D1 Computed tomography (CT)

Topic

Computed tomography is the most widely used morphological imaging technique in oncology, and its role spans the patient's entire oncological journey: it is used to stage the tumor based on size and extent, plan external-beam photon radiotherapy, assess tumor response to treatment, guide biopsies, and plan oncological surgery.

To fulfill these functions, computed tomography combines the physics of X-ray attenuation as the beams pass through the patient's body with a mathematical tomographic reconstruction process—using either filtered back-projection or iterative reconstruction—that converts the measured attenuation into cross-sectional images of the patient's body with a spatial resolution of between half a millimeter and one millimeter.

What computed tomography actually measures at each point in the cross-sectional image is the tissue attenuation at that location relative to the attenuation of water, expressed in Hounsfield units; this value serves as a physical proxy for tissue density and composition, ultimately enabling the differentiation of one tissue type from another within the image.

Within the Hounsfield unit scale, each tissue type occupies a distinct range: air has a value well below that of water—around negative one thousand Hounsfield units; fat has a value moderately below that of water—around negative one hundred Hounsfield units; water itself defines the reference value of zero Hounsfield units; soft tissue has a moderately positive value—between twenty and eighty Hounsfield units; and bone has a value well above that of water—between three hundred and one thousand Hounsfield units. The iodinated contrast agent sometimes administered during a computed tomography scan utilizes this same Hounsfield unit scale to make the tumor more visible: the contrast agent preferentially concentrates in hypervascularized tissues, and since the tumor is typically more vascularized than the surrounding healthy tissue, the agent amplifies the difference in Hounsfield units between the tumor and the normal parenchyma, enabling the CT scan to better distinguish one from the other.

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