Figure 16.25 shows the evolution of the detector and
source configurations [of CT]. The third generation configuration is
the most popular. All of the electrical connections are made
through slip rings. This allows continuous rotation of the
gantry and scanning in a spiral as the patient moves through
the machine. Interpolation in the direction of the axis of rotation
(the z axis) is used to perform the reconstruction for a
particular value of z. This is called spiral CT or helical CT.
Kalender (2011) discusses the physical performance of CT
machines, particularly the various forms of spiral machines.
Kalender obtained his PhD in 1979 from the University of Wisconsin’s famous medical physics program. He then went to the University of Tübingen in Germany. There, according to Wikipedia, “he took and successfully completed all courses in the pre-clinical medicine curriculum.” This is interesting, because just a few years earlier Russ Hobbie did the same thing in Minnesota.
Between 1971 and 1973 I audited all the courses medical students take in their first 2 years at the University of Minnesota. I was amazed at the amount of physics I found in these courses and how little of it is discussed in the general physics course.
With deep sadness, the ESR announces the passing of Prof. Willi Kalender on October 20, 2024 at the age of 75. A pioneering figure in diagnostic imaging and medical physics, Prof. Kalender significantly influenced the field through his groundbreaking research and leadership.
Diffusion MRI was born in the mid-1980s. Since then, it
has enjoyed incredible success over the past 40 years, both
for research and in the clinical field. Clinical applications
began in the brain, notably in the management of acute
stroke patients. Diffusion MRI then became the standard
for the study of cerebralwhite-matter diseases, through the
diffusion tensor imaging (DTI) framework, revealing abnormalities
in the integrity of white-matter fibers in neurologic
disorders and, more recently, mental disorders. Over time,
clinical applications of diffusion MRI have been extended,
notably in oncology, to diagnose and monitor cancerous
lesions in almost all organs of the body. Diffusion MRI
has become a reference-imaging modality for prostate and
breast cancer. Diffusion MRI began in my hands in
1984 (I was then a radiology resident and a PhD student in
nuclear and particle physics) with my intuition that measuring
the molecular diffusion of water would perhaps allow to
characterize solid tumors due to the restriction of molecular
motion and vascular lesions where in circulating blood
“diffusion” would be somewhat enhanced. This idea was
to become the cornerstone of diffusion MRI. This article
retraces the early days and milestones of diffusion MRI
which spawned over 40 years.
I knew Le Bihan when I worked at the intramural program of the National Institutes of Health in the late 1980s and early 1990s. To me, he was mainly Peter Basser’s French friend. Peter was my colleague who worked in the same section as I did (his office was the second office down the hall from mine), and was my best friend at NIH. Le Bihan describes the start of his collaboration with Basser this way:
During the “NIH Research Festival” of October 1990 I
met Peter Basser who had a poster on ionic fluxes in tissues
while I had a talk on our recent diffusion MRI results.
Peter appropriately commented that the correct way to deal
with anisotropic diffusion was to estimate the full diffusion
tensor , not just the ADC [apparent diffusion constant], as the approach of the time provided.
Basically, ADCs are not sufficient in the presence of
diffusion anisotropy, except in particular cases where the
main diffusion directions coincide with those of the diffusion
MRI measurements. To solve this issue Peter and I came
with a new paradigm, the Diffusion Tensor Imaging (DTI)
framework. By applying simultaneous diffusion-sensitizing
gradient pulses along the X, Y and Z axes the diffusion
MRI signal would become a linear combination of the
diffusion tensor components. From the diffusion MRI signals
acquired along a set of non-colinear directions, encoding
multiple combinations of diffusion tensor components
weighted by the corresponding b values, it would be possible
to retrieve the individual diffusion tensor components at
each location.
In Le Bihan’s Figure 3, he includes a photo of Basser, Jim Mattiello, and himself doing an early diffusion tensor imaging experiment. Le Bihan was the diffusion MRI expert and Mattiello (who worked in the same section as Basser and I did at NIH, and who I’ve written about before) was skilled at writing MRI pulse sequences. When they started collaborating, Basser knew little about magnetic resonance imaging, but he understood linear algebra and its relationship to anisotropy, and realized that by making the “b vector” a matrix he could obtain important information (such as its eigenvalues and eigenvectors) that would determine the fiber direction.
Denis Le Bihon (left), Peter Basser (center) and Jim Mattiello (seated), circa 1991.
Diffusion MRI works because spins that are excited by a radiofrequency pulse will then diffuse away from the tissue voxel being imaged, degrading the signal. The degradation is exponential and given by e–bD, where D is the diffusion constant and b is the “b-factor” that depends on the magnetic field gradient used to extract the diffusion information and the timing of the gradient pulse. I had always thought that this notation went way back in the MRI literature, but according to Le Bihon’s article he named the “b-factor” after himself (“B”ihon)!
