**PET Imaging in ALS**

model of amyotrophic lateral sclerosis. J Neurochem 2005;93(2):403–411, ISSN:

[83] Ilse Gijselinck, Kristel Sleegers, Christine Van Broeckhoven and Marc Cruts (2012). A Major Genetic Factor at Chromosome 9p Implicated in Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Lobar Degeneration (FTLD), Amyotrophic Lateral Sclerosis, Prof. Martin Maurer (Ed.), ISBN: 978-953-307-806-9, InTech, Available from: http:// www.intechopen.com/books/amyotrophic-lateral-sclerosis/a-major-genetic-factor-at-

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[84] Max Koppers, Michael van Es, Leonard H. van den Berg, Jan H. Veldink and R. Jeroen Pasterkamp (2012). Genetics of Amyotrophic Lateral Sclerosis, Amyotrophic Lateral Sclerosis, Prof. Martin Maurer (Ed.), ISBN:978-953-307-806-9, InTech, Available from: http://www.intechopen.com/books/amyotrophic-lateral-sclerosis/genetics-of-als [85] Emily F. Goodall, Joanna J. Bury, Johnathan Cooper-Knock, Pamela J. Shaw and Janine Kirby (2012). Genetics of Familial Amyotrophic Lateral Sclerosis, Amyotrophic Lateral Sclerosis, Prof. Martin Maurer (Ed.),ISBN: 978-953-307-806-9, InTech, Available from: http://www.intechopen.com/books/amyotrophic-lateral-sclerosis/genetics-of-fami‐

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[88] George E. Barreto, Janneth Gonzalez, Francisco Capani and Ludis Morales (2011). Role of Astrocytes in Neurodegenerative Diseases, Neurodegenerative Diseases - Processes, Prevention, Protection and Monitoring, Dr Raymond Chuen-Chung Chang (Ed.), ISBN: 978-953-307-485-6, InTech, Available from: http://www.intechopen.com/books/ neurodegenerative-diseases-processes-prevention-protection-andmonitoring/role-of-

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trophic-lateral-sclerosis/glial-cells-as-therapeutic-targets-for-als

0022-3042.

22 Update on Amyotrophic Lateral Sclerosis

lial-als

and-excitotoxicity

astrocytes-in-neurodegenerative-diseases

and-magnetic-resonance-neuroimaging

Bart Swinnen, Koen Van Laere and Philip Van Damme

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63545

#### **Abstract**

Amyotrophic lateral sclerosis is a neurodegenerative disorder that primarily affects the motor system, but extramotor involvement is common. Progressive muscle weakness and wasting, including bulbar and respiratory muscles, limit survival to 2–5 years after disease onset in most patients. The diagnosis is made on clinical grounds and is based on the presence of signs of upper and lower motor neuron loss in different body regions in the absence of other pathologies that can explain the symptoms and signs of the patient. Making an accurate diagnosis can be difficult in early disease stages. ALS is a heterogeneous disorder with variability in age at onset, in phenotypic presentation, in the extent of frontotemporal involvement and in the disease progression rate. There is a high unmet medical need for objective markers that aid in early diagnosis and in predicting disease outcome. In this chapter, the current knowledge about the diagnos‐ tic and prognostic value of 18F 2-fluoro-2-deoxy-D-glucose-PET in ALS is discussed. The potential of other targets and PET tracers to visualize different aspects of ALS disease pathology is described.

**Keywords:** amyotrophic lateral sclerosis, frontotemporal dementia, extramotor in‐ volvement, FDG PET, neuroinflammation, imaging biomarker

#### **1. Introduction**

Amyotrophic lateral sclerosis (ALS) is a relentlessly progressive disorder, however, with considerable variability in phenotype, disease progression and aetiology. Reliable prognosti‐ cation regarding unusually fast or slow progression is difficult in clinical practice. Since diagnosis is often delayed until patients are already a year into their disease, pharmacologi‐ cal treatment with riluzole is often postponed as well. Numerous therapeutic clinical trials

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

using other drugs have failed to show any benefit. These problems highlight the high need for reliable biomarkers in ALS. Ideally, such a biomarker should be easily accessible and affordable, as well as very sensitive and specific for ALS [1]. It should also be of prognostic value and should change with disease progression making it suitable for treatment monitor‐ ing [1]. Besides peripheral biomarkers in blood and cerebrospinal fluid, several neuroimag‐ ing biomarkers have been proposed, of which several nuclear or molecular imaging targets seem to be very promising. Although radionuclide imaging is not commonly used in clinical practice for ALS, recent studies suggest that various aspects of ALS pathology can be visualized and quantified.

