Chest CT Technique and Anatomy

00:01 So now let?s discuss some normal chest CT anatomy and technique to help give us a basis for looking at some pathology.

00:07 So, the radiation dose from a CT varies based on the type of machine, the type of scan, and the patient?s body size.

00:16 Every different machine has a slightly different type of radiation actually, the older machines emit somewhat more radiation than some of the newer machines do and the type of skin depending on how large the area of body is being scanned can also emit different types of radiation.

00:30 Larger patients tend to absorb more radiation than smaller patients do as well.

00:35 Radiation however is additive so it?s important to remember that multiple scans should be limited whenever possible.

00:41 So let?s review Hounsfeld units again.

00:44 A Hounsfeld unit is a measure of the density of a structure.

00:48 Density is the amount of radiation that a structure absorbs and Hounsfeld units range from about negative 1000 to about positive 1000. Air is the lowest Hounsfeld unit and it measures close to about negative 1000.

01:01 Bone is one of the highest Hounsfeld units and it measures almost positive 1000.

01:07 Everything else falls somewhere in between with water being at approximately zero. So, what are window levels? We briefly discussed this initially, it?s digital manipulation of the image that helps accentuate structures of various different Hounsfeld units.

01:22 Window levels can be changed by the radiologist at the work station and this is done as a post-processing mechanism so the CT scan doesn?t have to be obtained in different window levels.

01:32 So this is an example of the different types of window levels.

01:35 The image on the left is an example of soft tissue window and you can see that the muscles and the soft tissues are very well seen.

01:42 The image in the middle is an example of bony windows and you can see that the bones, the ribs, and the spine are very well seen on this, and the last image is one of lung windows and you can see the lung parenchyma quite well.

01:54 You can see the vessels throughout the lung parenchyma which you can?t see on the other images.

01:58 So these are the major types of window levels although there are many other window levels that fall in between these that can also be performed.

02:05 And how about CT planes? So, CT images can also be post-processed into different planes.

02:12 They?re acquired in the axial plane which is the one that we see here and that is as if you were looking at the patient from their feet up to their head. By convention, this side is always the right side of the patient and then this side is always the left side of the patient and this stands true throughout all of radiology.

02:33 It?s as if you were looking at the patient and the patient is facing you so the side on your left is always the right side of the patient, this is anterior and then this is posterior. So, this is an example of the sagittal plane.

02:49 This is as if you were looking at the patient from their side, so this is the front of the patient, this is the back of the patient, so you can see the thoracic spine is well seen here, this has been manipulated into bony window and then this is the sternum anteriorly.

03:06 And this last window plane is a coronal.

03:11 So this is as if you were looking at the patient from the front, so the patient is now looking at you and standing up.

03:16 So again by convention, this is the right side of the patient, this is the left side of the patient, these are the patient?s arms, and then this is the patient?s abdomen.

03:28 So let?s review some CT anatomy, we?ll start off with some axial mediastinal anatomy.

03:38 So, let?s take a look at all of these.

03:40 This is the post-contrast axial image through the chest, it cuts about mid-way through the mediastinum and you can see here, the superior vena cava.

03:50 This has the most contrast in it because the contrast is injected intravenously and tends to collect within the superior vena cava first.

03:58 Here we have the ascending aorta and then here we have the descending aorta.

04:05 In between here is the pulmonary artery, so we have the right main pulmonary artery and we have the left main pulmonary artery here.

04:17 Right here where the pulmonary artery bifurcates is the hilum and then the air-filled structures here are the two bronchi in addition to the lungs on each side here.

04:29 Coronal mediastinal anatomy is also important to recognize, so we have here the left ventricle with a thick muscle which is to be expected of the left ventricle.

04:46 You have contrast within the right atrium here and here you have a portion of the pulmonary artery, then you have a portion of the aorta.

04:55 Down here is the abdomen and you can see a portion of the liver.

04:59 So again, remember that this is just a single slice through a coronal CT, you will have multiple slices that slice through the chest but these are some of the more critical findings that you may see in terms of anatomy.

05:10 So again, you can look at each of these planes in different windows so let?s take a look at the coronal lung anatomy.

05:17 So here we have again a patient that you?re looking at, the patient is facing you. So to orient you a little bit again, this is the right side of the patient and then this is the left side of the patient, so we have both upper lobes, you can see here.

05:32 You can see a very faint minor fissure coming down right here and then a faint major fissure coming down right here, and so though as to separate the different lobes.

05:44 So in between the two, we have the right middle lobe and then inferiorly we have the right lower lobe.

05:49 So remember that the right lung is divided into upper, middle, and lower.

05:53 On the left side we only have two, we have the left upper lobe and we have the left lower lobe, and those are divided by the major fissure which is right here.

06:05 In between here, this air-filled structure is the trachea.

06:10 So here?s an example of axial lung anatomy.

06:16 See here we have the upper lobes which are seen superiorly and anteriorly as well, upper lobes on both sides here, here we have air-filled bronchi, we have one here and we have one here, and then we have an example of the major fissure right here which separates the right lower lobe, and then here we have an example of the major fissure on the left which separates the left lower lobe.

06:46 As we go further down, we would see an axial image of the minor fissure which is a little bit harder to see on axial images.

06:53 So let?s take a look at sagittal lung anatomy.

07:01 So again, remember that a sagittal view is as if you were looking at the patient from their side.

07:06 So these are two sagittal views, one from each side and here, we have posterior and then here, we have anterior.

07:16 So these are two patients as if their backs were towards each other.

07:20 Here, we have posterior for this patient and then here, we have anterior for this patient, and this shows us the anatomy of each lung.

07:28 So, this is taking a look at the patient?s right lung and you can see here the upper lobe, again, the middle lobe, and the lower lobe, and you can see these are divided by very fine fissures that can be somewhat difficult to see but as you scroll through, it would make it a little bit easier.

07:45 On the left we have just the two lobes and again, a large portion of the upper lung is the upper lobe and then we have a portion of the lower lung which is the lower lobe.

07:56 Here, we have a very small portion of the heart that?s being visualized which you would see better as you scroll through.

08:02 So the major fissure separates the lower lobes from the upper and middle lobes depending on which side you?re looking at.

08:10 The minor fissure is found only on the right side and that separates the right upper lobe from the right middle lobe.

08:17 So we?ve reviewed some normal CT anatomy and this will provide a good basis for us to proceed on to discussing some pathology.