Recent advances: Diagnostic radiology

British Medical Journal, July 17, 1999 by Jane Hawnaur

However, the price of faster and more versatile computed tomography may be a higher dose of radiation. This is not intrinsic to spiral scan technology, but results from the ability to obtain an increased number and complexity of scans.[15 16] Faster still is "ultrafast" computed tomography, which uses an electron beam to steer x rays around the patient and may have a role in screening for coronary artery disease by detecting calcification from which the presence of atherosclerosis can be inferred.[17 18]

Magnetic resonance imaging

Until recently, progress in body magnetic resonance imaging was also restricted by the long times needed to collect sufficient signal to form an image, and the inevitable physiological motion degraded the image quality. Improved performance of the hardware and new software for image acquisition and reconstruction have dramatically shortened scan times, increasing the robustness and cost effectiveness of this investigation.[19] Image acquisition during a breathhold overcomes problems of respiratory motion and has generated new enthusiasm for magnetic resonance imaging of the thorax and abdomen. The digital signals can be acquired, processed, and displayed in several ways. For example, cardiac-gated magnetic resonance imaging uses signals from the same point in successive electrocardiographic cycles to effectively freeze cardiac pulsation and provide excellent anatomical information in congenital and acquired heart disease. Conversely, a cine loop of myocardial pulsation and blood flow can be created from breathhold magnetic resonance imaging performed during several heartbeats. Perfusion imaging using breathhold contrast enhanced magnetic resonance imaging may show ischaemic myocardium that is active metabolically and thus potentially salvageable by revascularisation.[20] Treating this "hibernating myocardium" could improve left ventricular function and thus survival in patients with ischaemic myocardial disease. Magnetic resonance imaging also has the potential to show coronary artery stenoses non-invasively, although further development is required before the technique is sufficiently robust for clinical use.[21]

Ultrasonography

Advances in probe design, which enable endoluminal ultrasonography to be peformed--for example, in assessing the integrity of the anal sphincter--are familiar to many clinicians.[22] Ultrasound probes of small diameter and very high frequency can now be inserted into the coronary arteries so that plaques can be seen and the degree of stenosis assessed. These devices are mainly being developed by cardiologists for managing coronary atherosclerosis.[23] Ultrasound has also benefited from digital signal manipulation and post-processing. Volumetric ultrasound can be displayed as three dimensional or surface rendered images and has potential applications in obstetrics and gynaecology.[24] Measurement of broadband ultrasound attenuation provides a quantitative measure of fracture risk, equivalent to radiation bone densitometry techniques.[25] Conventional ultrasound uses the same frequency bandwidth for both the transmitted and received signal. Use of higher harmonic frequencies generated by propagation of the ultrasound beam through the patient improves the quality of the image in clinical applications.[26]