Tomographic tactic conquers planar imaging limitations

Imaging in medicine dates its origin to 1895, when Roentgen discovered the x-ray. This exciting and novel technology opened an unprecedented era in medicine, which continued through the 20th century and remains a major element of the day-to-day practice of radiology. Limited, modest efforts made during the mid-20th century to employ tomographic methodologies eventually led to the introduction of x-ray CT by Sir Godfrey Hounsfield in 1973.

The extraordinary power of this approach and its superiority over conventional planar imaging techniques became apparent within months after its application in several intracranial disorders. The value of plain-film radiography of the skull had already been questioned for decades. X-ray CT images of the head were embraced as the standard of care for a multitude of neurosurgical and neurological diseases as the technology spread throughout the medical community.

The concept of modern tomography was actually born at the University of Pennsylvania in the early 1960s, when Dr. David Kuhl and engineer Roy Edwards reconstructed tomographic images of the head and the body by using radiotracers. The impact of this modality in detecting central nervous system disorders became apparent when sophisticated SPECT instruments were designed and built in the late 1960s and early 1970s, then employed to assess a variety of diseases of the brain.

These images detected blood-brain barrier abnormalities using technetium-99m pertechnetate. Many lesions that were missed by conventional planar imaging techniques were readily detectable by SPECT. We became convinced then, as we remain today, that planar imaging in nuclear medicine is a suboptimal technique that cannot successfully compete with tomographic methodologies in today's practice.

As the technology for x-ray CT matured over the years, the superiority of tomographic imaging over planar radiology for assessing disease processes anywhere in the body became increasingly apparent to the practicing radiologist. Examination of anatomic sites such as the chest and abdomen by x-ray CT has added a major and exciting dimension to radiology. Although the success of this modality is more impressive in certain disorders and body locations, its impact has been strong in almost every organ system.

The introduction of MRI as a clinical modality in the 1980s further demonstrated the advantages of tomographic technologies in examining diseases with high sensitivity and specificity. The success of this modality in assessing certain pathologic processes in the head and neck region, cardiovascular system, abdomen and pelvis, and musculoskeletal system is unquestioned.

Similarly, the use of SPECT for imaging central nervous system and cardiac diseases has strengthened the role of this approach in the routine practice of nuclear medicine. FDG-PET imaging has clearly demonstrated that the high-contrast-resolution tomographic images provided with this technique are substantially superior to those achievable with conventional planar methods.

PLANAR IMAGING LIMITATIONS

Planar imaging, either with conventional radiography or scintigraphy, compresses a significant amount of volumetric data into a single plane. The accumulated information from superimposed structures substantially reduces the degree of contrast resolution, which is essential for defining normal and abnormal structures. This automatically leads to decreased sensitivity.

Tomographic imaging allows for a display of thin digital slices through an entire volume of data from a region, improving sensitivity for disease. Some have argued that in certain areas, such as the lungs or the appendicular skeleton, this gain from tomographic modalities will be small. Once a careful pretest clinical evaluation has determined the need for image-derived information for patient management decisions, however, using an inferior but less expensive technique with the potential for overlooking an abnormality or achieving suboptimal detection or characterization is unacceptable. Medical, ethical, and legal reasons dictate a higher standard.

Increasingly, quantitative data will guide accurate diagnosis and monitor response to treatment. This powerful approach can best be accomplished by tomographic techniques. Combining quantitative results from both structural and functional methodologies will substantially enhance the role of imaging for both research and clinical applications. Such information cannot be generated from planar studies.

Becoming "all tomographic," however, raises a number of concerns in both nuclear medicine and radiology.

- Feasibility and cost of switching from planar to tomographic imaging. Continued major advances in instrumentation for nuclear medicine and radiology make it likely that 3D imaging methods will in large part replace planar studies in the next 10 to 20 years. The costs of such instruments and associated tomographic studies will decrease as their routine use increases. The design of these machines will be further optimized for rapid image acquisition and processing as well as for patient throughput.

In nuclear medicine, the next decade will probably see a significant shift from conventional planar to tomographic imaging with PET. Similar trends will continue for radiological imaging by employing CT and MRI as the major modalities for detecting structural abnormalities.

- Radiation dose to the patient. Radiation dose will not substantially increase by switching from planar imaging, with single gamma-emitting radiolabeled compounds, to PET. Though concern is growing about increased radiation dose from a shift to CT imaging, the risk of the test that is requested should be weighed against the risk of disease. If CT delivers a larger radiation dose than a chest radiograph, for example, but detects a serious disease such as cancer, then the benefit-to-risk ratio justifies the test. The important parameter here is potential information gained per radiation risk.

- Personnel. The shift to tomographic imaging will require trained individuals at both the technical and professional levels. If the shift occurs over an extended period of time, adequate effort can implement a smooth transition.

- Image display and analysis software. The growing number of images provided by the new tomographic instruments necessitates more efficient display and analysis methods.

We speculate that in the next 10 to 20 years, most studies performed in both radiology and nuclear medicine will be performed by using tomographic instruments. Both practitioners and industry should take note and be prepared for this transformation.

Dr. Alavi is a professor of radiology and director of research education in the radiology department, Dr. Gefter is a professor of radiology and section chief of thoracic imaging, and Dr. Torigian is an assistant professor of radiology, all at the Hospital of the University of Pennsylvania.

Some planar studies will survive reconstructed future

Move to all-tomographic imaging will incorporate useful survivors from single-plane era

Some types of planar imaging techniques may survive the test of time and remain in the armamentarium of services provided. These will include studies in which the desired information will not require significant spatial or contrast resolution. The following lists are only partial.

Nuclear Medicine

Bone scintigraphy in the lower extremities

Renal scans

Gastric emptying studies

Hepatobiliary imaging

Thyroid examination for benign diseases

Brain death studies

Radiology

Radiography for limited device, line, or tube assessment

Radiography for radiodense foreign body or calcification detection

Portable bedside radiography of chest and abdomen

Radiography for initial fracture evaluation in appendicular skeleton