What is Radiography?
Radiography allows for visualization of internal structures because of their difference in density. For example, bone is more dense than air. The molecules that make a bone are packed tightly together; therefore, bone is dense. The molecules that make air are spread relatively apart; therefore, air is less dense than bone. Because of this difference in density, bone and air will appear differently on the final image. How does this work?
It starts in the x-ray tube. Through applications of physics, the tube generates x-rays.
The tube only makes x-rays when the technologist goes to his/her panel and tells the computer to take the x-ray. Having a radiograph will not make you radioactive. As soon as the technologist stops pushing his/her x-ray button, the x-rays go away.
How do these x-rays aimed at the knee lead to an image? Some x-rays are absorbed by the body part, and some go through the part to the image receptor. There are five different basic radiographic densities: metal, bone, soft tissue, fat, and air. Metal absorbs a lot of x-rays because metal is very dense. Air does not absorb many x-rays because air is not dense. Therefore, metal and air look differently on the image.
It starts in the x-ray tube. Through applications of physics, the tube generates x-rays.
- Technologist is the term used for the person taking the radiograph. A radiologist is the physician that reads the images.
- The technologist controls a number of factors to minimize the amount of x-rays used in the exam. X-rays are a source of ionizing radiation, but the amount used to obtain a radiograph is relatively small.
- The technologist should give the patient a lead shield to protect adjacent tissue.
- Finally, the technologist focuses the x-rays so that they only go through the part of interest.
- A patient’s physician can weigh the risks and benefits of taking an x-ray. In most cases, the benefit of the knowledge gained outweighs the risk of the small amount of x-rays. The physician knows most about an individual patient’s care and is best able to make this decision.
The tube only makes x-rays when the technologist goes to his/her panel and tells the computer to take the x-ray. Having a radiograph will not make you radioactive. As soon as the technologist stops pushing his/her x-ray button, the x-rays go away.
How do these x-rays aimed at the knee lead to an image? Some x-rays are absorbed by the body part, and some go through the part to the image receptor. There are five different basic radiographic densities: metal, bone, soft tissue, fat, and air. Metal absorbs a lot of x-rays because metal is very dense. Air does not absorb many x-rays because air is not dense. Therefore, metal and air look differently on the image.
The Normal Knee Radiograph
Now, you will learn about the appearance of a normal knee radiograph. The purpose of this section is to give you a basic introduction. From this module, you will not be able to diagnose or interpret images, but you will have a better sense of the necessity of radiographs to evaluate common knee injuries.
When a patient goes to the radiography department for an x-ray of his/her knee, several views of the part are needed to see all of the relevant anatomy. Therefore, more than one x-ray of the knee is usually taken. Some views show the patellofemoral joint better, while others show the joint space between the femur and tibia. Imaging facilities have protocols that tell the technologist which views to obtain of each body part. The technologist knows a wide range of views and can adapt depending on pathology and radiologist request.
Radiographs are excellent for looking at bony abnormalities. The soft tissues in the body have similar densities to one another; therefore, they appear similar on a radiograph. Radiographs provide some information about soft tissue, but the data is limited. Today you will focus on radiographs and bony anatomy.
In this section, you will learn three basic images of the knee:
When a patient goes to the radiography department for an x-ray of his/her knee, several views of the part are needed to see all of the relevant anatomy. Therefore, more than one x-ray of the knee is usually taken. Some views show the patellofemoral joint better, while others show the joint space between the femur and tibia. Imaging facilities have protocols that tell the technologist which views to obtain of each body part. The technologist knows a wide range of views and can adapt depending on pathology and radiologist request.
Radiographs are excellent for looking at bony abnormalities. The soft tissues in the body have similar densities to one another; therefore, they appear similar on a radiograph. Radiographs provide some information about soft tissue, but the data is limited. Today you will focus on radiographs and bony anatomy.
In this section, you will learn three basic images of the knee:
- AP Projection
- Lateral (usually medio-lateral projection)
- Sunrise view of the patella
The Normal Knee Radiograph - AP Projection
The first radiograph in the knee protocol will be the AP projection. Projection is a word used to describe the path of the x-rays. AP stands for Anterio-Posterior. Then, AP Projection means that the x-rays go through the front (anterior) of the knee and exit the back (posterior) of the knee.
An AP knee image should have all of the bone anatomy and soft tissue associated with the joint. The image should have the patient’s name, medical record number, and other information. The image should also have a marker--an R (right) or an L (left) so that the physician knows which knee was imaged. An AP x-ray of the knee can be taken with the patient sitting on the x-ray table or with the patient standing at an upright wall unit. The decision to take the image sitting or standing is determined by the patient’s pathology and by the protocol of the site. For example, if a trauma patient came into the ER with a broken femur, he/she would obviously have the image taken on the table, not standing. If a different patient walks into the outpatient clinic to have his/her arthritis evaluated, then the physician may find a standing, weight-bearing image of the joint helpful. When looking at the normal AP image:
|
|
|
The Normal Knee Radiograph - Lateral Projection
The second radiograph in the knee protocol will be the lateral projection. Usually, a Medio-Lateral Projection of the knee is obtained. In anatomical planes, medial means closer to the middle or center, and lateral means closer to the outer edge. For example, your nose is medial, and your ears are lateral. With regard to the Medio-Lateral Projection of the knee: The x-rays go through the inner side of the knee (medial) and exit the outer side of the knee (lateral).