#### **1.1. ALS and FTD: two overlapping disorders**

In ALS, the motor system is primarily affected [2]. Degeneration of upper motor neurons (UMNs) in motor cortex and lower motor neurons (LMNs) in the brain stem and spinal cord results in a progressive weakness and wasting of limb, bulbar and respiratory muscles, limiting survival to 2–5 years after disease onset.

In frontotemporal dementia (FTD), the degenerative process starts in the frontal and/or anterior temporal cortex [3]. Depending on the neuroanatomical regions affected, disease presentations include behavioural variant FTD (with changes in behaviour with apathy, loss of empathy, hyperorality, repetitive behaviours or disinhibition and executive dysfunction) or language variants of FTD (such as primary non-fluent aphasia and semantic dementia). The disease progressively affects other cognitive functions. Survival after disease onset ranges from 5 to 8 years.

ALS and FTD are considered to be extremes of a disease spectrum [4]. In about 10% of patients, both diseases co-occur. In another 30–40%, there is some degree of overlap, with mild motor involvement in patients with FTD, or mild cognitive/behavioural impairment in patients with ALS. About 50% of patients have pure FTD or pure ALS.

The only approved therapy for ALS is riluzole, which extends survival by only a few months, whereas for FTD no disease modifying therapies are available. The cornerstone of the man‐ agement of patients with FTD/ALS remains multidisciplinary care and supportive measures.

Ten percent of ALS patients and 40% of FTD patients have a positive family history. Mutations in a heterogeneous set of genes have been identified to cause this familial form of FTD/ALS [5]. The inheritance pattern usually is autosomal dominant. Mutations in *SOD1, TARDBP* and *FUS* cause ALS and sometimes ALS-FTD, and mutations in *GRN* and *MAPT* cause FTD, but rarely also ALS. However, by far the most common cause for ALS, FTD/ALS and FTD is the recently discovered mutation in *C9ORF72* [6, 7]. It underlies 30–50% of familial ALS (and ALS-FTD) and 20–25% of familial FTD [8].

At the neuropathological level, an overlap between ALS and FTD is present as well. Especially, cytoplasmic accumulations of TDP-43, the protein encoded by the *TARDBP* gene, are central to the overlap between ALS and FTD, as most patients with sporadic ALS and about half of patients with sporadic FTD have such pathology (FTD/ALS-TDP-43) [9]. In addition, several of the genetic forms of FTD/ALS have TDP-43 aggregates, including mutations in *TARDBP,* *C9ORF72* and *GRN*. TDP-43 is an RNA-binding protein with important functions in gene transcription, splicing, RNA transport and stress granule formation. Accumulations of FUS, another RNA-binding protein, with many structural and functional similarities to TDP-43, is also central to FTD/ALS as mutations in *FUS* cause ALS, rarely FTD, and FUS pathology is also observed in about 5% of sporadic FTD patients.

It has thus become clear that ALS is a neurodegenerative disease that primarily affects the motor system, but that a variable degree of extramotor involvement is present in most patients [4].

For treating physicians and for family members, it is important to uncover cognitive problems in ALS, but it is not always easy to perform extensive neuropsychological testing in patients with motor impairments. An imaging biomarker that reliably recognizes and measures both motor and extramotor involvement in ALS patients would be an important achievement for the management of ALS patients and for ALS research.

