterial conduits include the right IMA placed to the right coronary artery (RCA) or LAD, the right gastroepiploic artery to the RCA, or the use of free radial artery grafts. Reversed saphenous veins are generally used for grafting to distal branches of the RCA and circumflex coronary artery (LCX) or to diagonal branches of the LAD.

The long-term results of aortocoronary bypass surgery depend largely on the maintenance of graft patency. About 25% of venous grafts occlude within 1 year of surgery, whereas during the following 5 years there is a 2% annual occlusion rate, which increases to 5% yearly thereafter. Thus 50% to 60% of venous grafts are occluded after 10 years and only half of the remaining patent ones have no evidence of atherosclerosis. The mechanisms responsible for venous graft occlusion are believed to be thrombosis in the early weeks after surgery, followed by intimal hyperplasia during the first year and accelerated atherosclerosis in the later stages. Atherosclerotic changes develop comparatively in a smaller percentage of patients with IMA grafts. As a result, in situ arterial grafts occlude less frequently, up to 5% in the first year and 20% to 30% after 10 years, leading to an improved long-term survival.

These graft attrition statistics result in the need for accurate evaluation of bypass graft patency and function, which often is required several times during the lifetime of any given patient.

Imaging Modalities Capable of Evaluating Grafts

Selective x-ray coronary angiography is the gold standard for assessment of graft anatomy and has the added advantage of simultaneous visualization of the native coronary arteries. Use of a Doppler-tipped guidewire during angiography provides hemodynamic information about graft function by assessing flow pattern at rest and after pharmacologically induced hyperemia.

Selective coronary angiography is, however, invasive, uses ionizing radiation, iodine contrast, and has a small risk of complications such as coronary artery dissection, arrhythmia or stroke.

Two-dimensional (2D) Doppler echocardiography is restricted to evaluation of grafts placed on the LAD artery.

Near infrared fluorescence complex angiography and perfusion analysis is a novel real-time imaging technology used to assess the physiologic response to grafting during the intraoperative phase of CABG and which may be useful in predicting subsequent graft failure.

Cardiovascular magnetic resonance (CMR) and coronary computed tomography (CCT) are techniques which allow the direct evaluation of bypass grafts patency. A unique feature of CMR is that in addition to standard anatomic imaging, blood flow velocity and volume can be quantified within the grafts. Thus the true physiologic status of the functional unit represented by a graft and its recipient vessel can be determined noninvasively.

Cardiovascular Magnetic Resonance of Bypass Grafts

Over the years, several CMR techniques have been introduced to evaluate aortocoronary bypass grafts ( Table 26.1 ). The assessment strategy may include either anatomic (angiographic) or hemodynamic (flow volume, velocities, flow reserve) evaluation or a combination of these modalities. This is usually combined in clinical practice with evaluation of the myocardial function (cine CMR), tissue characterization (late gadolinium enhancement imaging for presence of myocardial fibrosis/infarction), and ischemia detection (pharmacologic first pass gadolinium contrast myocardial perfusion).

TABLE 26.1

Detection of Bypass Graft Patency According to Different Cardiovascular Magnetic Resonance Techniques

Reference	Technique	No. of Grafts	Sensitivity (%)	Specificity (%)	Accuracy (%)
White et al.	SE CMR	65	91	72	86
Rubinstein et al.	SE CMR	44	92	85	89
Jenkins et al.	SE CMR	60	90	90	90
Frija et al.	SE CMR	52	98	78	94
White et al.	Cine CMR	28	93	86	89
Aurigemma et al.	Cine CMR	45	88	100	91
Galjee et al.	SE CMR	98	98	85	96
	Cine CMR		98	88	96
	Combined		98	76	94
Kessler et al.	3D navigator	19	87	100	89
Vrachliotis et al.	3D CE MRA, ECG-triggered	44	93	97	95
Wintersperger et al.	3D CE MRA Non–ECG triggered	76	95	81	92
Kalden et al.	HASTE	59	95	93	95
Kalden et al.	3D CE MRA ECG-triggered		93	93	93
Bunce et al.	SSFP	79	84	45	78
Bunce et al.	3D CE MRA ECG-triggered		85	73	84
Langerak et al.	3D navigator Two observers	56	65–83	80–100	~80

CE MRA , Contrast-enhanced magnetic resonance angiography; ECG , electrocardiogram; HASTE , half-Fourier acquisition single-shot turbo spin echo; SE CMR , spin echo cardiovascular magnetic resonance; SSFP , steady state free precession.

Whereas pulse sequences developed for imaging native coronary arteries are also applied for imaging bypass grafts, CABG imaging is associated with specific problems (different vessel anatomy and flow patterns and the presence of metallic vascular clips, ostial markers and sternal wires).

A majority of published clinical CMR studies addressed imaging the proximal portion of vein grafts. Proximal segments are less affected by bulk cardiac motion compared with distal graft segments or native coronary arteries, resulting in fewer motion artifacts, whereas the lack of direct contact with epicardial fat or myocardium results in higher contrast to surrounding tissues. Unfortunately, graft stenosis often occurs at the site of anastomosis with the native vessel where CMR encounters artifacts and resolution problems similar to those in imaging the native coronary arteries. Arterial grafts imaging poses additional problems because of vessel tortuosity and smaller caliber as well as presence of metallic artifacts from hemostatic clips and sternal wires.

Bypass Graft Anatomic Imaging Techniques

Conventional Spin Echo and Gradient Echo Imaging

The assessment of saphenous vein aortocoronary bypass graft patency has been one of the early indications for CMR. More than two decades ago several groups reported the feasibility of assessing graft patency using conventional electrocardiogram (ECG)-triggered multislice spin-echo techniques. On spin-echo images, patent grafts with good blood flow appear in consecutive imaging planes as conduits with a signal void. In contrast, stenotic grafts with slow flow or occluded grafts display intermediate signal intensity ( Fig. 26.1 ). With selective x-ray angiography as the method of reference, the sensitivity of spin-echo CMR in predicting graft patency ranged from 90% to 98% with a specificity of 72% to 90%.

FIG. 26.1, Series of four transverse spin echo images of a sequential vein graft to the obtuse marginal branch of the circumflex (CX) artery (1) and a second vein graft to the left anterior descending artery (LAD) (2). Patent grafts show low signal intensity. (A) Most superior image demonstrating the CX graft originating from the ascending aorta and overriding the main pulmonary artery. (B) More descending course of the CX graft left of the pulmonary artery. (C) LAD graft originating from the more proximal part of the ascending aorta. (D) Course of both grafts at the level of the left atrium.

Using conventional gradient echo nonbreath-hold CMR, with a relatively long echo time (TE) and repetition time (TR), the sensitivity was in the same order of magnitude (88%–98%), with a somewhat higher specificity (86%–100%). On gradient echo images blood flow within patent grafts appears bright ( Fig. 26.2 ). On spin-echo images, signal voids from metal clips, stents or calcifications, can falsely mimic graft patency. These artifacts are accentuated, thus easier to detect, on gradient echo compared with spin-echo images, which decreases the number of false positive patent grafts (increases specificity). On the other hand, the number of false-positive occlusions might be expected to increase (decreased sensitivity).

FIG. 26.2, Transverse image obtained with gradient echo technique. Cross sections of patent grafts to the right coronary artery (RCA), left anterior descending coronary artery (LAD), and circumflex coronary artery (CX) are indicated and demonstrate a high signal intensity. A, Aorta; P, pulmonary artery.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here