PET·CT
andPET·MRI

Level 1 · Two ways of seeing

Chapter 01Structure is not activity

Most scans photograph structure: the shape, size and density of things. A tumour, an infection and a healing scar can look almost the same at that level.

Molecular imaging asks a different question: which cells are busy, and what are they doing? Hybrid scanners answer both questions in one visit.

Layer 1CT is the map

Computed tomography sends X-rays through the body from every angle and rebuilds a 3D map of density. Bone is bright, air is black, soft tissue sits in between. It is fast and sharp.

Layer 2PET is the weather

PET shows where an injected tracer has collected. With the sugar-like tracer FDG, busy tissue glows: the brain, the heart, the bladder where the tracer is excreted, and often tumours.

PET images are blurry and carry little anatomy. On their own, a hot spot has no address.

Layer 3MRI is the fine print

Magnetic resonance imaging uses a strong magnet and radio waves, with no ionising radiation. It separates soft tissues that look alike on CT: liver from tumour, grey matter from white matter, muscle from fat.

All togetherFusion gives the glow an address

A hybrid scanner stacks the layers in one session, on one table, in the same body position. PET says what is active. CT or MRI says where.

Every figure on this page is drawn by code from simplified models. None of it is real patient data.

Level 1 · Two ways of seeing

Chapter 02One slice, five answers

This is one synthetic slice through a chest at the level of the heart. Seen from the feet, the way radiologists read it, so the patient's right side is on the left of the image.

Watch how each scanner describes the same tissue.

CTDensity

Ribs and spine are white, lungs nearly black, and the heart and muscles sit in grey between. There is a small round nodule in the right lung. CT shows it clearly but cannot tell you if it is active.

PETActivity

The heart muscle glows, because it often burns sugar. So does the nodule, and so does a small lymph node in the middle of the chest that looked unremarkable on CT.

Notice how soft every edge is. PET resolution is a few millimetres at best.

MRIWater and fat

MRI signal comes from hydrogen nuclei in water and fat. Fat is bright here, muscle is grey, and the lungs are almost empty because air holds very few protons. Bone cortex gives no signal at all.

FusionTwo answers on one screen

Lay PET colour over the anatomy and both hot spots get an address: a nodule in the right lung and a node in the mediastinum.

Level 2 · The tracer

Chapter 04A molecule with two jobs

A radiopharmaceutical has two parts. The carrier decides where it goes in the body. The radioactive label lets the scanner see it.

The amounts are tiny. Tracer doses are far too small to change the biology they measure, which is the whole point of a tracer.

Step 1Start with glucose

Glucose is a ring of five carbons and one oxygen, with a sixth carbon hanging off. Hydroxyl (OH) groups stick out from the ring. Almost every cell in the body takes it up for energy.

Step 2Swap one group

At carbon 2, take away the OH group and attach a fluorine-18 atom. The result is 2-deoxy-2-[¹⁸F]fluoro-D-glucose: FDG.²³

The change is small enough that cells still pull FDG in as if it were glucose. It is big enough to stop it being fully burned, which you will see in Chapter 07.

Step 3Same label, new mission

Put a radioactive label on a different carrier and the scan shows different biology. One finds the PSMA protein on many prostate cancers.³⁰ Another binds somatostatin receptors on neuroendocrine tumours.³¹ Sodium fluoride settles where bone is remodelling.

"PET scan" is not one test. The tracer is chosen to match the clinical question.

Level 2 · The tracer

Chapter 05Inside the nucleus

Fluorine-18 has 9 protons and 9 neutrons. The number of protons fixes the element: 9 means fluorine, always. The number of neutrons picks the version, called the isotope.

The "18" is the mass number, protons plus neutrons. It is not a charge.

UnstableToo many protons

Stable fluorine is fluorine-19, with ten neutrons. Fluorine-18 is short of one, so it is unstable. To settle down, one proton turns into a neutron and throws out a positron and a neutrino. This is beta-plus decay.

p⁺ → n + e⁺ + ν_e9 protons become 8: fluorine-18 becomes stable oxygen-18.

LabBuild six PET isotopes

SupplyWhere the atoms come from

Fluorine-18, carbon-11, nitrogen-13 and oxygen-15 are made in a cyclotron, which fires protons at a target. Gallium-68 and rubidium-82 come from small generators: a longer-lived parent isotope sits in a shielded column and decays into the short-lived one, which staff wash out when needed.

Level 2 · The tracer

Chapter 06A clock made of chance

Each dot is one nucleus. Nobody can predict when a particular one will decay. It might be in a second or in a day.

