Cardiovascular system

Chapter 32 Cardiovascular system

Mark Cowling, Colin Monaghan

Introduction

This chapter will consider diagnostic angiography and venography. There are now several non-invasive methods available for evaluation of the cardiovascular system, such as Doppler ultrasound, computed tomography angiography (CTA) and magnetic resonance angiography (MRA). However, intra-arterial catheter angiography and venography remain important diagnostic tools and are likely to remain in clinical use for some years. Indeed, although it is invasive, intra-arterial catheter angiography has the benefit of being able to proceed directly to intervention should that be appropriate.

Equipment

Digital subtraction angiography (DSA) has been available for over 20 years. It is noteworthy that in the 2000 NCEPOD (National Confidential Enquiry into Perioperative Deaths)¹ report on interventional vascular radiology it was stated that 8% of hospitals in the UK were still undertaking vascular work on barium screening systems. No recent, more up-to-date data are available, but it seems likely that this situation is now much improved, and certainly it should be considered unacceptable to be performing complex vascular imaging and intervention without access to DSA.

All dedicated DSA units have the X-ray tube and image intensifier mounted on a ‘C-arm’, allowing oblique views to be obtained easily without moving the patient. When angiographic images are acquired (often referred to as an angiographic run), a number of images are obtained before the intra-arterial injection of contrast; these are used as mask images. Contrast is then injected and the arteries are opacified. The mask image is then subtracted from the contrast images. All detail on the mask, such as bone, is thus removed from subsequent images, leaving only the contrast opacifying the vessels on the image. Although the spatial resolution of this technique is a little less than that for film-based angiography, this is more than compensated for by the much greater contrast resolution. In other words, the finer detail is not obscured by overlying structures such as bone. Digital acquisition of the image data means that the subtraction process is performed by computer, and the subtracted images are available in real time.

In general DSA is an excellent technique, but there can be problems with image quality, particularly due to patient movement. One method of dealing with this is to use a facility termed pixel shifting. This involves using the computer to move the mask and contrast images relative to one another such that they are properly aligned, thereby removing misregistration artefact (Fig. 32.1A,B). This method of image processing is most suited to movement in relatively simple anatomical structures such as a limb, and also in situations where movement has only been slight. More extreme movements can be very difficult, if not impossible, to correct by pixel shifting. This is because pixel shifting involves a simple translation of image data in two dimensions, whereas patient movement actually occurs in three dimensions, often involving a degree of rotation, rather than pure translation. Modern automated methods of pixel shifting can be of benefit where more extreme patient movement has occurred; however, there are still limitations to the technique, and attention to good patient positioning and explanation remains critical to obtaining good results.

Figure 32.1 (A) Digital subtraction arteriogram of distal calf showing misregistration artefact due to patient movement; (B) after pixel shifting the image quality is much improved.

Further degradation of image quality can be encountered owing to patient breathing and bowel peristalsis during an acquisition. The former can be problematic in both the chest and the abdomen, and pixel shifting is of little value. It may prove necessary to perform the angiographic run again, but if a patient is very ill (the most common reason for being unable to suspend respiration adequately) it is often more helpful to acquire a larger number of masks than normal while the patient is breathing gently and ‘remask’ each image to improve the diagnostic quality. This involves changing to a different mask while looking at a single contrast image. The mask giving the least degree of misregistration artefact is chosen.

Misregistration due to bowel movement can cause marked degradation of images of the abdominal aorta and its branches, as well as the iliac arteries. In addition, it is nearly impossible to obtain images of diagnostic quality when undertaking mesenteric arteriography for gastrointestinal bleeding. Misregistration caused by gut peristalsis can be largely prevented by administering Buscopan (hyoscine-N-butylbromide) 20 mg either intravenously or through the arteriography catheter. This abolishes peristalsis for about 15 minutes, thereby improving the quality of arteriographic images in the abdomen and pelvis. With regard to images obtained during mesenteric angiography for acute gastrointestinal bleeding, misregistration of bowel loops can give the impression of contrast extravasation into the lumen where there is none. Buscopan can be very helpful and should be administered, but it is also important to review the images without subtraction in order to avoid misdiagnosis.

Other techniques have been used to try to avoid problems with gut misregistration. For example, bowel loops can be displaced laterally by using a balloon between the patient and the image intensifier to compress the abdomen. However, such methods can often no longer be used because of the presence of proximity sensors in the equipment that prevent it from moving if it is in contact with the patient or any other object.