#### **1.2. Need for an upper motor neuron marker**

using other drugs have failed to show any benefit. These problems highlight the high need for reliable biomarkers in ALS. Ideally, such a biomarker should be easily accessible and affordable, as well as very sensitive and specific for ALS [1]. It should also be of prognostic value and should change with disease progression making it suitable for treatment monitor‐ ing [1]. Besides peripheral biomarkers in blood and cerebrospinal fluid, several neuroimag‐ ing biomarkers have been proposed, of which several nuclear or molecular imaging targets seem to be very promising. Although radionuclide imaging is not commonly used in clinical practice for ALS, recent studies suggest that various aspects of ALS pathology can be

In ALS, the motor system is primarily affected [2]. Degeneration of upper motor neurons (UMNs) in motor cortex and lower motor neurons (LMNs) in the brain stem and spinal cord results in a progressive weakness and wasting of limb, bulbar and respiratory muscles, limiting

In frontotemporal dementia (FTD), the degenerative process starts in the frontal and/or anterior temporal cortex [3]. Depending on the neuroanatomical regions affected, disease presentations include behavioural variant FTD (with changes in behaviour with apathy, loss of empathy, hyperorality, repetitive behaviours or disinhibition and executive dysfunction) or language variants of FTD (such as primary non-fluent aphasia and semantic dementia). The disease progressively affects other cognitive functions. Survival after disease onset ranges from

ALS and FTD are considered to be extremes of a disease spectrum [4]. In about 10% of patients, both diseases co-occur. In another 30–40%, there is some degree of overlap, with mild motor involvement in patients with FTD, or mild cognitive/behavioural impairment in patients with

The only approved therapy for ALS is riluzole, which extends survival by only a few months, whereas for FTD no disease modifying therapies are available. The cornerstone of the man‐ agement of patients with FTD/ALS remains multidisciplinary care and supportive measures. Ten percent of ALS patients and 40% of FTD patients have a positive family history. Mutations in a heterogeneous set of genes have been identified to cause this familial form of FTD/ALS [5]. The inheritance pattern usually is autosomal dominant. Mutations in *SOD1, TARDBP* and *FUS* cause ALS and sometimes ALS-FTD, and mutations in *GRN* and *MAPT* cause FTD, but rarely also ALS. However, by far the most common cause for ALS, FTD/ALS and FTD is the recently discovered mutation in *C9ORF72* [6, 7]. It underlies 30–50% of familial ALS (and ALS-FTD)

At the neuropathological level, an overlap between ALS and FTD is present as well. Especially, cytoplasmic accumulations of TDP-43, the protein encoded by the *TARDBP* gene, are central to the overlap between ALS and FTD, as most patients with sporadic ALS and about half of patients with sporadic FTD have such pathology (FTD/ALS-TDP-43) [9]. In addition, several of the genetic forms of FTD/ALS have TDP-43 aggregates, including mutations in *TARDBP,*

visualized and quantified.

24 Update on Amyotrophic Lateral Sclerosis

5 to 8 years.

**1.1. ALS and FTD: two overlapping disorders**

ALS. About 50% of patients have pure FTD or pure ALS.

survival to 2–5 years after disease onset.

and 20–25% of familial FTD [8].

The diagnosis of ALS requires the presence of both UMN and LMN signs. LMN signs are often readily appreciated on clinical examination and electromyography (EMG) is a very sensitive method to confirm this and even detect subclinical LMN involvement. On the contrary, providing evidence of UMN involvement can be challenging. First, the clinical signs of UMN involvement (hyperreflexia, spasticity, pseudobulbar features, Hoffman's reflex and extensor plantar response) exhibit both a low sensitivity and interrater reliability. Second, there are no reliable tests to show that UMN involvement exists. This lack of a reliable method to detect UMN involvement and track the progressive loss of UMNs is an important blind spot in the ALS field.