Yet a crowd of them keeps perfect time. After one half-life, half are gone. After another, half of what was left.

The ruleHalving, again and again

A(t) = A₀ × 2^{−t / T½}One half-life leaves 50 %, two leave 25 %, three leave 12.5 %.

For fluorine-18 the half-life is about 110 minutes.²⁸ By the next morning almost none of the activity is left.

LabRace the clocks

LogisticsWhy 110 minutes is handy

It is long enough to make FDG in one cyclotron and drive it to hospitals around a region. It is short enough that the patient's activity fades within hours.

Oxygen-15 lasts about two minutes and rubidium-82 about 75 seconds, so they must be made or eluted right beside the scanner.

Two clocksPhysical and biological

Inside a patient, activity falls for two reasons at once: atoms decay, and the body excretes the tracer, mostly through the kidneys for FDG. The combined rate gives the effective half-life.

1 / T_eff = 1 / T_physical + 1 / T_biological

Level 2 · The tracer

Chapter 07Why sugar lights up

After injection, FDG travels in the blood like ordinary glucose. Protein channels in the cell membrane, the glucose transporters, let it in.

The trapA phosphate tag

Inside, the enzyme hexokinase adds a phosphate, exactly as it does to glucose. Glucose then moves on down the energy pathway. FDG-6-phosphate cannot, and it is too charged to leak back out easily.

So it accumulates. This is metabolic trapping, and it is why an FDG image is roughly a map of glucose use.^23,28

TumoursHungry cells

Many cancer cells take up far more glucose than their neighbours, often with extra transporters. Otto Warburg noticed in the 1920s that tumours ferment glucose even when oxygen is available, now called the Warburg effect.²⁵

CautionNot only cancer

The brain is a heavy glucose user, so it always glows. Heart uptake varies with diet. FDG is excreted in urine, so kidneys and bladder light up. Activated immune cells such as macrophages take up FDG too, sometimes more than tumour cells.²⁶

A bright spot is a question, not a diagnosis.

Level 3 · Catching antimatter

Chapter 08A positron's short life

The positron leaves the nucleus fast. It ricochets off electrons in the surrounding tissue, losing energy at every bump, and zig-zags to a stop.

BlurWhere it dies, not where it was born

PET can only find where the positron finally annihilates. The gap between birth and death is the positron range, a built-in blur.

For fluorine-18 the mean range in water is about 0.6 mm, with a maximum of 2.4 mm.¹⁰

LabCompare the isotopes

Values from Conti & Eriksson; the gallium-68 range is approximate, as published figures vary.¹⁰

ScaleDoes it matter?

Whole-body scanners resolve about 4 mm, so fluorine-18's range is barely measurable. Long-range emitters such as gallium-68 or rubidium-82 do blur visibly, and modern reconstruction can partly model it.¹⁰

Level 3 · Catching antimatter

Chapter 09Matter meets antimatter

Once it has slowed down, the positron meets an ordinary electron. Both disappear. Their mass turns completely into light. This is annihilation.

ArithmeticWhere 511 keV comes from

E = m c² = (9.109 × 10⁻³¹ kg) × (2.998 × 10⁸ m/s)²
= 8.19 × 10⁻¹⁴ J ≈ 511 keVThat is the rest energy of one electron. Two particles, so two photons of 511 keV each.

Every PET scanner in the world is tuned to this one energy.

MomentumBack to back

The pair was almost at rest, so the two photons must fly off in nearly opposite directions to cancel each other's momentum. That is what lets PET draw a straight line through the event.

Small printAlmost 180°

The pair always has a little leftover motion, so the angle is off by a fraction of a degree. This non-collinearity adds blur that grows with ring size: roughly 0.0022 × the ring diameter, or about 2 mm for an 80 cm ring.¹¹

Level 3 · Catching antimatter

Chapter 10Gamma rays into light

A 511 keV photon passes straight through most materials. To stop it, the ring is lined with thousands of small, dense crystals.

When a gamma ray hits one, the crystal gives off a brief flash of visible light. That property is called scintillation.

CrystalsHeavy and fast

Most modern scanners use lutetium-based crystals (LSO or LYSO). A recent digital scanner uses LSO needles 3.2 × 3.2 × 20 mm, packed by the thousand.¹⁴

Fast lutetium crystals made short timing windows possible, which opened the door to time-of-flight.¹⁴

SensorsLight into current

For decades, glass vacuum photomultiplier tubes turned the flash into a current. Newer scanners use silicon photomultipliers: chips covered in thousands of tiny cells, each firing a small avalanche of charge when struck by light.

