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Hello, my name is Mahesh. I'm a medical physicist,

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and a professor at Johns Hopkins University School of Medicine.

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It is my pleasure to speak on the CT physics,

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the series of topics I'm going to cover in this area.

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And my expertise is in the area of imaging physics.

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CT is one of my specialties.

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And here is a disclosure. This is a book I had written a few years ago.

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I'm in the process of editing it. This is the "MDCT Physics"

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written for clinicians.

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The way I want to do this lecture on CT physics is to

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group this discussion under these five main categories.

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Under the main categories such as CT technology overview,

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CT image quality,

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CT dosimetry, including the patient dose estimation.

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I also want to talk about separately on the cardiac CT and perfusion CT.

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Finally, under the CT physics, I want to

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talk the latest aspect of spectral CT physics that includes

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dual energy and photon counting CT.

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So first I just want to start giving a general overview of the

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CT technology. As you can see, this is a

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photograph of the current CT scanners.

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There's nothing great. These are the four major manufacturers

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which dominate the CT field, both in the US and globally.

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There are other vendors now also introducing

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CT scanners. I just wanted to show you the four major

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manufacturers which dominate the CT field in

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the US and around the world.

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In order to start about this, doing a technology overview, I

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want to talk about, just to give a basic introduction, this

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is already more than 56 years since CT technology was

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introduced. It evolved considerably since its invention in

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1970.

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But if you look back at the technology development,

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we see multiple milestones. Among them, there are

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two major evolutionary leaps occurred during

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1990s. That is the introduction of helical or spiral

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CT, which occurred in the early '90s.

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And later in late '90s or early 2000,

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the introduction of multi-detector CT came into picture.

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These two we consider as a major milestone, which open up

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the field of CT, and in fact, according to the

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recent surveys, among the top 30

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innovations in medical sciences, CT is one of the

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highly advanced innovation, which comes in the top

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30

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revolutions in medical science.

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Historically, the person who developed the early CT,

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Sir Godfrey Hounsfield, and Allan

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Cormack, got a Nobel Prize in medicine for development

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of CT back in 1979.

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The principle of CT reconstruction actually goes back to

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even 1917, when

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a French mathematician, Radon, introduced the concept of

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image reconstruction.

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But that took some time because the technological

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development needed to do computing this image reconstruction.

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So just want to give an idea of the

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history.

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This is a picture of Sir Godfrey Hounsfield

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and one of the experimental systems he used

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when he was developing the CT technology.

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Again,

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just for a digressive moment, the CT

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technology, actually, Hounsfield worked for a gramophone company,

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but the gramophone company, EMI, never thought this would be

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a major issue, so they sold it off, and that's the history now.

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That technology was purchased by Picker and later to

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GE, and now you see the history behind it.

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There are different variety of generations of CT.

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I'm not going to talk about the earlier generation one and two

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because they are no longer being used.

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The reason I just want to just jump into third generation CT is

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this is the principle we still use today.

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The third generation CT actually had a series of

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detectors, 700 to 1,000 detectors at that

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time,

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with an X-ray tube going around the patient.

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So basically, it created a wide beam.

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The detector collected the information, and reconstruction was done to

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create what the image object is in the image.

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So this third generation principle is exactly

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used even today with the multi-detector CT or photon counting

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CT, where there is a series of detectors

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opposite to the data connected, focused towards the X-ray tube, and they

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simultaneously collect the data when the X-ray tube is turned on.

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And we call it as a wide fan beam technology.

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Here, both the source and the detector rotates.

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There are other generation, fourth generation and other things which I'm not going

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to talk because these are no longer commercially available.

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So this is the type of images we are able to obtain now.

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Amazing technology which has revolutionized the diagnosis,

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like all of you know. CT is one of the most commonly used

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modalities, both in radiology and the

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fundamental in radiation oncology, where all the treatment plan for

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patient care, cancer care is done based on the CT

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imaging. And now in the nuclear medicine, we have the hybrid CTs,

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spec CT, and now with the cardiologists are using cardiac

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CT to diagnose early that detection of

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cardiovascular diseases. So it has really

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opened up the whole field. New fields have been developing

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on.In early '90s, one of

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the milestones which has led to the explosion of CT was

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the development of helical spiral CT.

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Three things came into picture which enabled for helical CT.

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This is the same principle we use currently.

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One is the development of slip ring technology, slip ring

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gantry, and the second is development of interpolation

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algorithms, and the third one is the development of high-power

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X-ray tube, which can be turned on for a longer time,

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and the heat produced by the X-ray tube was taken away, so the X-ray

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tube can be kept on.

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Prior to the helical CT, the way CT was done is the

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patient was scanned one rotation at a time because

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the connection of the wires connecting the X-ray

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tube and the detector kind of blocked continuous

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rotation. So what I meant is the development of

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slip ring technology really enabled the advancement

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of CT from the past 30 years. What I'm trying to show

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here is the photograph of an inside of a CT

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gantry where you can see here the X-ray

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tube is no longer connected with the physical wires.

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So it enables X-ray tube to rotate continuously.

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It doesn't have to rotate in one direction and unwind the wires,

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which was the case previously. But now, with the slip ring

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technology, the power to the X-ray tube is enabled by

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an outside entity. Here, the slip ring.

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These grooves will show how the current will passed on,

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and that is connected to the X-ray tube through

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electrical brushes. So the X-ray tube now is

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free-wheeling inside the gantry, which can continuously rotate.

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That is what enables us for doing the continuous rotation and continuous

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data connection.

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The detector at the same time also do not have any physical wire.

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They do have what is called as the slide contract, through which

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the data is pulled out.

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So

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the helical CT means the patient is continuously pushed through the

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table, through the gantry while the data is acquired.

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Which means if you look at just the trace the X-ray

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beam path, you're going to get more like a helix, not like

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a circular rotation, but more like a helix.

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Imagine you are trying to mount a photograph on a

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wall, and you have a screw, and you rotate the screw, and the screw go

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inside. That's exactly similar to helical CT,

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where the patient is rotate continuously transported through the

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gantry while the X-ray beam is turned on continuously.

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Therefore, the X-ray goes around the patient in a helical path.

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But this is important because the development of interpolation

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algorithm is very important because in order to do

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the image reconstruction, we want the data in all

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in one direction, one plane.

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With the helical rotation, the data is not in one plane.

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Therefore,

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these interpolation algorithms enables us to

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interpolate the data from adjacent helix to put

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the data in one plane. So basically, mathematically, we are projecting the

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data into the plane and then apply all the reconstruction

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principle to reconstruct the image in this particular plane.

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So the interpolation algorithms development was also key,

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and the person behind was a medical physicist by Willie Callender,

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is called as one of the father figure in our field, who kind of

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derived the first interpolation algorithm.