C T SCAN

   C T SCAN

The  first CT machine used for clinical purposes, developed by the late Sir Godfrey   Hounsfield ,was installed at the Atkinson Morley Hospital, London, in the early 1970,s In the intial days only sequential acquisition was possible .In 1980s,the development of slip-ring technology enabled continuous revolution of the X-ray un it ,which not only reduced the acquisition time per axial image to 1 sec ,but also allowed helical data to be acquired .The next step of development occurred in the late 1990s, when detectors were split into multiple thin rows along the z-axis to permit acquisition of multiple sections simultaneously (Multidetector  CT). This decreased acquisition time yet further and made it possible to routinely use thin sections. As a result, multid etector  CT is now a days able to provide near isotropic volumetric data sets in which spatial resolution is similar in all planes .200s saw the introduction of cardiac CT which is based on ECG-synchronization to freeze cardiac motion .The number of detector rows increased from 4 to some  where between 64 and 320 rows, depending on the manufacturer .Rapid tube rotation (<0.3s)and dual-source systems with two X-rays tubes were developed to optimize cardiac imaging .

Basic principle of C T is linear Attenuation of x rays

.CT No/HU value :           numerical value ,given to a tissue depending on degree of attenuation of X-ray by that tissue

Air                    :       -1000

Fat                   ;       -50 to -100

Water               :       0

Muscle             :       10-40

Blood               :       60

Contrast           :       130

Bone                :       >400

.Slice  thickness : Thickness of tissue scanned in one rotation

. Increase in the pitch leads to decrease in the scan duration and radiation but reduces the scan quality

. Decrease in the pitch leads to increase in the scan duration and radiation but improves the scan quality

Advantage of multislice CT/Multidetector C7

.Fatser acquisition

.Coverage of larger area

.Less movement artefacts

.Isotropic multiplanar reformats

.Improved vascular and cardiac imaging

.Potential for faster throughput of patients

DUAL SOURCE CT

Two X-ray sources are used operating at different potential (kv).The interaction between the two with tissues in the body is different and by comparing those differences in to images ,radiologists can differentiate , characterize ,isolate and distinguish body tissues and fluid ,leading to breakthroughs in medical imaging

Advantage

.Ability to scan at any heart rate

.No need to use beta –blockers

.Better temporal resolution -83ms

.Trauma ,oncology, obese patient

HIGH RESOLUTION CT SCAN (HRCT)

It is a type of  CT technique done for some specific diseases like

. Interstitial lung disease

.Brounchiectasis

.Miliary tuberculosis

.Temporal bone evaluation

CSF leaks

Principle

.Thin slice-thickness or collimation

.Narrow field of view

.Bone algorithm for image reconstruction

RADIATION DOSE IN A CT SCAN

CTDI (CT dose Index) : CT dose index (CTDI) IS A standardized measure of radiation dose output of a CT scanner which allows the user to compare radiation output of different CT scanners.

  

     CTDI represents the average absorbed dose, along the z-axis, from a series of contiguous irradiations .It is measured from one axial CT scan. and is calculated by dividing the integrated absorbed dose by the nominal total beam collimation .The CTDI is always measured in the axial scan mode for a single rotation of the X-ray source , and theoretically estimates the average dose with in the central region of a scan volume consisting of multiple ,contiguous CT scan for the case where the scan length is sufficient for the central dose to approach its asymptotic upper limit .

Dose Length Product(DLP)

DLP= CTDI vol  x scan length

The DLP reflects the total energy absorbed attributable to the complete scan acquisition. Thus, an abdomen –only CT exam might have the same CTDIvol as an abdomen /pelvis CT exam ,but latter exam would have a greater DLP, proportional to the greater  z-extent of the scan volume