Various markers of UMN involvement have been proposed so far. First, the appearance of the motor cortex on magnetic resonance imaging (MRI) has been shown to be altered in ALS patients. At the group level, the thickness of the motor cortex is decreased, especially on 7T MRI [10]. However, at the individual patient level, a clear atrophy can only be demonstrated in around 50% of ALS patients because of considerable overlap with healthy controls [11, 12]. Other parameters like T2 hypointensity and increased quantitative susceptibility mapping (QSM) are present in a high proportion of ALS patients. It probably reflects gliosis due to activated microglia, but still lacks convincing sensitivity and specificity [10, 13, 14]. Second, the appearance of the corticospinal tract (CST) on MRI has also been proposed as a potential UMN marker. T2 hyperintensity of the CST in the posterior limb of the internal capsule is present in almost 50% of ALS patients, while it is absent in other studies [14, 15]. Since only severe hyperintensity seems to be clearly related with ALS and this occurs only late in the disease course, it is of no value in the (early) diagnosis of ALS [16]. Diffusion tensor imaging (DTI) of the CST seemed to be very promising at first, but unfortunately also turned out to lack sensitivity and specificity [16–18]. Third, the functional assessment of the motor cortex and CST by means of transcranial magnetic stimulation (TMS), with motor evoked potentials (MEP) being the most important parameter, has been investigated as a potential UMN biomarker. Literature data are unfortunately discordant: while several studies reported a cortical hypoexcitability [19, 20], others found a clear hyperexcitablity [11, 21, 22]. Moreover, several studies showed that TMS cannot discriminate ALS patients from controls [18, 23], so better methods to detect UMN loss in ALS are needed.

#### **1.3. Need for a (differential) diagnostic test**

An early and certain diagnosis of ALS is of utmost importance for clinicians and patients. It allows an early initiation of riluzole therapy (as yet the only proven disease-modifying therapy for ALS), an accurate communication about the diagnosis and an early recruitment into clinical trials. Especially, in these early stages of the disease, the disease process may be amenable to therapy.

However, in daily clinical practice, the time between symptom onset and diagnosis (the diagnostic delay) is long, estimated to be 12–14 months in tertiary ALS referral centres [24]. There are various reasons for this delay, including patients-specific and doctor-specific delays. Aspecific presentations and phenotypic variability at onset contributes to the delay. Observa‐ tion of patients, with repeat clinical examinations and electrodiagnostic testing is a reliable method to correctly identify ALS patients that present early with only mild and focal motor symptoms, but this approach increases the diagnostic delay. Some cases pose a differential diagnostic problem with certain ALS mimicking diseases, also adding significantly to the diagnostic delay. Since most of these diseases lack abnormalities on CT or MRI imaging, they can impossibly be excluded by conventional imaging. Only a tool that allows to make a positive diagnosis of ALS early in the disease course could solve this problem.

#### **1.4. Need for a prognostic marker**

ALS is a heterogeneous disorder. Not only the genetic causes and the age at onset, but also the disease progression is highly variable. The median survival after disease onset is only 33 months and most patients die 2–5 years after disease onset. However, numerous cases of extremely long and extremely short survival have been reported, making up the extreme ends of a wide prognostic spectrum [25]. Making a reliable prognostic estimation is pivotal for both patients and their families and neurologists likewise, but still largely impossible these days. A variety of prognostic factors has been identified, such as age of onset, site of onset, rate of symptom progression, comorbid frontotemporal involvement and nutritional and respiratory status [26–31]. Although they are clearly of value, they all reflect divergent clinical disease parameters and do not directly reflect the underlying disease process. The first prognostic models taking into account the different known prognostic factors are underway [32]. Other, more pathophysiologically relevant biomarkers, such as pNFH levels [33, 34], seem to be promising. But also imaging biomarkers that reliably reflect the extent of motor and extramotor involvement have the potential to become a reliable prognostic determinant.

#### **1.5. Need for an in vivo marker of ALS pathophysiology**

Conventional neuroimaging has only revealed gross pathological insights by showing atrophy of the motor cortex and alterations of the CST. These imaging modalities can reveal structural changes in ALS at the group level with high spatial resolution. However, they lack the capacity to provide a reflection of the neuropathological process at the cellular or molecular level and are not applicable at the individual patient level. Imaging biomarkers that can visualize ALS disease pathology, such as neuroinflammation, neuronal death and ideally TDP-43 accumu‐ lation would greatly advance the field of ALS research. It would not only be valuable for diagnostic purposes, but would also be useful to monitor the evolution of disease over time and as a readout for treatment effects of disease-modifying therapies.

cortical hypoexcitability [19, 20], others found a clear hyperexcitablity [11, 21, 22]. Moreover, several studies showed that TMS cannot discriminate ALS patients from controls [18, 23], so

An early and certain diagnosis of ALS is of utmost importance for clinicians and patients. It allows an early initiation of riluzole therapy (as yet the only proven disease-modifying therapy for ALS), an accurate communication about the diagnosis and an early recruitment into clinical trials. Especially, in these early stages of the disease, the disease process may be amenable to

