Radiology is a very challenging and exciting field that is rapidly advancing both in its technical capabilities and diagnostic utility. There are different radiodiagnosis modalities including: plain radiography, computed tomography (CT), magnetic resonance imaging (MRI), angiography, etc... Each one of these different modalities has its own specific characteristics which make priority for one to the other in reaching the diagnosis of some diseases.

   Computed Tomography :

The limitation of all plain radiographic techniques is the two-dimensional representation of a three-dimensional structure. The linear attenuation coefficients of all the tissues in the X-ray beam form the image. Computed tomography (CT) obtains a series of different angular X-ray projections which are processed by a computer to give a section of specified thickness. The CT image comprises a regular matrix of picture elements (pixels). All of the tissues contained within the pixel attenuate the X-ray projections and result in a mean attenuation value for the pixel. This value is compared with the attenuation value of water and is displayed on a scale (the Hounsfield scale). Water is defined as 0 Hounsfield units (HU) and the scale is 2000 HU wide. Air typically has a HU number of -1000; fat is approximately -100 HU; soft tissues are in the range +20 to +70 HU, and bone is greater than +400 HU. All CT machines, of whatever generation, share similar components. The detectors are either gas ionization chambers, or scintillation crystals linked to photomultiplier tubes. The signal is digitized by an analogue to digital converter (ADC) in the gantry. The digitized signal is transferred to the image processing computer and subsequently displayed on the operator’s console. Images are usually photographed on medical recording film (hard copy) using optical or laser cameras. For long-term storage, the data is transferred to magnetic media (tape or disc) or to optical disc. No specific preparation is required for examinations of the brain, spine, musculoskeletal system and chest. Studies of the abdomen and pelvis almost always require opacification of the gastrointestinal tract, using a solution of dilute contrast medium (either water-soluble or a barium compound). Generally, 750-1000 ml given orally 30-60 minutes prior to imaging, with the final 300 ml taken as the patient enters the examination room. The large bowel may also be opacified by a solution of contrast medium administered rectally, either in a preparation room or on the CT table. Examinations of the female pelvis are often performed after the insertion of a vaginal tampon to facilitate interpretation. Identification of vascular structures may be made on the basis of anatomy alone, but the intravenous injection of water-soluble contrast medium may be required. Techniques vary according to the individual case, but the most common method is to inject a bolus of 50 ml followed by a rapid infusion of 50 ml, using contrast medium of 300-370 mg iodine/ml. Generally, all studies are performed with the patient supine, and images are obtained in the transverse plane. Occasionally, coronal images are obtained in the investigation of cranial abnormalities; in these cases the patient lies prone, with the neck extended, and the gantry is angled appropriately.

   Ultrasonography :

A sound wave of appropriate frequency (diagnostic range 3.5-10 MHz) is produced by piezo-electric principles. Both the size and shape of the emitting crystal and its resonant frequency are important factors in determining the course of the sound beam within the tissues to be examined. As the beam passes through tissues, two important effects determine image production. These are attenuation and reflection. The greater the attenuation of the sound beam through the tissue, the lower the resultant signal intensity received. Reflection of sound waves within the range of the receiver produces the image, the texture of which is dependent upon differences in acoustic impedance between different tissues. Ultrasound imaging systems are sensitive to the very small changes in acoustic impedance within soft tissue. Through the application of these basic principles, sophisticated hardware has been developed that converts the pulse-echo system, briefly described above, into a real-time two-dimensional sectional image. The addition of the facility to measure blood flow direction and velocity ultrasonically (using the Doppler principle) has led to the development and wide availability of duplex scanners. The effects of shadowing and enhancement within an ultrasound image are very important. Systems are designed assuming an average attenuation through a depth of tissue, and are balanced to give an even intensity of signal for deep and superficial tissues. An acoustic shadow occurs when a tissue within the measured depth has a higher than average attenuation; all tissues deep to this will appear with a falsely lower intensity (shadowed). Conversely, a tissue with a lower than average attenuation will cause all tissues deep to this to appear falsely high in intensity (enhanced). Fibrous tissue, calcification and gas all produce acoustic shadows, whereas fluid-filled structures often cause enhancement. If a selection of ultrasound transducers with varying frequencies, focusing mechanisms, and shapes and sizes is available, visualization of a wide range of tissues, from the neonatal brain to the soft tissues of the hand, becomes possible. Interpretation of the anatomy by static ultrasound images is more difficult than that by other imaging modalities because the technique is highly operator dependent and provides different information on tissue structure and form than other imaging methods.