Technique

Points of access for arteriography

Arteriography is most commonly performed by introducing a catheter through the common femoral artery in the groin. If this is not possible, the preferred route is to use the brachial artery at the level of the elbow joint. Alternatives include the radial artery, high brachial and axillary routes. Translumbar aortography, involving direct puncture of the abdominal aorta, is no longer practised in the UK.

The transfemoral approach

This involves the administration of local anaesthetic into the skin and deeper tissues, followed by the insertion of an arterial puncture needle. A suitable guide wire is introduced through the needle into the vessel. It is usual to observe the passage of the wire proximally through the iliac arteries into the abdominal aorta using fluoroscopy. This is helpful because it is possible for the wire to enter the inferior epigastric artery rather than the external iliac artery. This problem is immediately obvious if observed on fluoroscopy, and can be corrected. It should be noted, however, that as operators become more experienced and used to the ‘feel’ of the guide wire in the vessel, they may undertake little or no screening during this part of the procedure unless they encounter resistance to the passage of the wire.

The guide wire may fail to advance satisfactorily for a variety of reasons. Sometimes this can be resolved simply by repositioning the needle tip so that backflow of blood is improved, indicating that the needle tip has been positioned more ideally within the vessel lumen. However, on other occasions it may be necessary to screen over the needle tip while the operator is manipulating the puncture needle, and possibly even injecting contrast. At such times the primary beam is very near the operator’s hands. It is important that the screening radiographer remains vigilant and collimates as closely as possible to the needle tip to reduce the chances of the operator’s hands entering the primary beam.

Once the guide wire has been correctly introduced into the aorta the needle is removed, leaving the wire in place, and a suitable catheter or sheath is introduced over the wire. The next stage in the procedure will depend very much on the examination to be performed, and will be dealt with below.

Complications of the transfemoral route are minimal during diagnostic arteriography; however, the recommended upper limit of complications for audit purposes is as high as 3%.²

The transbrachial route

This route is very useful if the femoral pulses are impalpable, but if a purely diagnostic study is required, CTA should be considered. The complication rate associated with this route of access is in fact quite low, and in the past it would have been quite reasonable to use it routinely, and it may even have had advantages for outpatient or day-case angiography. However, it is used much less frequently than the transfemoral route, probably because it is technically more demanding and therefore a little more time-consuming, and also because of concerns about placing a catheter across the origin of the left vertebral artery. The technique is very similar to that described above for the transfemoral route. However, a vascular sheath is used to facilitate the administration of antispasmodic and anticoagulant drugs during the procedure, as these are considered to reduce the incidence of brachial artery occlusion.

Most arterial territories can be examined using the transbrachial route, although the manipulations required are often more difficult because the catheters tend to be longer. In general the left brachial approach will be used wherever possible, as this avoids placing the catheter across the origins of the great vessels, with the associated potential for formation of pericatheter thrombus and consequent embolic stroke. Most frequently the femoral arteries will be examined, which involves placing the catheter inferiorly into the descending thoracic aorta and distally into the abdominal aorta. Although the initial brachial puncture and vascular sheath insertion can usually be achieved without fluoroscopy, when passing a pigtail catheter proximally into the brachial and subsequently axillary artery it is not at all uncommon for the catheter and guide wire to enter branches such as the circumflex humeral arteries. It is therefore necessary to use fluoroscopy to follow the passage of the catheter and guide wire.

It can be difficult to screen sufficiently laterally, and careful positioning of the patient before the start of the procedure is important. More modern angiographic tables are able to pivot laterally; moving the table in this way can be very helpful. Once the catheter and guide wire have reached the origin of the subclavian artery, the operator will manipulate the catheter into the descending aorta. Depending on the tortuosity of the vessels this may be relatively difficult. One of the issues is the proximity of aerated lung to the aortic arch, which can make the catheter very difficult to see. Use of filters, collimators and sometimes magnification can be very helpful in improving visibility. In addition, on modern angiographic units with pulsed fluoroscopy it can occasionally be helpful to raise the pulse rate, which will improve image quality.

Complications of brachial puncture for diagnostic arteriography are also said to be low, with a rate of up to 0.3% requiring surgery being reported.³ Minor complications not requiring surgery and resolving spontaneously have been reported as 8–15%.^3,⁴

Other routes of access

Other routes of access are now much less commonly used. The axillary and high brachial routes were associated with a relatively high incidence of haematoma formation and occasional consequent nerve damage (up to 2.4%),^5,⁶

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Tags: Medical Imaging Techniques, Reflection and Evaluation

Mar 3, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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