Le Bihon describes how the clinical importance of diffusion MRI was demonstrated in 1990 when it was found that stroke victims showed a big change in the diffusion signal while having little change in the traditional magnetic resonance image. In fact, Le Bihon claims that the other big advance in MRI of that era—the development of functional MRI based on the blood oxygenation level dependent (BOLD) imaging—has not yet led to any clinical applications, while diffusion imaging has several.
Le Bihon’s article concludes
Diffusion MRI, as its additions, DTI and IVIM [IntraVoxel Incoherent Motion] MRI, has
become a pillar of modern medical imaging with broad
applications in both clinical and research settings, providing
insights into tissue integrity and structural abnormalities. It
allows to detect early changes in tissues that may not be visible
with other imaging modalities. Diffusion imaging first
revolutionized the management of acute cerebral ischemia
by allowing diagnosis at an acute stage when therapies can
still work, saving the outcomes of many patients. Diffusion
imaging is today extensively used not only in neurology
but also in oncology throughout the body for detecting and
classifying various kinds of cancers, as well as monitoring
treatment response at an early stage. The second major
impact of diffusion imaging concerns the wiring of the brain,
allowing to obtain non-invasively images in 3 dimensions of
the brain connections. DTI has opened up new avenues of
clinical diagnosis and research to investigate brain diseases,
revealing for the first time how defects in white-matter track
integrity could be linked to mental illnesses.
If you want to learn more about diffusion MRI, I recommend Le Bihon’s article. It provides an excellent introduction to the subject, with a fascinating historical perspective.
Dr. Katelyn Jetelina is the founder of “Your Local Epidemiologist,” a public health newsletter that reaches nearly 300,000 people in over 130 countries. Jetelina has a masters in public health and a PhD in epidemiology and biostatistics. She says that her “main goal is to translate the ever-evolving public health science so that people will be well-equipped to make evidence-based decisions.” This year she was named one of TIME magazine’s most influential people in health (that’s how I found out about her). You can find her website at https://yourlocalepidemiologist.substack.com.
These two science communicators gain their credibility because they read, understand, and can explain the scientific literature. Their views usually reflect the scientific and medical consensus. Another way to learn about that consensus is from various scientific and medical professional organizations.
Jetelina and Love both try to reach readers and listeners who may have legitimate questions and concerns about public health controversies. I admire this, and since the election I keep telling myself to be like Katelyn and Andrea; don’t be consumed by frustration and fury, and don’t attack those who disagree with you. But then I compose something like this blog post and I find myself writing with anger and hate. I guess I need both their newsletters to keep me from boiling over, and to serve as examples of how to discuss complex topics rationally.
I follow both Jetelina and Love on Twitter (I refuse to call it “X”). But during the presidential campaign I found Twitter to be a cesspool. I’ve been staying off social media since election day (except, of course, to publish my weekly blog post on Facebook). I’m thinking about deleting my Twitter account, but I’ll probably return to Facebook eventually. I haven’t yet gathered the courage to watch the evening news. I just can’t stomach it. I’m self-medicating by reading P. G. Wodehouse stories, and I’m trying to address my anger management issues. It isn’t easy.
I worked at the National Institutes of Health for seven years. It’s a wonderful institution, which I have tremendous respect for. It pains me to even hint that they might not be the most trustworthy source of health information available. But as I look to the future, I just don’t know. Let’s hope for the best and prepare for the worst by subscribing to Jetelina’s and Love’s newsletters. And in these difficult times I can offer you one bit of good news: both newsletters are free!
For those who want a little more physics mixed in with your public health (and who doesn’t?), I recommend my blog (hobbieroth.blogspot.com) associated with my textbook Intermediate Physics for Medicine and Biology, and my book Are Electromagnetic Fields Making Me Ill? (the answer to the title question is no!). I will do my best to give you the truth, but with the storm clouds I see on the horizon I can’t promise I’ll always give it to you cheerfully. I do promise to delete the profanity before I publish any posts.
A conversation with Dr. Katelyn Jetelina about her journey in the field of epidemiology.
The IOMP held a poster design contest to celebrate the event. The winning poster was created by Lavanya Murugan from Rajiv Gandhi Government General Hospital and Madras Medical College in Chennai, India. IDMP coordinator Ibrahim Duhaini (who works right here in Michigan at Wayne State University) wrote that “Her artwork beautifully captures the theme and spirit of this year’s IDMP and will continuously serve as an inspiration to others… Let us all commit to being beacons of inspiration for the next generation.” I couldn’t have said it better (but maybe Randy Travis could).