FilterIs it really 511 keV?

The size of each pulse tells the scanner how much energy was deposited. Only events in an energy window around 511 keV are kept, for example 435 to 585 keV on one long scanner.¹⁵ Photons that scattered in the patient lose energy and many are thrown out here.

Level 3 · Catching antimatter

Chapter 11Two hits, one line

If two detectors on opposite sides fire within a few nanoseconds, the scanner assumes both photons came from one annihilation. That is a coincidence.

It cannot tell where along the path the event happened, so it records the whole straight line between the detectors: a line of response.

TrueThe ideal case

Both photons escape the body without interacting and hit the ring. The line passes through the real event. Millions of these lines crossing at the same places build the image.

ScatterA photon changes course

One photon bounces off an electron in the body (Compton scattering) and arrives at the wrong detector. The recorded line misses the true event and adds haze, so it has to be estimated and subtracted.

RandomTwo strangers

Photons from two different annihilations happen to land inside the same timing window. The line joins nothing real. Randoms grow quickly as activity in the field of view rises.

ScaleMost photons are missed

A typical scanner covers only 15 to 26 cm of the body along its length. Only about 1 to 3 % of annihilations produce a line it actually detects.¹⁵ Keep that in mind: it is the reason total-body scanners exist (Chapter 31).

Level 3 · Catching antimatter

Chapter 12A race at the speed of light

Light covers about 30 cm in a nanosecond. If the event is off-centre, one photon reaches its detector slightly earlier than the other.

Measure that tiny difference and you know roughly where on the line it happened. This is time-of-flight PET.

FormulaDistance from time

Δx = c · Δt / 2Divide by two because moving the event towards one detector also moves it away from the other.

A timing uncertainty of 400 picoseconds means each event is placed within about 6 cm along its line.

LabSharpen the clock

TodayWhere scanners are now

A digital PET-CT with silicon photomultipliers measured 210 to 215 ps, about 3.2 cm.¹⁴ A SiPM-based PET-MRI reaches below 400 ps.⁷

TOF does not give a point. It narrows each line to a short segment, which lowers image noise for the same number of counts.

Level 4 · From counts to pictures

Chapter 13Rebuilding the image

Sort every line of response by its angle and by how far it passes from the centre. Stack them up and you get a sinogram, named because a single point source traces a sine wave through it.

The figure on the right is computed live in your browser from a small chest phantom.

Attempt 1Smear it back

The simplest idea is back-projection: spread each measurement evenly back along its line. Where many lines cross, things build up. The result is recognisable but blurry, with star-shaped streaks.

Attempt 2Guess, check, correct

Iterative methods start with a flat guess. They ask: if this were the real activity, what would the scanner have measured? They compare that with the actual data and nudge the guess. Repeat.

The classic version is MLEM. Grouping the data into subsets, called OSEM, made it about ten times faster and brought it into routine use.¹³

Trade-offWhen to stop

Each iteration sharpens edges but also amplifies noise. With few counts, the image turns speckled after enough iterations. Clinical systems stop early, smooth a little, and model time-of-flight and the detector's blur inside the algorithm.¹⁶

Level 4 · From counts to pictures

Chapter 14Photons that never arrive

On the way out of the body, many photons are absorbed or scattered. This loss is attenuation. Events deep inside a large patient lose far more than events near the skin.

TrickPosition does not matter

Both photons must escape. Their survival chances multiply:

e^−μ·a × e^−μ·b = e^−μ·(a+b) = e^−μ·LL is the total thickness of the body along that line.

So the loss depends only on the line, not on where the event sits on it. One measurement of the body's thickness along each line corrects every event on it.

UncorrectedA strange-looking body

Skip the correction and the skin looks hot, the lungs look too bright and the middle of the body looks cold. Readers sometimes view these non-attenuation-corrected images on purpose, to spot artefacts created by the correction.²⁸

MapWhere the correction comes from

The correction needs an attenuation map: how strongly each point of the body stops 511 keV photons. Early PET scanners measured it with rotating radioactive rods. PET-CT gets it from the CT. PET-MRI has to work it out from MR images, which is harder (Chapter 23).

Level 4 · From counts to pictures

Chapter 15Putting a number on it

Imagine the injected tracer spread perfectly evenly through the whole body. Every millilitre would hold the same activity, and you could call that level 1.

The standardised uptake value says how many times more, or less, a region holds than that even spread.

