Positron Emission Tomography

ROC7010 Imaging Physics 2: Nuclear Medicine Imaging

Written for Dr. Otto Muzik

By Kurt William Van Delinder

After first perusing this assignment, I did have concerns with the requirement to

observe two PET imaging procedures and complete the task of writing a written report. I

have spent a great deal of time in different oncology departments and have familiarized

myself with the PET imaging procedure through numerous reading assignments. Also, I

have hands on experience with CT, US, Fluoroscopy, and standard x-ray imaging. I was

uncertain of how valuable it would be to observe this imaging modality in person, when I

have had common everyday experience with the other imaging techniques.

After having taken part in the experience of attending the clinical visit, I can say

with certainty that my pre-conceived notions about observing the PET imaging

procedures were starkly incorrect. I admittedly enjoyed the visit and found it to be a great

educational experience for learning the Positron Emission Tomography procedure. I

arrived at approximately 8:00am on March 12th

and attended the Nuclear Medicine

department until roughly 11:00am. Throughout the duration of my visit, I partook in

consistent conversation regarding PET; asking questions and attempting to gain insight

into this imaging technique. The staff treated me exceptionally well; as if I were a tenured

medical professional, rather than a current student familiarizing myself with the field.

The staff employees, that I had interaction with, were very knowledgeable, professional

and friendly. They personally gave me a tour of the different environs related to PET.

This consisted of the PET-CT scanner, older PET Machine, Cyclotron, quality check lab,

patient rooms in which the radioactive nuclide is injected, chemistry lab, and any other

related rooms. Throughout my visit I wrote down numerous pages of notes but for the

sake of this written assignment, I will focus in on five aspects of PET that I found

personally interesting.

Firstly, the most interesting aspect to me regarding PET is the concept of injecting

a radioactive nuclide into the patient for the purpose of imaging. Of course, I read of this

application many years ago, but it is unique to be able to observe the procedure in person;

especially contrasting with the other main imaging modalities i.e. CT, MRI, US, etc. Due

to this administration of a live radioactive source, there are many unique protocols and

considerations to PET and NMI that arent necessarily required for other imaging

procedures. For example, the extra safety precautions and concerns that are required for

maintaining an in-house cyclotron and also, being able to produce the desired radioactive

isotope for imaging.

Progressing from my first point, the administration of a live radioactive substance,

leads me directly into my second, the utilization of different sources. As typically known,

the most common radioisotope employed within PET is F-18 (T

= 110 mins) and it is

bound to a sugar molecule entitled Fluoro-deoxyglucose. But, what was interesting to

find out was the application of many other different isotopes with many different half-

lives that can also be employed for PET. Not an exhaustive list but some other examples

are: 68-Ga with a T

= 68 mins, 11-C with a T

= 20 mins, 13-N with a T

= 9 mins, 15-

O with a T

= 123s and 82-Rb with a T

= 78s (1). Having a repertoire of various

radionuclides with different half-life duration times gives the health care practitioner the

possibility to partner the ideal radioactive source with that of the intended goal of the

imaging procedure.

Due to my background in health care, which is within the field of radiation

therapy, I often had to perform CT scans in simulation for planning and setup that are to

be used for the duration of a radiation therapy treatment. Often, the cancerous sites are

area specific, for example; Head & Neck, Lung, Breast, etc. So commonly, when I am to

perform a CT scan, there is some leeway as to the size of the area that can be scanned,

but typically the scan is confined to the approximate area that is going to be treated. This

is done to minimize radiation dose exposure. The risk of radiation exposure isnt a dire

concern as compared to the great health risk posed by being diagnosed with a cancerous

disease. However, depending on the imaging techniques purpose and with great

consideration to the patients age and previous exposure, a common working theme is to

cone down and minimize the imaging field, while giving appropriate boundary to the

main area that will be imaged. Observing the combined PET with CT scans, I noticed that

the CT scan sizes were very large. I observed full body, or at least close to full body

scans were performed. It should be noted that just undergoing a PET-CT will not

administer a substantial amount of radiation but combined with many other modalities the