The award-winning poster, a masterpiece, is shown below. In case you can’t read it, the quote in the center is by Curie: “Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.” Never has this quote been more relevant than now, as we face the dire health threats generated by climate change. I can identify many of the famous physicists and medical physicists in the poster. Can you? By the way, that little sticky note on the upper left of the frame contains a conversion factor indicating that one roentgen deposits 0.877 rads in dry air.
The winning poster of the design contest associated with the International Day of Medical Physics 2024.
Lavanya sent me her thoughts about the design of the poster.
Inspiration: Once, I gave up my dream of becoming an artist to pursue a career in Medical Physics.
This piece of art is a reflection of my study wall and myself, inspired by the world around me.
Technique: It’s a digital Art piece.
This artwork portrays a young girl immersed in her studies, surrounded by images of great scientists who have contributed to the field of radiation. The wall features news clips about Roentgen’s groundbreaking discovery and a picture of Marie Curie’s notebook, symbolizing power of radiating knowledge.
Everyone experiences uncertainty about their knowledge, future and career at some point. Believing in ourselves is the first step to achieving our goals. The individuals whose photos adorn the wall were once in our shoes, grappling with doubts and questioning their abilities. Yet, they persevered, never giving up and ultimately inspiring us in the field of radiation. Today, we proudly serve healthcare and humanity as Medical Physicists, standing on their shoulders.
I have included one of my favourite quotes from Marie Curie, a female scientist who has been inspiring women in research: “Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.”
Everyone fears radiation and its impact on mankind, but people like us choose to be radiation professionals regardless of the risks involved. This quote inspires us to understand the risks for the betterment of this field.
The message I wanted to convey through this art is to inspire the next generation of Medical Physicists to contribute their best to our field, following in the footsteps of the great minds of our past.
Lavanya is a medical physicist with over eight years of clinical experience in radiotherapy, nuclear medicine, and radiology. She excels in treatment planning, quality assurance, and treatment delivery. She’s also an artist, creating artwork under the pseudonym “Nivi.” You can find many of her pieces at her Instagram account. Below I show a few that are related to medical physics.
Lavanya calls this a “boredom doodle.” You can see a tiny version of it to the right of the Curie quote in her award winning poster.
“The main constituents of the atmosphere—oxygen and nitrogen—are transparent to both visible and thermal radiation, so they don’t contribute to eA [the fraction of the earth’s infrared radiation that the atmosphere absorbs]. Thermal energy is primarily absorbed by greenhouse gases. Examples of such gases are water vapor, carbon dioxide, and methane.”
I never discussed why oxygen and nitrogen are not greenhouse gasses, although water vapor and carbon dioxide are. Today, I’ll address this question.
Below is a list of gasses in our atmosphere and their abundance.
Nitrogen (N2) is diatomic; it consists of two nitrogen atoms bound together. Because the two atoms are the same, they share the electron charge equally. If there is no charge separation, then there is no dipole moment to oscillate at the frequency of the infrared radiation. Therefore, diatomic nitrogen—by far the most abundant molecule in our atmosphere, with nearly four out of every five molecules being N2—does not absorb infrared radiation.
It’s not a greenhouse gas.
Oxygen
About one out of every five molecules in the atmosphere is oxygen (O2), which is also diatomic with two identical atoms. Like nitrogen, oxygen can’t absorb infrared radiation.
Argon
Almost one out of every hundred molecules in the atmosphere is argon (Ar). Argon is a nonreactive noble gas, so it consists of individual atoms. A single atom cannot have a dipole moment, so argon can’t absorb infrared radiation. Neither can the other noble gasses: neon, helium, and krypton.
Carbon dioxide
The next most abundant gas is carbon dioxide (CO2), which makes up less than one tenth of one percent of the atmosphere. The above table lists the abundance of carbon dioxide as 0.03%, which corresponds to 300 parts per million (ppm). I must have gotten the 300 ppm value from an old source. Its concentration is now over 400 ppm and is increasing every year. The main cause of global warming is the rapidly increasing carbon dioxide concentration.
The carbon dioxide molecule has a linear structure; it has a central carbon atom surrounded by two oxygen atoms, one on each side, so the molecule forms a straight line. Perhaps instead of writing it as CO2 we should write OCO. The electrons of this molecule are more attracted to the oxygen atoms than the carbon atom, so the carbon carries a partially positive charge and the two oxygen atoms each are partially negative. But because of its linear structure, at equilibrium there is no net dipole moment. You can think of it as consisting of two dipoles with equal strength but oriented in opposite directions, so they cancel out.
Carbon dioxide has three types of “vibrationalmodes” (see the video at the end of this post). One is a symmetric stretch, where the two oxygen atoms move together outward or inward from the central carbon atom. This makes the OCO molecule first get longer and then shorter, but it still consists of two equal but opposite dipoles that add to zero.
Thus, this mode does not produce a dipole, so it cannot absorb infrared radiation.
