CTA Assessment of Saphenous Vein Grafts and Internal Thoracic (Mammary) Arteries

Key Points
Coronary artery bypass grafts are amenable to cardiac CT angiography assessment because they are larger in size than native arteries and are subject to little motion artifact and little calcification, factors that otherwise present major limitations to CCT.
The abundance of surgical clips on internal thoracic and mammary grafts is an impediment to adequate CCT angiography.
Unfortunately, the need to assess native coronary artery disease, which is always advanced in patients who have had a prior bypass, renders coronary graft assessment only a portion of the necessary evaluation.
However, CCT coronary graft angiography may be useful in well-chosen cases, such as mapping graft course in cases where reoperation is anticipated.

Coronary Bypass Graft Assessment

Coronary bypass grafts, both venous and arterial, are more readily evaluated by CT angiography (CTA) than are coronary arteries, because of:

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Their large diameter (more so for venous than arterial conduits)
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Their minimal motion, when compared with coronary arteries, because they are largely extracardiac
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Their general lack of calcification, versus the common and often extensive calcification of native coronary arteries
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Simpler courses (but not always) with little overlap. Some exceptions:
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  Posterior transverse sinus course of a venous graft or right internal mammary artery (RIMA) graft to the circumflex
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  Twisted course of vein grafts if multiple and adjacent
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  The presence of jump-grafts (sequential graft insertions; usually saphenous)
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  Internal thoracic artery positioned tightly against the chest wall, in which case the use of bone extraction software when post-processing may eliminate depiction of the internal mammary artery (IMA) graft

Potential Uses of Coronary CTA in Patients with Coronary Artery Bypass Grafts

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Nonvisualization of a coronary artery bypass graft (CABG) at the time of catheter angiography
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Chest pain in a patient with CABG
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Prior to repeat open heart surgery (CABG or valve surgery), to localize the bypass grafts relative to the sternum and intended sternotomy

Problems with Bypass Graft Assessment

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The large field of view. This is especially true if there is an internal thoracic artery to be imaged, because the field of view will extend from above the subclavian artery down to the diaphragm. This requires:
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  Longer breath-hold and greater chance for motion artifact
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  Greater radiation exposure
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Surgical clips abound around mammary arteries and may present a challenge and sometimes a problem when the adjacent lumen is being imaged.
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Sternal wires also may cause artifacts, as may ECG electrodes, which can confound assessment of anterior bypass grafts.
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The direction of flow within a graft cannot be established by its opacification. It is tempting to view opacification as indicative of anterograde flow. This becomes a problem, however, when there is a proximal severe-appearing lesion: assumption of anterograde filling implies nonocclusion, whereas retrograde filling of a proximally occluded graft may have been the case.
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If a left internal thoracic artery graft is present, the contrast will have to be injected via the right arm to avoid high-attenuation artifacts in the left subclavian artery; such artifacts may confound imaging of the ostium of the internal thoracic artery.
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If both the left and right internal thoracic arteries are used as conduits, then the ostium of one of the two will be obscured by subclavian vein overattenuation artifacts.
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Often a larger concentration of metallic clips is seen at the graft-native vessel anastomosis, resulting in a beam-hardening artifact and obscuring the depiction of the anastomosis.
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Cardiac CT (CCT) is better at assessing bypass grafts than native vessels, which are, of course, always diseased in the scenario of bypass grafting, often extensively, and may be calcified. Adequate and complete visualization of the native vessels distal to the bypass graft anastomosis may be (often is) problematic, because the native coronary arteries at this level tend to be smaller.
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Stenting within bypass grafts is still a challenge; however, stents within grafts often are better assessed than native stented vessels due to the larger size of the grafts and less cardiac motion.
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Interobserver agreement of quantification of lesions is poor: less than 50%.

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For more information, see Tables 8-1 and 8-2 .

TABLE 8-1

Cardiac CT Assessment of Coronary Artery Bypass Grafts

AUTHOR	JOURNAL	YEAR	CT	NO. PTS/NO. GRAFTS	NONASSESS. (%)	GRAFT TYPES (SVG:IMA)	LESION	SENSITIVITY (%)	SPECIFICITY (%)	PPV (%)	NPV (%)
Achenbach et al.	Am J Cardiol	1997	EBCT	25	4	55:1	Occlusion	100	100
					16		Stenosis	100	97
Chiurlia et al.	Am J Cardiol	2005	16-CT	52	99	117:47	Occlusion	100	100
							Stenosis	96	100
Martuscelli et al.	Circulation	2004	16-CT	96		166:85	Occlusion	100	100
							Stenosis	90	100
				84			Anastomosis	97	100
Nieman et al.	Radiology	2003	4-CT	109	16			93	42	70	81
Ropers et al.	Am J Cardiol	2001	MSCT	65	0	162:20	Occlusion	97	98	97	98
					38		Stenosis	75	92	71	93
Schlosser et al.	JACC	2004	16-CT	51	0	91:40		96	95	81	99
Anders et al.	Eur J Radiol	2006	16-CT	32	18	74:19	Occlusion	100	98
							Stenosis	81	87
Salm et al.	Am Heart J	2005	16-CT	25	8	53:14	Occlusion	100	100	100	100
					8	53:14	Stenosis	100	94	50	100
Moore et al.	Clin Radiol	2005		50		0%	Occlusion	100	100
						0%	Stenosis	100	99
Burgstahler et al.	Int J Cardiol	2006	16-CT		0	43 g	Occlusion	100	100		100
					5		Stenosis	100	93		100
Schuijf et al.	Am J Cardiol	2004	16-CT		1	43 g	Occlusion	96	100		99
					15		Stenosis	100	96		100
Pache et al.	Eur Heart J	2006	64-CT		0	93 g	Occlusion	100	100		100
					6		Stenosis	100	100		100
Stauder et al.	Eur Radiol	2006	16-CT	20
					22.5	80 art	Stenosis	96	97	96	97
					7.3	180 svg	Stenosis	99	94	92	99
					31	Native	Stenosis	92	77	88	85
Malagutti et al.				52/109		Grafts		99	96
						Native		89	93	50
Ropers et al.				Graft occlusion Graft stenosis Native arteries	0 0 9			100 100 86	100 94 76	100 92 44	100 100 96