dose can be significant. For example: A patient undergoes a PET-CT scan along with

another radiation diagnostic technique and then finds out that they have developed a

malignant form of disease. The patient now undergoes a CT scan within sim for

Radiation Oncology planning purposes. Image guided radiation therapy is commonly

performed as a CBCT or x-ray port imaging fields will then be utilized for treatment

localization. Of course, the actual treatment itself, is radiation based and administers a

very large radiation dose. I do have an appreciation of the designed application of using a

full body PET-CT in order to investigate the possible occurrence of a secondary

malignancy, but in the example I have explained, which isnt farfetched the total

radiation exposure would be of great concern.

The fourth interesting aspect that I learned about PET is the stark difference in

time that it takes to scan a person as compared to CT. CT scans are very fast, they can be

performed in the order of a few minutes. Due to the quick nature of performing a CT,

there isnt really any real protocol for a poorly scanned patient, just mathematical

algorithms to improve the overall image quality. If there is a problem, the scan is simply

re-scanned. The duration to perform a PET scan is much longer, approximately 30-45

minutes. Upon hearing the scan duration, the next logical question is to ask what happens

if the patient moves during the scan? In PET, they have the ability to repeat certain

sections of the total body scan. So, commonly, once a full scan is performed, the

technicians re-scan areas that may showcase artifacts or simply poor imaging quality.

This was very interesting to me having spent most of my time working with CT.

Lastly, my final concept regarding PET would have to be related to the imaging

modalities application in health care. Before observing the procedure and discussing the

imaging technique, my initial understanding was that PET was only used in the diagnosis

of a few abnormal illnesses but generally wasnt commonly performed. By glancing at

the DMCs PET schedule and observing an average of approximately 8-10 scans a day

for a workload of 40 patients per week, I realized that this isnt so. Generally, PET scan

imaging procedures are used for five loosely grouped areas of disease: Conditions

affecting the brain, Heart, types of cancer, Alzheimers disease, and various neurological

diseases (2). Conditions affecting the brain refer to patients who have memory

disorders of an undetermined cause, suspected or proven brain tumors or seizure

disorders unresponsive to medical therapy (2). Pet scans used for heart conditions focus

on the ability to determine blood flow; allowing the capacity to detect strokes, myocardial

infarctions and coronary artery disease. PET can also be used to diagnose cancer and

determine metastasis or reoccurrence. Not all cancers can be detected but a list of a few

are: melanoma, lung carcinoma, breast carcinoma, lymphoma, liver mets from colon

carcinoma, rectal esophageal etc (3). For Alzheimers disease, PET is able to display a

biochemical change. Neurological diseases are referred to as neurological syndromes

within the main central nervous system and can be affiliated with many different and

often rare diseases. The problems are often caused by abnormal antibodies in the blood

transferred to the spinal fluid.

In summary, I found my clinical visit to the PET department to be both

entertaining and largely educational. It was refreshing to be able to learn about this

imaging modality by observing the processes and techniques in person. Studies have

often shown that being able to learn in an interactive environment increases your ability

to memorize the information and retain it over a greater duration of time. This is

commonly believed to be due to a greater region of the brain being activated while in the

process. Whether this is true or not I cant decisively say but, I enjoyed having my head

out of a book, even if it was just for a small duration of time.

Sources:

Radiologieplzen.eu, Information Portal Department of Medical Imaging, Plzen, Czech Republic, WWW Document, (http://radiologieplzen.eu/wp-

content/uploads/PETCT-A-SPECTCT--ENGLISH-version-PPTminimizer.ppt).

University of Buffalo Department of Nuclear Medicine, Positron Emission Tomography PET, WWW Document,

(www.santarosa.edu/~yataiiya/4D/PET%20Presentation.pp).

(BNL) Brookhaven National Laboratory, The Physics of Positron Emission Tomography (PET), WWW Document,

(www.bnl.gov/ncss/files/ppt/NucChemSummerSchool-072106-v2.ppt).