Diagnostic imaging makes huge technological progress

The history of radiology is over 100 years old, having begun with Wilhelm Conrad Rontgen's discovery of x-rays in 1895. The last two decades, however, have seen the practice of diagnostic radiology change almost beyond recognition.¹ This transformation is largely due to technical advances in equipment that resulted from fruitful interactions among the basic sciences, clinical medicine, and manufacturers.

Diagnostic imaging is now an extremely large discipline, and its practitioners are involved in all areas of patient care. Traditional x-ray film is being replaced by soft-copy images. We have ultrasound, CT, MR, and interventional radiology. Nuclear medicine has taken off, thanks to the wider availability of PET scanners, and safer agents have replaced traditional contrast media. Doctors use noninvasive imaging methods for diagnosis and minimally invasive image-guided procedures during treatment. But how did we get here?

CROSS-SECTIONAL IMAGING

The invention of the CT scanner in the 1970s was arguably the most significant development in radiology since the discovery of x-rays. CT introduced radiologists to both cross-sectional imaging and digital imaging. The advent of spiral scanning, demonstrated first as a work-in-progress clinical tool at the 75th RSNA meeting in 1989, has substantially increased the clinical value of CT.²

The development of spiral CT was made possible by advances in computing. Data no longer had to be taken elsewhere for analysis, but could be reconstructed almost instantaneously. Spiral CT involves simultaneous x-ray source rotation and patient table translation, which significantly reduces the time required for raw data acquisition.

Multislice CT systems offer improved spatial resolution and faster scanning times, multiplanar reformatting, 3D reconstructions, and virtual endoscopy. Radiologists routinely perform CT pulmonary angiography; CT of kidney, ureter, and bladder; and CT colonography. Technical improvements in scanners have improved the accuracy of tumor staging and the initial evaluation of trauma patients.

Multislice CT systems are now at the cutting edge in terms of speed, patient comfort, and resolution. Examinations are faster and more patient-friendly, as more anatomy can be scanned in less time. Demands for higher speed scanning have led to increased computing power and better x-ray tubes that function longer at a higher power range-in the region of 20 to 60 kW and 80 to 140 kV.

MR imaging is still a relatively new technique. Sir Peter Mansfield and Paul C. Lauterbur received the Nobel Prize for Physiology or Medicine in 2003 in recognition of their pioneering research in MRI. The modality is now used to investigate neurologic and musculoskeletal disorders and to examine patients with cancer.

Magnet design and scanning speeds have improved over the past 20 years. Fast imaging techniques emerged in the mid-1980s with the introduction of rapid acquisition and relaxation enhancement (RARE) imaging (now more commonly called fast or turbo spin-echo imaging)³ and gradient-echo sequences.⁴ Faster scanning has increased MR's clinical usefulness as a diagnostic tool. Cardiac and vascular MR imaging provides good resolution of rapidly moving structures. This capability is essential in imaging the heart, and it allows high-quality noninvasive peripheral arterial studies.

MR contrast agents have progressed from research studies to clinical use in the past two decades. Current products rely on new principles of enhancement for organ-specific imaging and on novel physicochemical properties to boost the concentration of gadolinium delivered. Blood pool agents are being developed for MR angiography, which is emerging as a viable noninvasive means of assessing vessels.

The hunt for new and better MR sequences continues.⁵ Functional imaging studies that acquire both anatomic and physical information are also gaining ground. Diffusion-weighted and perfusion-weighted MR protocols are improving accuracy in the assessment of stroke patients. And open-style MR scanners with higher field strengths make image-guided interventional procedures possible.

ULTRASOUND AND INTERVENTION

Diagnostic ultrasound has evolved from the combined efforts of mechanical, electrical, and biomedical engineers, physicists, computer technologists, clinicians, sonographers, researchers, university and government administrators, and adventurous, perceptive commercial enterprises. Advances in electronic, microprocessor, and computer technology have increased the diagnostic capabilities of ultrasound scanners. High-resolution ultrasound imaging of superficial structures is now possible, and smaller, portable scanners can be used at a patient's bedside.

Many investigators have been intrigued by 3D fetal imaging. The first such images, constructed on advanced computers from raw 2D data, were inferior to those produced on conventional 2D scanners.⁶ The time required to generate the 3D pictures also made them impractical for routine clinical use. More accurate assessment of antenatal fetal abnormalities is now possible with ultrasound, and endovaginal and endorectal scanning have become routine practice. Doppler technology has also improved, allowing more accurate assessment of blood vessels in the carotid region, for example, and in the evaluation of suspected deep vein thrombosis.

Ultrasound-guided interventions such as biopsies and abscess drainage, pioneered in 1974, have also become routine. Interventional radiology itself is not a modern idea, however. The subject of image-guided procedures arose soon after the discovery of x-rays and continued throughout the 20th century. Prof. Alexander Margulis coined the term "interventional radiology" in 1967 to describe the growing body of manipulative procedures performed by a physician skilled in radiological techniques and experienced in clinical problems.⁷ CT-guided abscess drainage and image-guided biopsies are performed routinely in radiological practice. Radiologists treat aortic aneurysms, and minimally invasive treatments are expected to be the future standard of care.

