Computed Tomography

A Revolutionary Imaging Modality

Despite competition with Magnetic Resonance Imaging (MRI), CT remains the modality of choice for many diagnostic problems in the area of neck and trunk

Dr RK Mathur Director- Radiology (PAN MAX) Max Healthcare

Computed Tomography (CT) has undergone dramatic developments since its first introduction into clinical practice. Despite competition with Magnetic Resonance Imaging (MRI), CT remains the modality of choice for many diagnostic problems in the area of neck and trunk. The advent of new scanning techniques such as spiral or helical CT and more recently Multi Detector Row CT (MDCT) has even reversed the trend towards MRI with a return to CT in some areas.

The advantage of CT over conventional radiography is its high contrast resolution, achieved on one hand through the elimination of the scattered radiation which interferes with the measuring arrangement and on the other, through higher sensitivity of the detector compared to that of the X-ray film. As a result, the accuracy with which the density of the object can be measured is improved by atleast a factor of 100, allowing the differentiation of minute differences in soft tissue structures in the body. Thanks to this excellent contrast resolution, CT has justifiably been described as the most important advancement in diagnostic radiology since the discovery of X-rays. With a gradual increase in the speed of data acquisition and addition of multiple detector rows over the last few years, CT has achieved the unique distinction of being able to generate volume data sets of the body.

Handling these large volumes of data necessitated the use of higher order computers and advanced software. Clinical advantages with the help of these technical advancements have addressed and positively impacted practically every aspect of patient care. The speed and accuracy of current MDCT scanners has modified the diagnostic work of patients with trauma, unexplained chest and abdominal pain, besides being a problem solving modality for a myriad of clinical problems. High-speed volume data collection supported by complementary software yields multidimensional diagnostic information of the area scanned.

The use of IV contrast further enhances the diagnostic yield to cover almost all vascular subsystems of the body. An excellent example is evaluation of chest pain in the emergency room. A single CT study with appropriate contrast usage will yield accurate information of the state of the lungs, the pulmonary arteries, aorta and the coronary arteries, thus ruling out life threatening disease processes with great savings in time and cost. The ability of MDCT to acquire and display isotropic images in multiple planes greatly aids the radiologist in visualising areas in the abdomen which may not be best seen on axial images alone. This also assists the pre-surgical planning to remove tumours and assess the exact location of an abnormality. In short, a surgeon gets a look inside the abdomen prior to the actual surgical procedure.

Neurology Vs CT

Since inception, CT has played a pivotal role in the diagnosis of neurological disease. However, MR has replaced CT as the modality of choice for head and spine except for a limited role in acute trauma and stroke where speed of scanning is of essence in restless patients.

CT has been routinely employed for the evaluation of the mediastinum and lungs. Any abnormality detected on a chest X-ray needing further evaluation undergoes a CT scan. Enlarged glands are one of the commonest causes of an abnormal chest X-ray and CT can not only demonstrate their exact location and size but also with judicious use of contrast media their character. High Resolution Computed Tomography (HRCT) helps optimise the demonstration of parenchymal lung architecture with use of some modifications to the routine CT technique. The use of thin collimation 1mm or less, fast scan time and image recon with a high spatial frequency algorithm are pre-requisites for this technique. Structures as small at 0.1- 0.2mm can be seen on HRCT.

Architectural distortions in the pulmonary lobule in a broad category of diseases under the group of Interstitial Lung Disease can only be assessed by HRCT. It offers guidelines to treatment and serial scans document the response to treatment. CT guided Fine Needle Aspiration Cytology (FNAC) is now a routine procedure to obtain specimen for laboratory diagnosis of pulmonary nodules. (See Fig A )

In Oncology

CT plays a pivotal role in the diagnosis and management of cancer. In suspected cases of cancer, CT accurately delineates the disease in multiple organ systems in the body. It defines a roadmap for the surgeon by demonstrating local invasion and vascularity of tumors and in cases of doubt provides guidance for needle biopsy. Special software works on CT data sets to help in planning radiation therapy. Follow up CT scans at appropriate intervals help in monitoring response to the treatment. The role of CT as a screening procedure for early detection of cancer in the lung remains controversial. Issues of radiation dose and cost V/s benefit remain unresolved.

Some of the early applications of CT were gradually taken over by MRI. However, the ability of CT to demonstrate bone in great detail necessitates the shift between MRI and CT in a select group of patients. Minor and hairline fractures are best demonstrated by CT. Software supporting 3D reconstruction is now universally available. In complex fractures in face, spine and pelvis a model can be prepared for the orthopaedic or plastic surgeon to plan, repair or conduct reconstructive surgery.

Tech Buzz

A unique software enables the radiologist to see inside hollow viscera like trachea, colon and in some cases even vessels. The data sets are compiled and a virtual scopy with fly-through sequence can be created. A look beyond visual range in obstruction to trachea and colon is a distinct clinical advantage.

Some developments in CT technology have been spurred by the promise held out in early generation scanners to visualise IV contrast flowing inside the vessels. It was routine to be able to demonstrate the renal arteries. For example, on a single detector row Helical CT scanner, however the challenge that was posed was to be able to demonstrate long segments of large and peripheral arteries and eventually coronary arteries. These challenges have been largely met by the 64DR CT scanner now in widespread use. CT angiography of the heart has evolved into a broadly applicable clinical examination in the out-patient setting that can replace invasive cardiac catherisation in selected patient populations. The procedure is inherently challenging as its target is the continuously moving heart. Current 64DRCT scanners produce amazing pictures of the heart with details of the coronary arteries which quite recently defied imagination. It is excellent at demonstrating fatty or atherosclerotic blockages and calcium deposits. Its ability to rule out coronary artery disease is nearly 100 per cent. When disease is present it can determine the degree of severity as mild, moderate or severe. -photo-caption. This categorisation lends direction to further management.

Cardiac Matters

Non-invasive cardiac imaging has made a significant contribution towards evaluation of coronary grafts and stents in patients who have undergone coronary artery bypass surgery and angioplasty with stenting. The accuracy of this procedure has been established in several controlled studies. The promise of stent analysis has resulted in the fabrication of CT compatible stents which allow clear details of the stent lumen devoid of any artifacts. Experience gained through extensive use of this technology has thrown up further challenges. The number of CT procedures is rising exponentially and concerns have been raised about the radiation dose especially in cardiac CT. Equipment manufacturers have been seized of this concern and a new technique of prospective cardiac gating will reduced dose by 70 per cent in new scanners. Scanning patients with irregular cardiac rhythm and software to remove calcium from the wall of coronary arteries are works in progress. It is visualised that these breakthroughs are round-the-corner and that the complete data of a cardiac study may be acquired in just one second by a revolutionary flat panel detector in the foreseeable future. This technology however, comes with a hefty price tag and the economics and politics of such an expensive technology will unfold itself in the next few years.

rakesh.mathur@maxhealthcare.com