FormulaBody-weight SUV

SUV = C_tissue ÷ (A_injected ÷ body weight)kBq/mL ÷ (kBq ÷ g), assuming 1 g of tissue ≈ 1 mL. Activity is decay-corrected to the same time.

CautionOne number, many influences

SUV shifts with the time between injection and scan, blood glucose, body composition, scanner calibration and reconstruction settings. Small lesions read low because of blur, the partial-volume effect.

European guidance standardises these steps, and the EARL programme harmonises scanners so SUVs can be compared between centres.^27,28 There is no universal SUV that means cancer.

Level 5 · CT, the anatomy engine

Chapter 16X-rays in a spin

An X-ray tube and a curved row of detectors face each other across the patient and spin together. A fan of X-rays passes through the body, and each detector records how much got through.

PhysicsFading through tissue

I = I₀ · e^{−∫μ dx}Each ray dims according to everything it passes through. Bone dims it a lot, air hardly at all.

One angle gives one shadow profile. It hides depth, just like a normal X-ray.

RotationHundreds of shadows

Collect profiles from all the way round and you get a sinogram again, the same idea as in PET. Reconstruction turns it into a cross-section. Hounsfield described the first system in 1973.²⁰

Two flavoursLow dose or diagnostic

In PET-CT the CT can be a low-dose scan, enough to locate findings and correct attenuation, or a full diagnostic CT, often with contrast. A low-dose CT is not meant to replace a dedicated CT report.³⁰

Level 5 · CT, the anatomy engine

Chapter 17Hounsfield's grey scale

CT pixels are not just shades of grey. Each one holds a calibrated number, the Hounsfield unit, which compares the tissue with water and air.

HU = 1000 × (μ − μ_water) ÷ (μ_water − μ_air)Water is 0 by definition. Air is −1000.

LandmarksA ruler for the body

Lung sits near −850, fat around −100, water 0, muscle and blood a few tens above zero, and dense bone several hundred to over +1000. The scale on the right marks the values used in this phantom.

LabWindow and level

The full range is far wider than the eye can separate in grey, so readers choose a window: a width of HU to spread across black-to-white, centred on a level.

Level 5 · CT, the anatomy engine

Chapter 18One table, two machines

A PET-CT is two scanners mounted back to back in one housing. The table slides the patient through the CT first, which takes seconds, then through the PET ring for several minutes. Same table, same position: the images are already lined up. This alignment is co-registration.¹

Double dutyCT becomes the attenuation map

CT measures attenuation at X-ray energies of tens of keV. PET needs it at 511 keV. The conversion uses a two-part line: one slope for air-to-water tissues and a gentler one for bone, because bone absorbs low-energy X-rays especially strongly.¹⁷

This is why PET-CT dropped the old radioactive rod sources: the CT does the job faster and with less noise.

FusionColour on grey

The corrected PET is shown in a colour scale over the grey CT. Now the hot nodule sits clearly in the right lung, and the node in the mediastinum.

CatchBreathing

CT freezes one moment of the breathing cycle. PET averages over many minutes of breathing. Near the diaphragm, the two can disagree, which can misplace a lesion or distort the correction.

Dense metal and concentrated contrast can also mislead the CT-based correction. Checking the uncorrected PET helps catch both.²⁸

Level 6 · MRI, the magnetic engine

Chapter 19A body full of compasses

You are mostly water, and every water molecule has two hydrogen nuclei. Each is a single proton that behaves like a tiny spinning magnet. Normally they point every which way and cancel out.

Field onA slight majority

Inside a strong magnet, slightly more protons settle along the field than against it. The excess is only a few in a million, but with so many protons it adds up to a measurable net magnetisation pointing along the field.

Clinical scanners use 1.5 or 3 tesla, tens of thousands of times stronger than the Earth's magnetic field.

WobbleThe Larmor frequency

The protons don't line up perfectly. They wobble around the field direction like spinning tops, at a rate set by the field strength: the Larmor frequency.

f = (γ / 2π) · B₀ , γ / 2π = 42.58 MHz per teslaProton value from CODATA.²²

Level 6 · MRI, the magnetic engine

Chapter 20Kick, then listen

A short radiofrequency pulse at exactly the Larmor frequency tips the net magnetisation over, out of line with the field. Off any other frequency, the protons ignore it. That is the "resonance" in magnetic resonance.

SignalA spinning magnet makes a current

Tipped over, the magnetisation sweeps round at the Larmor frequency. A rotating magnet near a coil of wire induces a voltage, the same principle as a bicycle dynamo. That faint radio echo is the MR signal.