However, in daily clinical practice, the time between symptom onset and diagnosis (the diagnostic delay) is long, estimated to be 12–14 months in tertiary ALS referral centres [24]. There are various reasons for this delay, including patients-specific and doctor-specific delays. Aspecific presentations and phenotypic variability at onset contributes to the delay. Observa‐ tion of patients, with repeat clinical examinations and electrodiagnostic testing is a reliable method to correctly identify ALS patients that present early with only mild and focal motor symptoms, but this approach increases the diagnostic delay. Some cases pose a differential diagnostic problem with certain ALS mimicking diseases, also adding significantly to the diagnostic delay. Since most of these diseases lack abnormalities on CT or MRI imaging, they can impossibly be excluded by conventional imaging. Only a tool that allows to make a positive

ALS is a heterogeneous disorder. Not only the genetic causes and the age at onset, but also the disease progression is highly variable. The median survival after disease onset is only 33 months and most patients die 2–5 years after disease onset. However, numerous cases of extremely long and extremely short survival have been reported, making up the extreme ends of a wide prognostic spectrum [25]. Making a reliable prognostic estimation is pivotal for both patients and their families and neurologists likewise, but still largely impossible these days. A variety of prognostic factors has been identified, such as age of onset, site of onset, rate of symptom progression, comorbid frontotemporal involvement and nutritional and respiratory status [26–31]. Although they are clearly of value, they all reflect divergent clinical disease parameters and do not directly reflect the underlying disease process. The first prognostic models taking into account the different known prognostic factors are underway [32]. Other, more pathophysiologically relevant biomarkers, such as pNFH levels [33, 34], seem to be promising. But also imaging biomarkers that reliably reflect the extent of motor and extramotor

Conventional neuroimaging has only revealed gross pathological insights by showing atrophy of the motor cortex and alterations of the CST. These imaging modalities can reveal structural

diagnosis of ALS early in the disease course could solve this problem.

involvement have the potential to become a reliable prognostic determinant.

**1.5. Need for an in vivo marker of ALS pathophysiology**

better methods to detect UMN loss in ALS are needed.

**1.3. Need for a (differential) diagnostic test**

26 Update on Amyotrophic Lateral Sclerosis

**1.4. Need for a prognostic marker**

therapy.

So far, positron emission tomography (PET) has not been commonly used in ALS. However, recent studies show that various radioligands have potential to be useful imaging biomarkers in ALS. These can be of value in the diagnosis, in predicting outcome and in imaging disease pathology in ALS. In this chapter, an overview of the PET studies in ALS is given and future perspectives on the use of PET in ALS are discussed. On overview of all tracers used in ALS

**Figure 1. PET tracers used in ALS**. Using PET imaging six different neurological systems or cell populations can be assessed using different tracers. General glucose metabolism in the grey matter is assessed by [18F]-FDG. Cortical amy‐ loid deposition can be assessed using [11C]-PIB to distinguish ALS from Alzheimer's disease. The extrapyramidal sys‐ tem can be investigated using [18F]-fluorodopa, [18F]-FPCIT or [123I]-FP-CIT. Neuronal integrity can be visualized using tracers for the GABA-A receptor ([11C]-flumazenil) or the 5-HT1A receptor ([11C]-WAY100635). Microglial cells can be highlighted using tracers targeting the translocator protein (TSPO), like [18F]-DPA-174, [11C]-(R)-PK11195 or [11C]- PBR28. Astrocytes are visualized by tracers for the MAO-B enzyme ([11C]-DED).

so far is provided in **Figure 1**. Apart from the commonly used tracers for indirect neuronal functioning, such as glucose metabolism ([18F]-FDG) and perfusion, more specific receptor or protein deposition tracers used in other neurodegenerative diseases, like Parkinson's disease ([18F]-FP-CIT) and Alzheimer's disease ([11C]-PIB), have also been investigated in ALS patients. Recently, tracers with an affinity for specific cell types, like neurons ([11C]-flumazenil, [11C]- WAY100635), microglial cells (TSPO ligands) and astrocytes ([11C]-DED), can also highlight specific pathophysiological processes of ALS.
