June 01, 2008

X-Rays: The Legacy

Roentgen's X-rays have been used in fields as varied as medicine and metallurgy. Its use has been boosted with developments and modifications in technology. Mammography, digital subtraction angiography, xeroradiography, CT scan, tomography, barium enema (or meal), Intra Venous Urography and oral choecystography are important ramifications of X-ray usage.

Mammography is an useful tool in the detection of breast cancer. Long wavelength X-rays (=> low frequency=> low energy, since E=hv; h being Plank's constant, v being frequency of radiation) are used in mammography, which enables surgeons or radiologists to see if there were any cyst or a cancerous neoplasm in the breast. It is usually done on women above 30 years in whom breast tissue is less dense. In ladies below 30 years in whom the breast is more dense, ultrasound is recommended, as the picture quality of mammograph suffers.

In xeroradiography, a coat of amorphous selenium which is layered on aluminium-oxide coated aluminium plate, is charged uniformly, by a device called scorotron. Whenever and wherever incident X-ray photons impinge on these, charges disappear. The magnitude of discharging of the charges depends on the energy of the photons. Toner particles now are attracted by static electricity of the charges. Other techniques are also used at the same time, so that the adherence of toner to the plate is faithful. Next the pattern of the toner is transferred to a paper, which typically can be re-used. It's use has now fallen out of favor.

If you wanted to visualize renal stones or wanted to investigate ureteric obstruction or anything related to the state of the urinary tract, you could easily do it by performing an Intravenous Urography or IVU. In this procedure the patient is given an intravenous injection of an iodinated radio-opaque contrast dye. The dye circulates in the systemic circulation and is filtered in the kidney, delineating the details. We can now see the structures because we can see the dye. Similarly, oral cholecystography (OCG) employs radiopaque contrast media to be taken orally. It then goes to the liver and thence to the gall bladder.

But how can we see blood vessels in the brain? Even if we inject a contrast, the digital subtraction angiographyshadow of the dense skull bones will superimpose, spoiling everything. We can circumvent it in digital subtraction angiography (MRI angiography is another option). In DSA, we take an X-ray of the head (suppose); we then take another snapshot after giving the patient a shot of a contrast dye. If we add the negative of the former with the positive of the latter, the shadow due to the skull will cancel out. This is the principle of DSA. A photo is shown on the left.

Tomography of olden times, before the advent of CAT scan, was performed in an ingenious way. The patient was positioned between the X-ray camera and the photo plate (cassette). If you rotated both the camera and the cassette in opposite directions keeping the suspected lesion of the patient as the imaginary center, then all else but the suspected lesion will become blurred. The suspected lesion can be viewed easily, despite the fact that it could have been buried deep into the lung parenchyma. Modern CT scan is much more powerful and complicated.

X-ray never failed to amaze us. Now its grandchildren seems set to rule another decade or so, despite the ionization stigma it has on its shoulders.

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