Improvements in catheter, coil, and stent design have contributed to the growth of interventional radiology during the last 20 years. The first stents were made of nitinol. Subsequent improvements included the self-expandable Z stent, a knitted tantalum stent, and balloon-expandable stents. After intravascular coronary stents were developed in the 1980s,⁸ the use of stents in peripheral vasculature to keep vessels patent after angioplasty became common.^9,10

Stents have been used in the biliary system since 1978, but this application has become more, common in the past 20 years. Today they are used in the peripheral vascular system, in the veins during transjugular intrahepatic portosystemic shunt procedures, in the tracheobronchial tree and biliary tract for stenosis caused by tumors, and in the urinary tract for urethral strictures and ureteric strictures.

DIGITAL X-RAYS

Work with digital subtraction angiography began in the late 1970s.^11-13 Digital imaging techniques advanced further in the '80s when analog-to-digital converters and computers were incorporated into conventional fluoroscopic systems. Conversion of x-ray energy patterns into digital signals, using scanning laser-stimulated luminescence, paved the way for a new digital radiography technique. Scientific investigations led to proof-of-concept studies and the first prototypes.¹⁴

Investigators also sought to develop a complete digital diagnostic system for radiography and fluoroscopy. A prototype device capable of converting x-ray photons to digital image data was patented in 1995. The inventors used a multilayer structure, consisting of a thin-film detector array, selenium x-ray semiconductor, dielectric layer, and top electrode to process the data.¹⁵ Two years later, another group designed a real-time digital detector for projection radiography. This unit was fitted with an active matrix flat-panel device instead of conventional x-ray image intensifiers for fluoroscopy.¹⁶

Digital radiography and fluoroscopy technology are now replacing conventional x-ray detector technologies in many hospitals. It seems that the flat-panel detector is now a key technology for diagnostic radiology and heralds the beginning of a new era of diagnostic x-ray systems.

Digital technology is also challenging the dominance of screen-film techniques in mammography, one of the most common x-ray examinations. But manufacturers must convince users that digital mammography systems produce sufficiently high-resolution images to justify the financial outlay.

Digital systems have a hard act to follow, as screen-film systems have evolved considerably. The design of second-generation mammography units in the 1980s reduced exposure time, making patients more confident about the procedure. Motorized compression devices helped simplify mammography procedures and opened the way for population-based screening. Additional design improvements included an add-on component for breast biopsies, automated guns for large-core needle biopsy, and stereotactic biopsy.¹⁷ Introduction of a bimetallic x-ray tube in the early 1990s, coupled with a rotating molybdenum and rhodium focal track, enabled better penetration of breast tissue with another decrease in radiation exposure.

First reports of digital mammography can be traced back to 1987,^18,19 but it wasn't until 1996 that digital spot medium field-of-view acquisition systems become available for mammography. Digital spot view mammography makes stereotactic biopsies faster and more accurate, shortens examination times, and improves patient comfort and convenience. The first full-field digital mammography system entered the market in 2000. Given sufficient technological improvements and cost savings, full-field digital mammography may replace screen-film as the technology of choice for breast imaging.

Among the novel approaches to breast imaging that have been tested is digital tomosynthesis mammography, which offers the potential to improve detection rates of breast lesions and to predict whether they are benign or malignant.²⁰ This technique may be particularly useful for imaging dense breasts if overlap in breast structures can be eliminated. Contrast-enhanced subtraction mammography enables images to be evaluated over time.²¹

Development of special monochromatic x-ray sources has aided advances in dual-energy mammography, an emerging radiographic technique for the acquisition of a material-selective image by weighted subtraction of low- and high-energy digital x-ray images. Dual-energy subtraction is achieved by exploiting the energy dependence of the x-ray attenuation components in the image. This allows the removal of background morphology to enhance presentation of details that would otherwise be obscured.^22,23

The introduction of PET scanning and, more recently, molecular imaging has led to impressive advances in functional imaging. This area of imaging will likely play a major role in the future, as researchers try to evaluate functional and pathologic changes observed in tissues. Such an approach could revolutionize cancer diagnosis and follow-up.

The way radiologists work has changed beyond recognition. Greater clinical involvement and multidisciplinary teamwork are leading to better patient care and outcomes. It is crucial that radiologists do not become overwhelmed by the massive technological advances but continue to play a central role in patient management and treatment in this digital age.

DR. BUSCH is deputy director of the Roentgen Museum in Remscheid, Germany. DR. BANERJEE is a consultant radiologist at Birmingham Heartlands and Solihull Hospitals NHS Trust in the U.K. DR. THOMAS is a consultant radiologist at Princess Royal University Hospital in Orpington, U.K.

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