T1Standing back up

After the pulse, the magnetisation regrows along the field. The time constant is T1. Fat recovers quickly, free water slowly.

T2Losing the beat

Meanwhile, the protons drift out of step with each other and the signal fades. That time constant is T2, and it is much shorter than T1. Fluid keeps in step longest.

PositionWhere did the echo come from?

Extra coils called gradients make the field slightly stronger on one side than the other, so frequency now depends on position. That was Lauterbur's insight.²¹ The echoes fill a data grid called k-space, and a Fourier transform turns it into the image.

Switching gradients rapidly is what makes MRI so loud.

Level 6 · MRI, the magnetic engine

Chapter 21Contrast you can tune

The same tissue can look bright or dark depending on timing. Two dials matter most: the repetition time between pulses (TR) and the echo time before listening (TE).

S ≈ PD · (1 − e^−TR/T1) · e^−TE/T2A simplified spin-echo equation. PD is proton density.

LabMake your own contrast

Short TR and TE: fat bright, fluid dark (T1-weighted). Long TR and TE: fluid bright (T2-weighted).

DiffusionHow freely water moves

Diffusion-weighted imaging measures how far water molecules wander. In tightly packed tumours or fresh strokes they are hemmed in, so the signal stays bright. MRI offers many such functional measures, which is a big part of PET-MRI's appeal.¹⁸

Weak spotWhy lungs are hard

Lungs are mostly air: few protons, and countless air-tissue boundaries that scramble the signal quickly. CT remains better for spotting small lung nodules, one of PET-CT's lasting strengths.³³

Level 7 · PET-MRI

Chapter 22Why it was hard

A photomultiplier tube amplifies light by firing electrons down a chain of plates in a vacuum. Each plate knocks loose more electrons, building a cascade.

Electrons in flight are steered by magnetic fields. Inside an MRI magnet, they curl off course and the cascade collapses.⁵

Both waysMutual interference

The trouble runs in both directions. The MRI's switching gradients and radio pulses can disturb PET electronics, and PET hardware inside the bore can spoil the MR signal or the field's uniformity. Each side had to be shielded from the other.⁵

FixSolid-state sensors

Semiconductor light sensors have no electrons flying through a vacuum, so the field barely affects them. The first whole-body system used avalanche photodiodes with LSO crystals, fitted between the gradient coil and the RF body coil of a 3 T magnet.^5,6 A later design used silicon photomultipliers fast enough for time-of-flight.⁷

DesignsSequential or simultaneous

One approach places a PET-CT-style PET scanner and an MRI in the same room with a shared rotating table, scanning one after the other. The other builds the PET ring inside the magnet for true simultaneous acquisition.⁵

Performance tests found the integrated scanner performed on a par with standalone PET and MRI systems.^5,6

Level 7 · PET-MRI

Chapter 23Attenuation without X-rays

A PET-MRI has no CT, but PET still needs an attenuation map. The problem: MR signal depends on protons and relaxation times, not on the electron density that stops 511 keV photons.

Cortical bone is the worst case. It attenuates strongly, yet gives almost no MR signal.¹⁸

DixonFour classes

The standard whole-body fix uses a Dixon MR sequence, which separates water and fat signals. The body is sorted into air, lung, fat and soft tissue, and each class gets a fixed value, for example 0.086 cm⁻¹ for fat and 0.10 cm⁻¹ for soft tissue.¹⁸

Bone is counted as soft tissue, so uptake in and near bone comes out too low.

Bone backBetter guesses

Newer methods add bone using ultrashort or zero echo-time sequences, anatomical atlases or deep learning. In a multi-centre brain study, the best methods came within about ±3 % of CT-based correction, while the basic Dixon approach averaged around −11 %.¹⁹

Also watchArms and metal

The MR field of view can be narrower than the PET's, cutting off the arms on the map. Metal implants leave signal voids that can be misread as air. Both distort the correction locally.^32,33

Level 7 · PET-MRI

Chapter 24The simultaneous advantage

In an integrated PET-MRI, both scanners watch the same tissue at the same moment. Several MR sequences can run during the PET acquisition without adding time.⁵

MotionFreeze the breathing

A lesion near the diaphragm moves a centimetre or more with each breath, smearing its PET signal. MR can track that motion in real time, and the PET counts can be shifted back to where they belong.