Carbon dioxide can also undergo an asymmetric vibration, in which one of the oxygen atoms is moving inward or outward, and the other is moving outward or inward. In this case, the molecule maintains the same length, but the position of the oxygen atoms oscillate back and forth, with one being closer to the carbon atom and then the other.
Now the two dipoles don’t cancel, so there’s a net dipole moment. (Think of the dipole moment as the charge times the distance; Even if the partial charge on each atom does not change, the different distances of each oxygen atom from the central carbon atom will alter the net dipole moment.) So, this mode of vibration will absorb infrared radiation. Carbon dioxide is a greenhouse gas.
Just for completeness, CO2 also has bending modes, where the two oxygen atoms move back and forth in a plane parallel to the line of the molecule (see the video).
Again, these modes induce a dipole that can oscillate in synchrony with infrared radiation and are therefore greenhouse active. Carbon dioxide is the primary contributor to climate change.
The earth is lucky that carbon dioxide has such a low concentration in its atmosphere. I wonder what would happen if most of our atmosphere consisted of CO2 instead of oxygen and nitrogen. Oh, wait… we don’t have to wonder. The atmosphere of Venus is 96% CO2, and Venus has an average surface temperature of 464°C (well above the boiling point of water). Wow!
Water vapor
Water vapor (H2O) is a special case. Its abundance in the atmosphere is not constant. It can vary from nearly zero to about 4%, depending on the humidity. A molecule of water is also different than carbon dioxide because it is not a linear molecule. Figure 6.18 in Intermediate Physics for Medicine and Biology shows the structure of a water molecule, with its oxygen atom having a partial negative charge and its hydrogen atoms being partially positive. Even when at rest, a molecule of water has a dipole moment. The water molecule has several vibrational modes, all of which cause this dipole moment to change, and it’s therefore an absorber of infrared radiation.
In the last post, I mentioned that feedback loops affect the climate. Water vapor provides an example. As the atmosphere heats up, it can hold more water vapor (see Homework Problems 65 and 66 in Chapter 3 of IPMB). More water vapor means more infrared absorption. More infrared absorption means more heating of the atmosphere, which means the atmosphere can hold more water vapor, which means more infrared absorption and heating, and so on. A positive feedback loop is sometimes called a vicious cycle.
Some of the water in the atmosphere is in the form of clouds. Clouds play a complex role in climate change. They can block the sunlight and therefore contribute to cooling. But it’s complicated.
Methane
Methane (CH4) is a very active infrared absorber. The methane molecule consists of a central carbon atom with partial negative charge, surrounded by a tetrahedron of four hydrogen atoms each with a partial positive change. Like carbon dioxide, when in equilibrium methane has no net dipole moment. However, methane has many complicated rotational and vibrational modes, in part because it consists of so many atoms. Many of those modes result in a changing dipole moment, similar to what we saw for carbon dioxide. So, methane can absorb infrared radiation and is an important greenhouse gas. Molecule for molecule, methane is a much stronger greenhouse gas than carbon dioxide. The only reason it doesn’t contribute more to global warming is that its concentration is so low.
Sulfur dioxide
A molecule of sulfur dioxide (SO2) is a lot like a molecule of water, with a bent shape. In this case, the central sulfur atom carries a partial positive charge and the two oxygen atoms are partially negative. Water is a stable molecule but sulfur dioxide is chemically reactive. If it is present in a high concentration it’s hazardous to your health. In that case, its contribution as a greenhouse gas will be the least of your problems. It’s often emitted when burning fossil fuels (especially coal), and is considered an air pollutant.
Sulfur dioxide can interact with water vapor to form tiny droplets called aerosols. These aerosols can remain in the air for years and reflect incoming sunlight (somewhat like clouds do). In this way, sulfur dioxide can have a cooling effect in addition to its greenhouse gas warming effect. On the whole, the aerosol cooling dominates, so sulfur dioxide cools the earth. It’s often released during volcanic eruptions, which can lead to cooler summers and colder winters for a few years.
Hydrogen
There is a tiny bit of hydrogen gas (H2) in the atmosphere, but like oxygen and nitrogen it’s diatomic so it doesn’t absorb infrared radiation.
Nitrous oxide
Finally, nitrous oxide (laughing gas, N2O) is similar in structure to sulfur dioxide and water. Like sulfur dioxide, it’s a form of air pollution and can be a greenhouse gas too (although its concentration is so small that it doesn’t make much contribution to global warming). Our atmosphere consists mostly of nitrogen and oxygen. We are fortunate that the most common form these elements take in the atmosphere are diatomic N2 and O2. Imagine what would happen if chemistry was slightly different, so that a large fraction of our atmosphere was N2O instead of N2 and O2. Yikes!