   Magnetic Resonance Imaging :

Magnetic resonance imaging (MRI) combines a strong magnetic field and radiofrequency (RF) energy to study the distribution and behavior of hydrogen protons in fat and water. The spinning proton of the hydrogen nucleus can be thought of as a tiny bar magnet, with a north and south poles. In the absence of an external magnetic field, the magnetic moments of all of the protons in the body are randomly arranged. However, when the patient is placed in a strong magnetic field these magnetic moments align either with or against the field lines of the magnet. There is a small excess of magnetic moments which align with the field so that a net magnetic vector is established. RF energy is used to generate a second magnetic field, perpendicular to the static magnetic field of the machine. The result of this second magnetic field is to rotate or flip the protons away from the static magnetic field; the amount of rotation depends on the quantity of RF energy absorbed. Once the RF field is switched off, the protons experience only the effects of the static magnetic field and flip back to their original position. During this return to equilibrium, a process which is called relaxation, protons emit the RF energy which they had acquired. This energy is detected by the antenna in the MRI machine, digitized, amplified, and finally, spatially encoded by the array processor. The resulting images are displayed on the operators console and can be recorded on hard copy (for viewing) or transferred to magnetic tape or optical disc (for storage). MRI systems are graded according to the strength of the magnetic field they produce. High-field systems are those capable of producing a magnetic field strength of 1-2 Tesla (T), mid-field systems operate at 0.35-0.5 T, and low-field systems produce field strength of less than 0.2 T. Mid- and high-field systems use superconducting magnets in which the coils of copper wire are kept in a superconducting state by being immersed in an insulated helium bath. Electromagnets are fitted in resistive systems are limited by heating factors to 0.35 T. The third type uses permanently magnetized metal cores and is of low field strength. MRI does not cause any recognized biological hazard. Patients who have any form of pacemaker or implanted electro-inductive device must not be examined. Other prohibited items include ferromagnetic intracranial aneurysm clips, certain types of cardiac valve replacement, and intra-ocular metallic foreign bodies. Generally, it is safe to examine patients who have extra-cranial vascular clips and orthopedic prostheses, but these may cause local artefacts. Loose metal items must be excluded from the examination room. The preparation for an MRI examination is simple. Patients wear metal-free clothes and must answer a rigorous safety questionnaire. Anti-peristaltic agents (e.g., parenteral hyoscine N-butylbromide or glucagon) are often used in abdominal and pelvic examinations. Software techniques counteract respiratory motion for chest and abdominal imaging. Electrocardiogram (ECG) gating is used for cardiac studies. MR images may be obtained in any orthogonal or non-orthogonal plane. There is a wide range of pulse sequences, each of which provides a different image contrast. An intravenous injection of contrast medium as (gadolinium complex) may be given to enhance tumors, inflammatory and vascular abnormalities.

   Angiography :

Access to the arterial system in order to produce an arteriogram (angiogram) is usually obtained by puncture and catheterization of a femoral artery under local anesthesia. Radiographic contrast medium is then injected into the vessel in the area under examination. If, for some reason, access via the femoral artery is not possible (e.g., owing to iliac occlusive disease or the presence of a graft), alternative sites, such as the brachial or axillary artery, can be used. Translumbar aortography (TLA), a method of arteriography that involves direct percutaneous puncture of the aorta, is now less commonly employed as it does not allow selective catheterization of aortic branches and hence percutaneous interventional vascular procedures cannot be performed. The development of new technology has meant that the aorta and the main upper and lower limb arteries can be visualized from an intravenous injection of contrast medium (into the brachial vein, superior vena cava, or right atrium), obviating the need for arterial puncture in some patients. This technique employs digital subtraction angiography (DSA), whereby unwanted background information is subtracted, leaving only an image of the blood vessels. Images of arteries obtained by injection into a vein are referred to as intravenous DSA examinations (IV DSA). DSA images of arteries can be obtained by direct intra-arterial injection (IA DSA). Manual photographic subtraction of background information can also be performed with conventional (non-digital) arteriography. Subtraction, either photographic or digital, is used in cases in which fine vascular detail is required and can be simply recognized by the fact that, in contrast to an unsubtracted film, the arteries appear black as opposed to white. Different radiographic projections are sometimes employed to visualise best the vasculature, e.g., in the aortic arch an anteroposterior view may not clearly show the origins of the vessels arising from the arch as they are very close to each other and may be superimposed. A left anterior oblique position opens the arch, allowing better visualization of the origins of the brachiocephalic, left carotid and subclavian arteries. The veins may be visualized in the same way as the arteries, e.g., by direct puncture and catheterization (via the femoral vein in most instances). The veins of the upper and lower limbs are imaged by injecting contrast medium via an 18G or 20G needle placed in a peripheral vein, e.g. the dorsum of the foot or hand, or the antecubital fossa. Alternatively, if imaging from an arterial injection over a prolonged period of time, the arterial, capillary and venous phases can be recorded and venous anatomy visualized. This is a particularly useful way of imaging the portal venous system without necessitating direct trans-splenic or transhepatic puncture.