A lateral knee image should have all of the bone anatomy and soft tissue associated with the joint. The image should have the patient’s name, medical record number, and other information. The image should also have a marker--an R (right) or an L (left) so that the physician knows which knee was imaged. Again, a lateral of the knee can be taken with the patient on the x-ray table or standing upright depending on the circumstances. When looking at the normal lateral image:
|
The Normal Knee Radiograph - Sunrise/Horizon View
The third radiograph in the knee protocol will be the Sunrise view. This image will show the patella free of superimposition. It looks like a sun floating above the horizon. This view demonstrates the patellofemoral joint space. There are several ways to acquire the Sunrise view, but the patient does not stand for any of them.
For this image, it is important for the patient to relax his/her quadriceps femoris muscles. These muscles form the quadriceps tendon which attaches to the patella. If the muscles are contracted, then they can move the patella and show a false normal appearance. The technologist will position the patient so that his/her legs are supported. Having the muscles relaxed will allow the patella to go into its neutral position. When looking at the normal Sunrise view:
|
|
In summary, there are many different radiographic views that demonstrate knee anatomy. This section focuses on the AP projection, Lateral projection, and Sunrise view. The protocol depends on the patient’s pathology and clinical site preferences. The radiologic technologist can adapt to provide the optimal images necessary of the knee. As a side note, computed tomography (CT) uses the same x-ray physics and the differing densities of tissue to create an image.
Knee Radiographs and Common Running Injuries
Radiography is helpful for evaluating fractures. A common running injury discussed in this module is the stress fracture. Radiologists go to school for years and have expertise to guide them in the hunt for fractures. They have the experience to know what is normal and what is abnormal. Today, you will look at two radiographic stress fracture cases as examples.
When viewing an image that contains a stress fracture, radiologists look for disturbances in the cortex. It should be contiguous along the edge of the bone. Also, they look for any opacities or lucencies that are abnormal. Irregularities in the trabeculae (pattern of bone matrix) and soft tissue signs can be red flags.
When viewing an image that contains a stress fracture, radiologists look for disturbances in the cortex. It should be contiguous along the edge of the bone. Also, they look for any opacities or lucencies that are abnormal. Irregularities in the trabeculae (pattern of bone matrix) and soft tissue signs can be red flags.
Radiographic Case Study #1: Stress Fracture
The first case for review contains an AP and Lateral radiograph of the left knee. There is a stress/fatigue fracture in the distal femur (part of the femur that is near the knee). It is seen because the area is more opaque than the surrounding bone. It may be more obvious on the lateral image.
Radiographic Case Study #2: Stress Fracture
The second case for review contains an initial AP radiograph and a follow-up AP radiograph two months later. There is a stress/fatigue fracture in the proximal tibia (part of the tibia near the knee). On the initial image, it is seen by a slight opacity and disruption in the trabeculae (pattern of bone matrix). On the follow-up image, the area is more apparent because of an opacification related to the bone’s healing process.
Radiography Self-Check For review, take the Radiography self-check to the right. What Next? You have learned that radiography is a helpful modality to evaluate the knee. X-rays are particularly useful to view the bones of the knee. What about soft tissue injuries? Next, you will learn about MRI and its uses. |
References
1. Radiographs and MRI Images: Permission to use these images has been granted to Melissa Culp by Jordan Renner, MD and the Division of Radiologic Science at UNC-Chapel Hill. These images are to be used for this instructional module only and are not to be reproduced.
2. Imaging Content written by: Melissa Culp, BS, RT(R)(MR)
Additional Resources
1. Frank ED, Long BW, Smith BJ, eds. Merrill’s Atlas of Radiographic Positioning and
Procedures. 12 ed. St. Louis, MO: Elsevier; 2012.
2. Kelley LL, Petersen CM, eds. Sectional Anatomy for Imaging Professionals. 3rd ed. St. Louis, MO: Elsevier; 2013.
3. Westbrook C, Roth CK, Talbot J, eds. MRI in Practice. 4th ed. Chichester, West Sussex, UK: Wiley-Blackwell; 2011.
2. Imaging Content written by: Melissa Culp, BS, RT(R)(MR)
Additional Resources
1. Frank ED, Long BW, Smith BJ, eds. Merrill’s Atlas of Radiographic Positioning and
Procedures. 12 ed. St. Louis, MO: Elsevier; 2012.
2. Kelley LL, Petersen CM, eds. Sectional Anatomy for Imaging Professionals. 3rd ed. St. Louis, MO: Elsevier; 2013.
3. Westbrook C, Roth CK, Talbot J, eds. MRI in Practice. 4th ed. Chichester, West Sussex, UK: Wiley-Blackwell; 2011.