BonusThe magnet shortens positron range

A moving charge in a magnetic field curls into a spiral. Simulations of a 3 T PET-MRI found the range of high-energy positrons shrank by a factor of 3 to 4 across the field, while growing along it.¹²

Many measuresOne visit

Metabolism from PET, cellularity from diffusion, blood flow from dynamic contrast, fine anatomy from T1 and T2: all lined up in time and space. This is often called multiparametric imaging.³²

Level 8 · In the clinic

Chapter 26Which scan for which question?

The choice follows the clinical question, the tracer and what is available locally. Here are six teaching scenarios. Scroll through them, or tap one.

1 · ChildMany scans ahead

Children are more sensitive to radiation and often need repeated follow-up. Removing the CT component cuts the dose substantially, so PET-MRI is attractive where available.³⁴

2 · LungStaging lung cancer

FDG PET-CT is the established choice. CT's view of small lung nodules is hard to beat.^33,35

3 · ProstatePSMA and the pelvis

PSMA PET finds prostate cancer spread with high sensitivity.³⁰ MRI is already the reference for the prostate gland itself, so PSMA PET-MRI can combine both in one visit.³²

4 · MemoryBrain metabolism

FDG brain PET shows patterns of reduced glucose use linked to different dementias, and MRI shows atrophy and blood-vessel damage. Brain imaging is where PET-MRI attenuation correction is most mature.^19,33

5 · NETNeuroendocrine tumours

Somatostatin-receptor PET maps these tumours.³¹ Liver metastases are common and MRI excels in the liver, which is where PET-MRI added most findings in comparison studies.³⁹

6 · Head & neckLocal extent

Tumours here sit among muscles, nerves and glands packed into a small space. MRI's soft-tissue contrast helps define local spread, a common use for PET-MRI.³³

Level 8 · In the clinic

Chapter 27A tracer for every question

Readers often start with a spinning maximum-intensity projection: for every line of sight, show only the brightest point. It gives a whole-body overview in seconds.

The normal pattern is different for every tracer. Knowing it is half of reading the scan.

FDGGlucose use

Brain always bright, heart variable, kidneys and bladder from excretion, liver a soft grey. The example lesion here is a single hot node.²⁸

PSMAProstate-specific membrane antigen

Salivary and tear glands, kidneys, liver, spleen and bowel take up PSMA ligands normally. The brain stays dark.³⁰

SSTRSomatostatin receptors

The spleen is usually the brightest normal organ, with kidneys, liver, adrenals and the pituitary also visible.³¹

NaFBone turnover

Sodium [¹⁸F]fluoride outlines the whole skeleton, brightest where bone is actively remodelling, plus kidneys and bladder from excretion. Healing fractures and arthritis light up as well as metastases.

Level 8 · In the clinic

Chapter 30Radiation and magnets

Two different hazards. PET and CT use ionising radiation, measured as effective dose in millisieverts. MRI uses none, but its magnet is always on.

LabBuild a dose

For adults, FDG gives about 0.019 mSv per MBq injected.³⁶ Choose an activity and a CT type, and compare with everyday doses.³⁷

RulesJustify, then optimise

Every exposure needs a clinical reason that outweighs the small risk, and the dose should be as low as the task allows. Newer, more sensitive scanners let centres lower the injected activity or shorten scans.^16,27 Children get weight-adjusted doses, and PET-MRI removes the CT share altogether.³⁴

MagnetThree MRI hazards

The static field can pull iron objects into the bore at speed. Switching gradients are loud and can tingle nerves. Radiofrequency energy warms tissue. Every patient is screened, and implants are labelled MR Safe, MR Conditional or MR Unsafe.³⁸

Level 8 · In the clinic

Chapter 31What comes next

Remember that a standard scanner catches only 1 to 3 % of annihilations? The obvious fix is a longer ring.

Long106 centimetres

One commercial scanner joins four rings for a 106 cm axial field of view, covering the major organs in one position with timing near 228 ps.¹⁵

Total194 centimetres

EXPLORER covers the whole adult body. Its team reported diagnostic images from about 25 MBq, scans of around a minute, and imaging up to 10 hours after injection. They had predicted up to 40-fold gains in effective count rate for total-body work.¹⁶

AheadFaster clocks, smarter software

Timing keeps improving. If detectors reached 10 picoseconds, Δx = cΔt/2 would place each event within about 1.5 mm. Artificial intelligence is being used to clean up low-dose images and to build bone-aware attenuation maps for PET-MRI.

And the same molecules used for imaging, such as PSMA and somatostatin ligands, can carry therapeutic isotopes. Scan first, then treat the same target: theranostics.